test: the format and ASCII rules stop being a habit here too

libavr's guidance binds this repo, and its gate checked every chip's codegen
without ever checking whether the sources it compiled were clang-format clean
or ASCII. `libavr_format_test()` does both now, over this tree alone - the
oracle's assembly needs no exclusion, being in neither glob, which is the right
answer for a vendored reference whose text is the artifact.

The sizes this repo prints were already gated: `sizes.py check-readme` is the
shape the rest of the fleet has now copied.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.clang-format

@@ -7,8 +7,9 @@ TabWidth: 4
 UseTab: ForIndentation
 AlignEscapedNewlines: DontAlign
 AllowShortFunctionsOnASingleLine: Empty
-AlwaysBreakTemplateDeclarations: true
+BreakTemplateDeclarations: Yes
 BreakBeforeBraces: Custom
 BraceWrapping:
   AfterFunction: true
+InsertBraces: true
 ...

.clangd

@@ -0,0 +1,38 @@
+# Editor accommodations for the second frontend. No compilation database is
+# named here: this repo rides as a submodule in its consumers, and this file
+# travels with it — a consumer's own database then covers these sources, with
+# that project's loader flags. The checkout that is opened as a folder names
+# its build tree in .vscode/settings.json instead.
+CompileFlags:
+  Add:
+    # clang has no 24-bit integer and GCC's are keywords, not macros, so the
+    # editor needs a stand-in for avr::uint24_t. The next width up is the only
+    # one available — clang rejects _BitInt(24) on this target.
+    - -D__uint24=unsigned long
+    - -D__int24=long
+    # clangd forwards the driver's system includes but not its own header
+    # directory, so <stdint.h> resolves to avr-libc's, which still gates the
+    # limit and constant macros on the C++98 opt-in.
+    - -D__STDC_LIMIT_MACROS
+    - -D__STDC_CONSTANT_MACROS
+    # isr::emit spells a vector number into [[gnu::signal(N)]], which clang
+    # rejects rather than ignores — enough of them in one TU to reach the
+    # default limit of 19 inside the headers and truncate the parse.
+    - -ferror-limit=0
+  Remove:
+    # Codegen shaping the loader TUs carry and clang has no spelling for.
+    - -fira-algorithm=*
+    - -fno-split-wide-types
+    - -fno-tree-ter
+    - -fno-ivopts
+    - -fno-move-loop-invariants
+    # The build promotes warnings for the compiler that has to be right about
+    # them; in the editor the flag paints a second frontend's opinions in the
+    # colour reserved for things that do not compile.
+    - -Werror
+Diagnostics:
+  Suppress:
+    # clang's AVR `signal` attribute takes no arguments and it knows none of
+    # progmem, naked or OS_main. A misspelling is what the build is for.
+    - attribute_wrong_number_arguments
+    - unknown-attributes

.vscode/extensions.json

@@ -0,0 +1,6 @@
+{
+	"recommendations": [
+		"llvm-vs-code-extensions.vscode-clangd",
+		"ms-vscode.cmake-tools"
+	]
+}

.vscode/settings.json

@@ -0,0 +1,37 @@
+{
+	// clangd is the language server; the cpptools engine would parse every file
+	// a second time and disagree, since nothing tells it about a cross
+	// compiler.
+	"C_Cpp.intelliSenseEngine": "disabled",
+
+	// --query-driver lets clangd ask the cross compiler for its own system
+	// includes and target. The database is named here rather than in .clangd
+	// because that file travels with the driver into a consumer's submodule,
+	// where a build tree of this repo's own need not exist.
+	"clangd.arguments": [
+		"--compile-commands-dir=${workspaceFolder}/build/atmega328p-generated",
+		"--query-driver=**avr-g++*",
+		"--header-insertion=never"
+	],
+
+	// The presets are the build interface; the toolchain file inside the
+	// libavr submodule is the one place the compiler is chosen. The submodule
+	// carries no local/toolchain for it to discover, so the prefix is named
+	// here, for the window that opens this folder.
+	"cmake.useCMakePresets": "always",
+	"cmake.environment": {
+		"LIBAVR_TOOLCHAIN": "D:/dev/libavr/local/toolchain/avr-gcc-16.1.0-mingw"
+	},
+	"cmake.configureOnOpen": true,
+	"cmake.options.statusBarVisibility": "compact",
+
+	"files.watcherExclude": {
+		"**/build/**": true,
+		"**/libavr/**": true
+	},
+
+	"files.associations": {
+		".clangd": "yaml",
+		".clang-format": "yaml"
+	}
+}

CMakeLists.txt

@@ -2,64 +2,73 @@ cmake_minimum_required(VERSION 3.28)
 
 project(tsb_libavr LANGUAGES CXX)
 
-# libavr from a local checkout (LIBAVR_ROOT) or the forge; the toolchain file
-# comes from the same checkout via CMakePresets.json.
-include(FetchContent)
+# libavr rides as the pinned submodule; LIBAVR_ROOT (cache or environment)
+# overrides it for tandem development against a working tree. The toolchain
+# file comes from the submodule via CMakePresets.json either way.
 if(NOT LIBAVR_ROOT AND DEFINED ENV{LIBAVR_ROOT})
 	set(LIBAVR_ROOT $ENV{LIBAVR_ROOT})
 endif()
 if(NOT LIBAVR_ROOT)
 	set(LIBAVR_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/libavr)
 endif()
-if(LIBAVR_ROOT)
-	FetchContent_Declare(libavr SOURCE_DIR ${LIBAVR_ROOT})
-else()
-	FetchContent_Declare(libavr GIT_REPOSITORY git@git.blackmark.me:avr/libavr.git GIT_TAG main)
+if(NOT EXISTS ${LIBAVR_ROOT}/CMakeLists.txt)
+	message(FATAL_ERROR "libavr not found at ${LIBAVR_ROOT} - run: git submodule update --init libavr")
 endif()
-FetchContent_MakeAvailable(libavr)
+add_subdirectory(${LIBAVR_ROOT} libavr-build)
+
+include(${LIBAVR_ROOT}/cmake/checks.cmake)
 
 if(PROJECT_IS_TOP_LEVEL)
 	add_compile_options(-Werror) # warnings are errors for the port's own code
 	enable_testing()
 
+	# Rules 11 and 33 over this repo's own sources. The oracle's assembly needs
+	# no exclusion: it is neither formatted nor ASCII-checked, being in neither
+	# glob, which is the right answer for a vendored reference whose text is
+	# the artifact.
+	libavr_format_test()
+
 	# The behavioral tests drive the real wire protocols over a simavr pty
 	# (as the host tools do) and actually flash the device. The runners are
-	# host programs built at configure time against libsimavr; if they or
-	# Python are missing, only the size tests run.
-	find_program(_host_cc NAMES cc gcc)
+	# host programs built at configure time against libsimavr (C++23 - what
+	# the distribution's compiler speaks in full); if they or Python are
+	# missing, only the size tests run.
+	find_program(_host_cxx NAMES c++ g++)
 	find_package(Python3 COMPONENTS Interpreter)
-	if(_host_cc AND Python3_FOUND)
+	if(_host_cxx AND Python3_FOUND)
 		set(PB_DEVICE ${CMAKE_BINARY_DIR}/pureboot_device)
 		execute_process(
-		    COMMAND ${_host_cc} -O2 -I/usr/include/simavr -I/usr/include/simavr/parts
-		            -o ${PB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pureboot_device.c
+		    COMMAND ${_host_cxx} -std=c++23 -Wall -Wextra -O2
+		            -I/usr/include/simavr -I/usr/include/simavr/parts
+		            -o ${PB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pureboot_device.cpp
 		            -lsimavr -lsimavrparts -lelf -lutil
 		    RESULT_VARIABLE _pbdev_res ERROR_VARIABLE _pbdev_err)
 		if(NOT _pbdev_res EQUAL 0)
-			message(STATUS "pureboot_device not built (${_pbdev_err}) — protocol tests skipped")
+			message(STATUS "pureboot_device not built (${_pbdev_err}) - protocol tests skipped")
 			unset(PB_DEVICE)
 		endif()
 		if(LIBAVR_MCU STREQUAL "atmega328p")
 			set(TSB_DEVICE ${CMAKE_BINARY_DIR}/tsb_device)
 			execute_process(
-			    COMMAND ${_host_cc} -O2 -I/usr/include/simavr -I/usr/include/simavr/parts
-			            -o ${TSB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/device.c
+			    COMMAND ${_host_cxx} -std=c++23 -Wall -Wextra -O2
+			            -I/usr/include/simavr -I/usr/include/simavr/parts
+			            -o ${TSB_DEVICE} ${CMAKE_CURRENT_SOURCE_DIR}/test/device.cpp
 			            -lsimavr -lsimavrparts -lelf
 			    RESULT_VARIABLE _dev_res ERROR_VARIABLE _dev_err)
 			if(NOT _dev_res EQUAL 0)
-				message(STATUS "tsb_device not built (${_dev_err}) — protocol tests skipped")
+				message(STATUS "tsb_device not built (${_dev_err}) - protocol tests skipped")
 				unset(TSB_DEVICE)
 			endif()
 		endif()
 	endif()
 endif()
 
-# The ELF is only a container (symbols, section headers) and is never flashed —
+# The ELF is only a container (symbols, section headers) and is never flashed -
 # and the host tool's load_image() dispatches on extension, so handing it one
 # would silently program the header bytes. Every loader image therefore gets
 # both flashable forms beside it at link time: .hex for avrdude, and .bin for
 # the host tool's raw path (which is what the reloc and update tests convert to
-# on the fly). .eeprom is dropped — EEPROM content is its own update.
+# on the fly). .eeprom is dropped - EEPROM content is its own update.
 function(add_image_outputs name)
 	add_custom_command(TARGET ${name} POST_BUILD
 	                   COMMAND ${CMAKE_OBJCOPY} -O ihex -R .eeprom
@@ -68,35 +77,39 @@ function(add_image_outputs name)
 	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.bin)
 endfunction()
 
-# The TinySafeBoot protocol reimplemented on libavr in three variants that trade
+# The TinySafeBoot protocol reimplemented on libavr in variants that trade
 # clarity for size. Each links into the ATmega328P boot section (BOOTSZ selects
-# its size; BOOTRST vectors a reset to its base) with -nostartfiles — a polled
-# loader has no use for the crt or the vector table. The naked entry sits in
-# .vectors, laid first, and runs. The boot base is FLASHEND+1 minus the section
-# size; the linker section-start and the source's boot_bytes agree. tsb_app is
+# its size; BOOTRST vectors a reset to its base) with -nostartfiles - a polled
+# loader has no use for the crt or the vector table. The entry sits in
+# .vectors, laid first, and runs - avr::startup::entry on the policy tier,
+# the experiment tiers' own naked stubs elsewhere, each documented in its
+# source. The boot base is FLASHEND+1 minus the section size; the linker
+# section-start and the source's boot_bytes agree. tsb_app is
 # the application's reset vector, pinned to 0 here so the loaders jump to a
 # named function; --pmem-wrap-around lets relaxation turn that absolute jump
 # into the wrapped rjmp AVR's modulo-flash PC actually executes.
-# All three implement the full oracle feature set (see oracle/README.md):
+# All four implement the full oracle feature set (see oracle/README.md):
 # watchdog bail, one-wire half-duplex, config-page activation timeout, password
 # gate, emergency erase, config/flash/EEPROM read-write. They differ only in how,
-# and the size gradient is the cost of that "how" — see dev/lessons.md.
-#   tsb_asm   — the tricks tier's C++ with exactly two routines in asm (the
-#                 bounded rx and the page-store loop — the two whose remaining
-#                 cost is the C ABI itself): 510 B in the 512 B section the
-#                 hand-written 500 B oracle occupies. Everything else, from
-#                 bring-up to dispatch, is C++ on libavr.
-#   tsb_tricks — no asm at all: the whole-loader register allocation lives in
+# and the size gradient is the cost of that "how".
+#   tsb_asm    - the tricks tier's C++ with exactly two routines in asm: the
+#                 bounded rx and the page-store loop, the two whose remaining
+#                 cost is the C ABI itself. Everything else, bring-up to
+#                 dispatch, is C++ on libavr.
+#   tsb_tricks - no asm at all: the whole-loader register allocation lives in
 #                 global register variables (Y walks the page pointer), every
 #                 helper is a tiny noinline primitive placed by the
 #                 global-register store rules, pages stream straight to
-#                 SPM/EEPROM, and the bring-up is the two reset-non-default
-#                 registers only. 526 B in the 1 KB section (BOOTSZ=10) — 14
-#                 over the oracle's section, from 168 over at this tier's first
-#                 floor.
-#   tsb_pure  — pure idiomatic libavr, one function per command, TU-local
-#                 (internal linkage), streaming (no SRAM page buffer): 836 B in
-#                 the 1 KB section.
+#                 SPM/EEPROM.
+#   tsb_pure   - pure idiomatic libavr, one function per command, TU-local
+#                 (internal linkage), streaming (no SRAM page buffer).
+#   tsb_policy - the policy floor: pureboot's rules (no asm, no register
+#                 variables) with every pureboot lesson applied, and the
+#                 measured evidence that the 512 B fit is a property of the
+#                 mechanisms philosophy #5 bans.
+#
+# What each measures is oracle/README.md's table, which is the one place the
+# four numbers and the hand-written loader's own are compared.
 #
 # add_tsb_variant(<name> <boot-section-bytes>)
 function(add_tsb_variant name bytes)
@@ -124,14 +137,19 @@ endfunction()
 # chips build pureboot alone.
 if(LIBAVR_MCU STREQUAL "atmega328p")
 	add_tsb_variant(tsb_asm 512)
+	add_tsb_variant(tsb_policy 1024)
 	add_tsb_variant(tsb_pure 1024)
 	add_tsb_variant(tsb_tricks 1024)
+	# The policy tier's floor is measured with the loop flags pureboot's size
+	# work found (a loader's loop bodies all contain calls); the other tiers
+	# keep the flag set their recorded floors were measured with - none.
+	target_compile_options(tsb_policy PRIVATE -fno-move-loop-invariants -fno-tree-ter)
 endif()
 
-# pureboot — the pure-constraint port (see pureboot/README.md): one source,
+# pureboot - the pure-constraint port (see pureboot/README.md): one source,
 # no inline assembly, no global register variables, every libavr chip,
 # fitting each chip's smallest boot sector. The geometry and the
-# pureboot_add_loader() deployment function live in pureboot/CMakeLists.txt —
+# pureboot_add_loader() deployment function live in pureboot/CMakeLists.txt -
 # the unit a downstream project consumes; everything below is this port's
 # own build: the stock loaders, their tests, and the size matrix. The
 # distinct binary dir keeps the `pureboot` target's output name free.
@@ -139,7 +157,7 @@ add_subdirectory(pureboot pureboot-cmake)
 
 # The stock loader: the family-default deployment (crystal/RC clock, the
 # chip's natural link, default pins). The activation window stays a cache
-# variable — re-timing a deployed loader is a self-update with a re-timed
+# variable - re-timing a deployed loader is a self-update with a re-timed
 # build. pureboot9 is that re-timed build, and what the update test installs.
 set(PUREBOOT_TIMEOUT 8 CACHE STRING "pureboot activation window, seconds")
 pureboot_add_loader(pureboot TIMEOUT ${PUREBOOT_TIMEOUT})
@@ -153,10 +171,25 @@ if(PROJECT_IS_TOP_LEVEL)
 	if(Python3_FOUND)
 		add_test(NAME pureboot.pi
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/check_pi.py
-		                 ${CMAKE_OBJDUMP} ${CMAKE_NM} $<TARGET_FILE:pureboot> ${PUREBOOT_BASE_HEX})
+		                 ${CMAKE_OBJDUMP} ${CMAKE_OBJCOPY} ${CMAKE_CXX_COMPILER} ${LIBAVR_MCU}
+		                 $<TARGET_FILE:pureboot>
+		                 ${CMAKE_BINARY_DIR}/CMakeFiles/pureboot.dir/pureboot/pureboot.cpp.obj
+		                 ${PUREBOOT_BASE_HEX})
 		add_test(NAME pureboot.planner
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_planner.py
 		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py)
+		add_test(NAME pureboot.scan
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_scan.py
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py)
+		# CMakePresets.json is generated; hand edits drift the moment the
+		# generator reruns, so the gate holds the pair together.
+		add_test(NAME presets.generated
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/tools/make_presets.py
+		                 --check)
+		add_test(NAME pureboot.handshake
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_handshake.py)
+		add_test(NAME pureboot.updatelink
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_update_link.py)
 	endif()
 
 	# The protocol test flashes this fixture through the loader with the real
@@ -175,6 +208,38 @@ if(PROJECT_IS_TOP_LEVEL)
 		                 ${CMAKE_BINARY_DIR}/pbtest-work)
 		set_tests_properties(pureboot.protocol PROPERTIES TIMEOUT 180)
 
+		# The activation window as a measured duration: application installed,
+		# line idle, the first transmit is the application's banner - its
+		# cycle is the window the source declares, held to +/-2 % (one
+		# mis-counted cycle per poll is a 10 % shift).
+		add_test(NAME pureboot.window
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbwindow.py
+		                 --device ${PB_DEVICE} --loader $<TARGET_FILE:pureboot>
+		                 --mcu ${PUREBOOT_SIM_MCU} --hz ${_pb_stock_hz}
+		                 --base ${PUREBOOT_BASE_HEX} --page ${PUREBOOT_PAGE}
+		                 --baud ${_pb_stock_baud} --app $<TARGET_FILE:pbapp>.bin
+		                 --seconds ${PUREBOOT_TIMEOUT}
+		                 --tool ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 --workdir ${CMAKE_BINARY_DIR}/pbwindow-work)
+		set_tests_properties(pureboot.window PROPERTIES TIMEOUT 300)
+
+		# The half-duplex loader's window, same gate: its poll runs through
+		# readable()'s release-line test, whose outlined call re-shapes the
+		# whole loop - a per-class cycle count (poll_cost() in pureboot.cpp)
+		# that only the built image can prove, chip by chip.
+		if(PUREBOOT_HAS_USART)
+			add_test(NAME pureboot.window.halfduplex
+			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbwindow.py
+			                 --device ${PB_DEVICE} --loader $<TARGET_FILE:pureboot_hd>
+			                 --mcu ${PUREBOOT_SIM_MCU} --hz ${_pb_stock_hz}
+			                 --base ${PUREBOOT_BASE_HEX} --page ${PUREBOOT_PAGE}
+			                 --baud ${_pb_stock_baud} --app $<TARGET_FILE:pbapp>.bin
+			                 --seconds ${PUREBOOT_TIMEOUT}
+			                 --tool ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+			                 --workdir ${CMAKE_BINARY_DIR}/pbwindow-hd-work)
+			set_tests_properties(pureboot.window.halfduplex PROPERTIES TIMEOUT 300)
+		endif()
+
 		# The position-independence acceptance test: the identical image,
 		# installed one slot lower, must serve the full command set.
 		add_test(NAME pureboot.reloc
@@ -186,8 +251,30 @@ if(PROJECT_IS_TOP_LEVEL)
 		set_tests_properties(pureboot.reloc PROPERTIES TIMEOUT 180
 		                     ENVIRONMENT "PB_OBJCOPY=${CMAKE_OBJCOPY}")
 
+		# The seal against the one command that proves it: erasing the page the
+		# loader runs from, refused unsealed and honoured sealed. Destroys the
+		# loader by design, so it gets a device of its own.
+		add_test(NAME pureboot.selfwrite
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbselfwrite.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud}
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbselfwrite-work)
+		set_tests_properties(pureboot.selfwrite PROPERTIES TIMEOUT 180)
+
+		# The seal against a link that damages bytes on purpose: every header
+		# field flipped after sealing must be refused, and the identical flip
+		# applied before sealing must be obeyed.
+		add_test(NAME pureboot.glitch
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbglitch.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud}
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbglitch-work)
+		set_tests_properties(pureboot.glitch PROPERTIES TIMEOUT 180)
+
 		# Entering the loader from a running application with no reset
-		# between, over a page buffer the application dirtied — the case the
+		# between, over a page buffer the application dirtied - the case the
 		# loader declines to guard and the host repairs. Hardware forbids the
 		# state here (SPM runs only from the boot section); simavr does not,
 		# which is what makes it constructible.
@@ -218,7 +305,7 @@ if(PROJECT_IS_TOP_LEVEL)
 		endif()
 
 		# The self-update end-to-end: the re-timed build (same source, only
-		# the timeout differs — a byte-different image) replaces the resident
+		# the timeout differs - a byte-different image) replaces the resident
 		# through --update-loader, with every power-fail phase rehearsed from
 		# the runner's flash dumps.
 		pureboot_add_loader(pureboot9 TIMEOUT 9)
@@ -234,13 +321,13 @@ if(PROJECT_IS_TOP_LEVEL)
 	endif()
 
 	# The size matrix: every configuration axis that could move the image
-	# size — the serial backend (different code), the USART instance
-	# (different registers), the clock (different constants), and the baud
-	# through the shapes its bit timing takes — each combination must still
-	# fit the chip's slot budget. Pins are size-neutral (port and bit are
-	# immediate operands) and the timeout is a constant, so neither adds an
-	# axis. The stock build is one point of this matrix and already has its
-	# test.
+	# size - the serial backend (different code), the USART instance
+	# (different registers), the clock (different constants), the baud
+	# through the shapes its bit timing takes, and the pins through the one
+	# thing they decide (whether a bit-banged link has to release the USART
+	# that owns them) - each combination must still fit the chip's slot
+	# budget. The timeout is a constant and adds no axis. The stock build is
+	# one point of this matrix and already has its test.
 	function(pureboot_size_variant name)
 		pureboot_add_loader(${name} ${ARGN})
 		add_test(NAME ${name}.size
@@ -248,43 +335,64 @@ if(PROJECT_IS_TOP_LEVEL)
 		                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
 	endfunction()
 
-	# The autobaud loader (pureboot/autobaud.md): one clock-agnostic image per
-	# chip, no clock × baud axis, size-tested against the same per-chip budget on
-	# every chip.
-	#
-	# Only the unified version is built. Its two predecessors —
-	# pureboot_autobaud_pure.cpp at 508 B and pureboot_autobaud_reg.cpp at 512 —
-	# had 4 B and 0 B of margin on the 1284P, and the fix for the activation hang
-	# costs ~22, which puts them at 530 and 534. Neither can ship, so the choice
-	# the branch existed to offer is settled by measurement rather than taste.
-	# The sources stay for the record; autobaud.md carries the numbers.
-	function(pureboot_autobaud_variant name source)
-		pureboot_add_autobaud(${name} ${source})
-		add_test(NAME ${name}.size
-		         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:${name}>
-		                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
-	endfunction()
-	pureboot_autobaud_variant(pureboot_autobaud_uni pureboot_autobaud_uni.cpp)
+	# The autobaud loader: one clock-agnostic image per chip, so it has no
+	# clock x baud axis of its own - the matrix below sweeps those for the
+	# fixed-baud builds, and this one binary has to serve all of them at run
+	# time. Size-tested against the same per-chip budget as every other variant.
+	pureboot_add_loader(pureboot_autobaud SERIAL autobaud)
+	add_test(NAME pureboot_autobaud.size
+	         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:pureboot_autobaud>
+	                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
+	# The measured unit's home is wire contract, not layout accident: the
+	# host reads the bit period from it (--info's measured clock). In the
+	# GPIOR home the image must carry no RAM copy at all; in the RAM home it
+	# is the loader's only RAM object, at the very start of SRAM.
+	add_test(NAME pureboot_autobaud.unit
+	         COMMAND ${CMAKE_COMMAND} -DOBJDUMP=${CMAKE_OBJDUMP} -DELF=$<TARGET_FILE:pureboot_autobaud>
+	                 -DRAM_START=${PUREBOOT_RAM_START} -DGPIOR=${PUREBOOT_UNIT_GPIOR}
+	                 -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_unit.cmake)
 
 	# One point of the exhaustive matrix, named from its resolved parameters
-	# so the enumeration cannot collide with itself. Unreachable rates drop
-	# out here rather than aborting the configure.
-	function(pureboot_matrix_point hz baud link)
+	# so the enumeration cannot collide with itself. `pins` is empty for the
+	# default pair, or the index of the USART whose own pins a bit-banged
+	# link sits on. Unreachable rates drop out here rather than aborting the
+	# configure.
+	# The optional trailing argument is the one-wire shape of the same link:
+	# ONE_WIRE folds a software point onto its RX pin (the default, or the
+	# named USART's RXD), HALF_DUPLEX is the hardware USART's turn-around.
+	function(pureboot_matrix_point hz baud link pins)
+		set(_name pbm_${hz}_${baud}_${link})
 		if(link STREQUAL "software")
 			pureboot_baud_feasible(${hz} ${baud} 1 _ok)
 			set(_args SERIAL software)
+			if(NOT pins STREQUAL "")
+				list(APPEND _args RX ${PUREBOOT_USART${pins}_RX} TX ${PUREBOOT_USART${pins}_TX})
+				set(_name ${_name}_on${pins})
+			endif()
+			if(ARGC GREATER 4 AND ARGV4 STREQUAL "ONE_WIRE")
+				if(NOT pins STREQUAL "")
+					set(_args SERIAL software RX ${PUREBOOT_USART${pins}_RX} TX ${PUREBOOT_USART${pins}_RX})
+				else()
+					list(APPEND _args RX pb0 TX pb0)
+				endif()
+				set(_name ${_name}_1w)
+			endif()
 		else()
 			pureboot_baud_feasible(${hz} ${baud} 0 _ok)
 			set(_args USART ${link})
+			if(ARGC GREATER 4 AND ARGV4 STREQUAL "HALF_DUPLEX")
+				list(APPEND _args HALF_DUPLEX)
+				set(_name ${_name}_hd)
+			endif()
 		endif()
 		if(_ok)
-			pureboot_size_variant(pbm_${hz}_${baud}_${link} CLOCK ${hz} BAUD ${baud} ${_args})
+			pureboot_size_variant(${_name} CLOCK ${hz} BAUD ${baud} ${_args})
 		endif()
 	endfunction()
 
 	# Clock points: the shipped-fuse floor (CKDIV8), the calibrated RC, and
 	# the crystal the stock build assumes (the tiny13's ladder is its own RC
-	# menu — it has no crystal option).
+	# menu - it has no crystal option).
 	if(LIBAVR_MCU MATCHES "^attiny13")
 		set(_matrix_clocks 1200000 4800000 9600000)
 		set(_full_clocks 128000 600000 1200000 4800000 9600000)
@@ -295,34 +403,40 @@ if(PROJECT_IS_TOP_LEVEL)
 	endif()
 
 	# The exhaustive cross product: every clock a deployment plausibly runs
-	# — the internal oscillators, the shipped CKDIV8 floor, the plain
-	# crystals and the UART crystals — against every rate, against every
+	# - the internal oscillators, the shipped CKDIV8 floor, the plain
+	# crystals and the UART crystals - against every rate, against every
 	# backend. Beyond the ladder the list carries the slow rates a
 	# sub-megahertz oscillator is left with, which no ladder rate reaches
 	# (16000 Bd is the only rate the 128 kHz oscillator holds exactly); at
 	# the fast clocks those same rates also select the software UART's
 	# 16-bit _delay_loop_2 bit spin (two words more setup at each of its five
 	# sites), the largest image the space produces and a shape the ladder
-	# default — always the *fastest* rate a clock reaches — never picks.
+	# default - always the *fastest* rate a clock reaches - never picks.
 	#
-	# Bounded to one chip per size-bearing class: flash addressing (the
-	# word-addressed 1284), hand-over shape (the patched vector on the tinies
-	# and m48s), page size, and USART inventory. Everything else in the image
-	# is chip-independent code, so a further chip buys builds and no
-	# coverage; every chip outside the set carries the compact matrix.
+	# Every chip runs the full cross product: the size-bearing classes (flash
+	# addressing, hand-over shape, page size, USART inventory) are what make
+	# the image differ, and a chip outside them is expected to match its class
+	# - but "expected" is what a matrix is for, and the whole sweep is cheap
+	# enough to run rather than reason about. PUREBOOT_FULL_MATRIX is what
+	# selects it; the compact matrix below is the per-commit default.
 	get_property(_full_bauds GLOBAL PROPERTY PUREBOOT_BAUD_LADDER)
 	list(APPEND _full_bauds 16000 4800 2400 1200)
-	set(_matrix_spot attiny13a attiny85 atmega48pa atmega8a atmega168pa
-	                 atmega328p atmega164a atmega644a atmega1284p)
-	if(DEFINED ENV{PUREBOOT_FULL_MATRIX} AND LIBAVR_MCU IN_LIST _matrix_spot)
+	if(DEFINED ENV{PUREBOOT_FULL_MATRIX})
 		foreach(_matrix_hz IN LISTS _full_clocks)
 			foreach(_matrix_baud IN LISTS _full_bauds)
-				pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software)
+				pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software "")
+				pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software "" ONE_WIRE)
 				if(PUREBOOT_HAS_USART)
-					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 0)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software 0)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software 0 ONE_WIRE)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 0 "")
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 0 "" HALF_DUPLEX)
 				endif()
 				if(PUREBOOT_HAS_USART1)
-					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 1)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software 1)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software 1 ONE_WIRE)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 1 "")
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 1 "" HALF_DUPLEX)
 				endif()
 			endforeach()
 		endforeach()
@@ -341,16 +455,103 @@ if(PROJECT_IS_TOP_LEVEL)
 		endforeach()
 		list(GET _matrix_clocks -1 _matrix_top_hz)
 		pureboot_size_variant(pureboot_sw_wide CLOCK ${_matrix_top_hz} BAUD 9600 SERIAL software)
+		# The pin axis at the widest software image - the slowest ladder rate
+		# against the fastest clock, whose bit spin needs the 16-bit delay
+		# loop - with the USART release on top of it. The exhaustive sweep
+		# above carries the same axis across its whole cross product.
+		if(PUREBOOT_HAS_USART)
+			pureboot_size_variant(pureboot_sw_wide_on_usart0 CLOCK ${_matrix_top_hz} BAUD 9600
+			                      SERIAL software RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_TX})
+		endif()
+		if(PUREBOOT_HAS_USART1)
+			pureboot_size_variant(pureboot_sw_wide_on_usart1 CLOCK ${_matrix_top_hz} BAUD 9600
+			                      SERIAL software RX ${PUREBOOT_USART1_RX} TX ${PUREBOOT_USART1_TX})
+		endif()
 	endif()
 	if(PUREBOOT_HAS_USART1)
 		pureboot_size_variant(pureboot_usart1 USART 1)
 	endif()
 
-	# One configured deployment end to end — a real board's shape rather
+	# The pin axis at its fixed points, in both matrix modes. The autobaud
+	# loader carries no clock and no baud, so the sweep has nothing to vary
+	# for it - yet it is the tightest image in the space, and on a USART's
+	# own pins it pays the release too: that combination is the one that
+	# overflowed the 1284's slot. The software build on those pins is the
+	# same deployment the mute test drives.
+	if(PUREBOOT_HAS_USART)
+		pureboot_size_variant(pureboot_sw_on_usart0 SERIAL software
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_TX})
+		pureboot_size_variant(pureboot_autobaud_on_usart0 SERIAL autobaud
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_TX})
+	endif()
+	if(PUREBOOT_HAS_USART1)
+		pureboot_size_variant(pureboot_sw_on_usart1 SERIAL software
+		                      RX ${PUREBOOT_USART1_RX} TX ${PUREBOOT_USART1_TX})
+		pureboot_size_variant(pureboot_autobaud_on_usart1 SERIAL autobaud
+		                      RX ${PUREBOOT_USART1_RX} TX ${PUREBOOT_USART1_TX})
+	endif()
+
+	# The OSCCAL axis at its fixed points: the stock shape, and the tightest
+	# image in the space with the trim on top - the axis adds one register
+	# write, and these points hold both of its addressing encodings to every
+	# chip's budget.
+	pureboot_size_variant(pureboot_osccal OSCCAL 0x9c)
+	pureboot_size_variant(pureboot_autobaud_osccal SERIAL autobaud OSCCAL 0x9c)
+	if(PUREBOOT_HAS_USART)
+		pureboot_size_variant(pureboot_autobaud_osccal_on_usart0 SERIAL autobaud OSCCAL 0x9c
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_TX})
+	endif()
+
+	# The one-wire axis at its fixed points, in both matrix modes (the
+	# exhaustive sweep carries the same shapes across its cross product):
+	# the software link folded onto one pin, the tightest autobaud image
+	# likewise - on the default pin and on the USART's own RXD, whose
+	# release the now-driven shared pin needs where a receive-only link
+	# would not - and the hardware USART's half-duplex turn-around, stock
+	# and at the widest fixed-baud shape.
+	# The two spellings deliberately split across the two points: HALF_DUPLEX
+	# folds TX onto RX, RX == TX states the same thing directly.
+	pureboot_size_variant(pureboot_1w SERIAL software RX pb0 HALF_DUPLEX)
+	pureboot_size_variant(pureboot_1w_autobaud_osccal SERIAL autobaud OSCCAL 0x9c RX pb0 TX pb0)
+	if(PUREBOOT_HAS_USART)
+		pureboot_size_variant(pureboot_1w_on_usart0 SERIAL software
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_RX})
+		pureboot_size_variant(pureboot_1w_autobaud_osccal_on_usart0 SERIAL autobaud OSCCAL 0x9c
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_RX})
+		pureboot_size_variant(pureboot_hd HALF_DUPLEX)
+		list(GET _matrix_clocks -1 _hd_top_hz)
+		pureboot_size_variant(pureboot_hd_wide CLOCK ${_hd_top_hz} BAUD 9600 HALF_DUPLEX)
+	endif()
+	if(PUREBOOT_HAS_USART1)
+		pureboot_size_variant(pureboot_usart1_hd USART 1 HALF_DUPLEX)
+	endif()
+
+	# The trim byte, observed through the wire from the first prompt - one
+	# chip per OSCCAL addressing class: extended I/O on the 328P (data 0x66,
+	# an sts - DS40002061B section 36), plain I/O on the 85 (data 0x51, an out -
+	# Atmel-2586 section 21).
+	if(LIBAVR_MCU MATCHES "^(atmega328p|attiny85)$" AND DEFINED PB_DEVICE)
+		if(LIBAVR_MCU STREQUAL "atmega328p")
+			set(_osccal_addr 0x66)
+		else()
+			set(_osccal_addr 0x51)
+		endif()
+		get_target_property(_osccal_hz pureboot_osccal PUREBOOT_HZ)
+		get_target_property(_osccal_baud pureboot_osccal PUREBOOT_BAUD)
+		add_test(NAME pureboot.osccal
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbosccal.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_osccal> ${PUREBOOT_SIM_MCU}
+		                 ${_osccal_hz} ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_osccal_baud}
+		                 ${_osccal_addr} 0x9c ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbosccal-work)
+		set_tests_properties(pureboot.osccal PROPERTIES TIMEOUT 120)
+	endif()
+
+	# One configured deployment end to end - a real board's shape rather
 	# than the stock assumption: the ATmega328P on its shipped 1 MHz fuses,
 	# the software UART on hand-picked pins (TX = PB1, RX = PB5), the ladder
 	# baud (9600). The full protocol suite runs against it, fixture
-	# application included, over the runner's GPIO bridge — proving the
+	# application included, over the runner's GPIO bridge - proving the
 	# configuration plumbing produces a working loader, not just one that
 	# fits.
 	if(LIBAVR_MCU STREQUAL "atmega328p" AND DEFINED PB_DEVICE)
@@ -374,6 +575,85 @@ if(PROJECT_IS_TOP_LEVEL)
 		set_tests_properties(pureboot.custom PROPERTIES TIMEOUT 180)
 	endif()
 
+	# Hand-over with the USART that owns the loader's pins left enabled - the
+	# state an application reaches by jumping in without a reset, and the one
+	# that made a bit-banged loader on PD0/PD1 (where the Uno's USB bridge
+	# lands) receive and obey while answering nothing. Run where it was found
+	# on silicon; the runner supplies the pin ownership simavr has no model
+	# for, which is what lets this fail when the release is gone.
+	if(LIBAVR_MCU STREQUAL "atmega328p" AND DEFINED PB_DEVICE)
+		get_target_property(_mute_hz pureboot_sw_on_usart0 PUREBOOT_HZ)
+		get_target_property(_mute_baud pureboot_sw_on_usart0 PUREBOOT_BAUD)
+		get_target_property(_mute_link pureboot_sw_on_usart0 PUREBOOT_LINK)
+		add_executable(pbapp_handover test/pbapp.cpp)
+		target_link_libraries(pbapp_handover PRIVATE libavr)
+		target_compile_definitions(pbapp_handover PRIVATE PUREBOOT_CLOCK_HZ=${_mute_hz}
+		                           PUREBOOT_BAUD=${_mute_baud} PUREBOOT_HANDOVER)
+		add_custom_command(TARGET pbapp_handover POST_BUILD
+		                   COMMAND ${CMAKE_OBJCOPY} -O binary
+		                           $<TARGET_FILE:pbapp_handover> $<TARGET_FILE:pbapp_handover>.bin)
+		add_test(NAME pureboot.mute
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbmute.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_sw_on_usart0> ${PUREBOOT_SIM_MCU} ${_mute_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_mute_baud}
+		                 $<TARGET_FILE:pbapp_handover>.bin
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbmute-work ${_mute_link})
+		set_tests_properties(pureboot.mute PROPERTIES TIMEOUT 180)
+
+		# The same hand-over against the one-wire deployment on that USART's
+		# RXD: RXEN forces the shared pin's direction, so a loader that only
+		# released the transmit-side hold would read the wire and answer into
+		# a pin it cannot drive. The host runs with the --one-wire echo
+		# discard, which the bridge's shared-line model feeds for real.
+		get_target_property(_mute1w_link pureboot_1w_on_usart0 PUREBOOT_LINK)
+		add_test(NAME pureboot.mute.onewire
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbmute.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_1w_on_usart0> ${PUREBOOT_SIM_MCU} ${_mute_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_mute_baud}
+		                 $<TARGET_FILE:pbapp_handover>.bin
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbmute-1w-work ${_mute1w_link})
+		set_tests_properties(pureboot.mute.onewire PROPERTIES TIMEOUT 180)
+
+		# The full protocol suite over one shared pin: the loader folded onto
+		# PB0, the bridge following the pin's direction, the fixture
+		# bannering as a guest on the same line, and the host discarding its
+		# own echo throughout.
+		get_target_property(_1w_hz pureboot_1w PUREBOOT_HZ)
+		get_target_property(_1w_baud pureboot_1w PUREBOOT_BAUD)
+		get_target_property(_1w_link pureboot_1w PUREBOOT_LINK)
+		add_executable(pbapp_1w test/pbapp.cpp)
+		target_link_libraries(pbapp_1w PRIVATE libavr)
+		target_compile_definitions(pbapp_1w PRIVATE PUREBOOT_CLOCK_HZ=${_1w_hz}
+		                           PUREBOOT_BAUD=${_1w_baud} PUREBOOT_SOFT_SERIAL
+		                           PUREBOOT_RX=pb0 PUREBOOT_TX=pb0)
+		add_custom_command(TARGET pbapp_1w POST_BUILD
+		                   COMMAND ${CMAKE_OBJCOPY} -O binary
+		                           $<TARGET_FILE:pbapp_1w> $<TARGET_FILE:pbapp_1w>.bin)
+		add_test(NAME pureboot.onewire
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbtest.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_1w> ${PUREBOOT_SIM_MCU} ${_1w_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_1w_baud} ${PUREBOOT_EEPROM}
+		                 $<TARGET_FILE:pbapp_1w>.bin ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pb1w-work ${_1w_link})
+		set_tests_properties(pureboot.onewire PROPERTIES TIMEOUT 180)
+
+		# The hardware USART's half-duplex turn-around, end to end: every
+		# reply byte runs drive-line, TXC-hold, release - against simavr's
+		# RXEN-gated receiver, which drops input to a disabled receiver the
+		# way silicon does. The pty is a two-wire transport, so the host
+		# needs no echo discard here; the off-chip tie itself is the
+		# hardware bench's item.
+		add_test(NAME pureboot.halfduplex
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbtest.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_hd> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud} ${PUREBOOT_EEPROM}
+		                 $<TARGET_FILE:pbapp>.bin ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbhd-work)
+		set_tests_properties(pureboot.halfduplex PROPERTIES TIMEOUT 180)
+	endif()
+
 	# The second USART, driven for real on one chip: instance selection is
 	# compile-checked everywhere, but only a live session proves the loader
 	# initialized and polls the USART it claims to. The fixture application
@@ -397,10 +677,10 @@ if(PROJECT_IS_TOP_LEVEL)
 		set_tests_properties(pureboot.usart1 PROPERTIES TIMEOUT 180)
 	endif()
 
-	# The autobaud variants driven end to end over the software-UART bridge (both
-	# under review — pureboot/autobaud.md): the host sends the 0xC0 calibration
-	# pulse, the loader times it, locks, and programs. Run on the near-flash 328P
-	# and the word-addressed 1284P — the two flash-addressing classes — and each
+	# The autobaud loader driven end to end over the software-UART bridge:
+	# the host sends the 0xC0 calibration pulse, the loader times it, locks,
+	# and programs. Run on the near-flash 328P
+	# and the word-addressed 1284P - the two flash-addressing classes - and each
 	# at two clocks with the one binary, which is the clock-agnostic property
 	# autobaud exists for (test/pbautobaud.py). The fixture application banners
 	# over the same software link at the first clock's rate.
@@ -412,14 +692,51 @@ if(PROJECT_IS_TOP_LEVEL)
 		add_custom_command(TARGET pbapp_autobaud POST_BUILD
 		                   COMMAND ${CMAKE_OBJCOPY} -O binary
 		                           $<TARGET_FILE:pbapp_autobaud> $<TARGET_FILE:pbapp_autobaud>.bin)
-		foreach(_variant uni)
-			add_test(NAME pureboot.autobaud_${_variant}
+		add_test(NAME pureboot.autobaud
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbautobaud.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_autobaud> ${PUREBOOT_SIM_MCU}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} $<TARGET_FILE:pbapp_autobaud>.bin
+		                 1000000 9600 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbautobaud-work)
+		set_tests_properties(pureboot.autobaud PROPERTIES TIMEOUT 240)
+
+		# The tightest deployment in the space, end to end: the autobaud
+		# loader folded onto the USART's own RXD with the OSCCAL trim baked
+		# - one-wire calibration, the receive-side release, and the host's
+		# echo discard, over the same two-clock sweep. One chip carries it;
+		# the shape is chip-independent.
+		if(LIBAVR_MCU STREQUAL "atmega328p")
+			get_target_property(_ab1w_link pureboot_1w_autobaud_osccal_on_usart0 PUREBOOT_LINK)
+			add_executable(pbapp_autobaud_1w test/pbapp.cpp)
+			target_link_libraries(pbapp_autobaud_1w PRIVATE libavr)
+			target_compile_definitions(pbapp_autobaud_1w PRIVATE PUREBOOT_CLOCK_HZ=1000000
+			                           PUREBOOT_BAUD=9600 PUREBOOT_SOFT_SERIAL
+			                           PUREBOOT_RX=pd0 PUREBOOT_TX=pd0)
+			add_custom_command(TARGET pbapp_autobaud_1w POST_BUILD
+			                   COMMAND ${CMAKE_OBJCOPY} -O binary
+			                           $<TARGET_FILE:pbapp_autobaud_1w> $<TARGET_FILE:pbapp_autobaud_1w>.bin)
+			add_test(NAME pureboot.autobaud.onewire
 			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbautobaud.py
-			                 ${PB_DEVICE} $<TARGET_FILE:pureboot_autobaud_${_variant}> ${PUREBOOT_SIM_MCU}
-			                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} $<TARGET_FILE:pbapp_autobaud>.bin
+			                 ${PB_DEVICE} $<TARGET_FILE:pureboot_1w_autobaud_osccal_on_usart0>
+			                 ${PUREBOOT_SIM_MCU} ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE}
+			                 $<TARGET_FILE:pbapp_autobaud_1w>.bin
 			                 1000000 9600 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
-			                 ${CMAKE_BINARY_DIR}/pbautobaud-${_variant}-work)
-			set_tests_properties(pureboot.autobaud_${_variant} PROPERTIES TIMEOUT 240)
-		endforeach()
+			                 ${CMAKE_BINARY_DIR}/pbautobaud-1w-work ${_ab1w_link})
+			set_tests_properties(pureboot.autobaud.onewire PROPERTIES TIMEOUT 240)
+		endif()
+
+		# The autobaud window: the calibration poll budget, at the measured
+		# 10 cycles a poll (pbwindow.py pins the constant the README's
+		# seconds arithmetic uses; the budget itself is the clock-free knob).
+		add_test(NAME pureboot.window.autobaud
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbwindow.py
+		                 --device ${PB_DEVICE} --loader $<TARGET_FILE:pureboot_autobaud>
+		                 --mcu ${PUREBOOT_SIM_MCU} --hz 1000000
+		                 --base ${PUREBOOT_BASE_HEX} --page ${PUREBOOT_PAGE}
+		                 --baud 9600 --app $<TARGET_FILE:pbapp_autobaud>.bin
+		                 --autobaud-polls 4000000 --link sw
+		                 --tool ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 --workdir ${CMAKE_BINARY_DIR}/pbwindow-autobaud-work)
+		set_tests_properties(pureboot.window.autobaud PROPERTIES TIMEOUT 300)
 	endif()
 endif()

ide/README.md

@@ -0,0 +1,78 @@
+# Atmel Studio
+
+`master` carries `bootloader.atsln`, so this branch does too: `ide/bootloader.atsln`
+builds the loaders from the same sources Ninja does, to a **byte-identical
+`.text`** — the stock 328P pureboot deployment and the `tsb_asm` tier in its
+512-byte section (`check-flags.py` below is what holds the flag sets equal, so
+the sizes are Ninja's own). CMake remains the build system; the solution is
+here so the port opens in Studio as its predecessor did.
+
+## The two projects, and why two
+
+pureboot is a chip × backend × clock × baud matrix — `pureboot_add_loader()`
+resolves a deployment into compile definitions — and a `.cppproj` is one binary
+at one set of flags, so a project can only ever be one point of it. `pureboot`
+is that point: the stock 328P deployment, USART0 at 115200 on a 16 MHz crystal,
+an 8-second activation window. `tsb_asm` is the TinySafeBoot tier that occupies
+the same 512-byte section `master`'s `tsb` project targeted.
+
+The other three tsb tiers (`tsb_pure`, `tsb_tricks`, `tsb_policy`) are not here.
+They differ from `tsb_asm` in their source file, their section size, and — for
+`tsb_policy` — two loop flags; nothing about that is a Studio concern, and what
+they exist to demonstrate is a size gradient only the CMake size tests measure.
+Adding one is a copy of `tsb_asm/tsb_asm.cppproj` in its own directory, with its
+name, its GUID, its source path and its `--section-start` changed (`0x7c00` for
+the 1 KiB tiers), plus four lines in the solution.
+
+`avrdevice` is a project property, so each project gets its own directory:
+Studio builds into `<project dir>/<Configuration>` whatever `OutputDirectory`
+says, and two projects sharing a directory would share one object file.
+
+## Debug keeps `-Os`
+
+Both configurations compile at `-Os`; Debug adds only `-gdwarf-4`. The `.text`
+is therefore identical in both, which is the point — a loader's section is a
+**correctness** bound and not a budget. `-Og` builds this same source to 590 B,
+and linking it at `--section-start=.text=0x7e00` on a 32 KiB part puts 78 bytes
+past flash end **without a diagnostic**: `rcall`/`rjmp` targets there wrap
+modulo flash size, so the image dies right after activation. A debug
+configuration that silently produces that is worse than none, and DWARF costs no
+flash, so the optimisation level stays where correctness needs it.
+
+## What Studio needs from the machine
+
+libavr from the **submodule**, found at
+`$(MSBuildProjectDirectory)\..\..\libavr\include` — correct by construction, and
+anchored to the project because a plain relative path resolves against the
+generated makefile's directory (the configuration's output directory), not the
+project's. There is no `LIBAVR_ROOT` escape hatch: a variable exported in a
+shell is invisible to Studio launched from the Start menu, and the failure reads
+as a missing `libavr/libavr.hpp` — which is what the submodule answers.
+
+A GCC 16.1 toolchain registered as flavour `avr-g++-16.1.0`, nothing older
+reaching `-std=c++26`.
+
+## Generating and gating
+
+One generated file is required before a project will load at all, and one command
+per project checks the flags have not drifted (both from libavr's
+`tools/atmelstudio/`):
+
+```sh
+for name in pureboot tsb_asm; do
+    python libavr/tools/atmelstudio/componentinfo.py \
+        "ide/$name/$name.componentinfo.xml" --device ATmega328P
+    python libavr/tools/atmelstudio/check-flags.py \
+        --solution ide/bootloader.atsln --project "$name" --target "$name" \
+        --compile-commands build/atmega328p-generated/compile_commands.json \
+        --log "build/as-$name.log"
+done
+```
+
+`--project` because one reference describes one binary; `--target` because
+`pureboot.cpp` is compiled by every point of the size matrix and the flags
+differ per point, so the basename alone does not name a reference. Release is
+what the gate compares — the presets define no debug build, and Debug differs
+from Release only in `-gdwarf-4`.
+
+Legacy (the yazoalfa-era submodules) stays on `master`.

ide/bootloader.atsln

@@ -0,0 +1,28 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Atmel Studio Solution File, Format Version 11.00
+VisualStudioVersion = 14.0.23107.0
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{E66E83B9-2572-4076-B26E-6BE79FF3018A}") = "pureboot", "pureboot\pureboot.cppproj", "{99067222-32D5-49E3-B4F8-5ABA0F7722B7}"
+EndProject
+Project("{E66E83B9-2572-4076-B26E-6BE79FF3018A}") = "tsb_asm", "tsb_asm\tsb_asm.cppproj", "{6618D3BE-7EB3-49A2-9113-F128E396FF06}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|AVR = Debug|AVR
+		Release|AVR = Release|AVR
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{99067222-32D5-49E3-B4F8-5ABA0F7722B7}.Debug|AVR.ActiveCfg = Debug|AVR
+		{99067222-32D5-49E3-B4F8-5ABA0F7722B7}.Debug|AVR.Build.0 = Debug|AVR
+		{99067222-32D5-49E3-B4F8-5ABA0F7722B7}.Release|AVR.ActiveCfg = Release|AVR
+		{99067222-32D5-49E3-B4F8-5ABA0F7722B7}.Release|AVR.Build.0 = Release|AVR
+		{6618D3BE-7EB3-49A2-9113-F128E396FF06}.Debug|AVR.ActiveCfg = Debug|AVR
+		{6618D3BE-7EB3-49A2-9113-F128E396FF06}.Debug|AVR.Build.0 = Debug|AVR
+		{6618D3BE-7EB3-49A2-9113-F128E396FF06}.Release|AVR.ActiveCfg = Release|AVR
+		{6618D3BE-7EB3-49A2-9113-F128E396FF06}.Release|AVR.Build.0 = Release|AVR
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+EndGlobal

ide/pureboot/pureboot.cppproj

@@ -0,0 +1,118 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="14.0">
+  <PropertyGroup>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectVersion>7.0</ProjectVersion>
+    <ToolchainName>com.Atmel.AVRGCC8.CPP</ToolchainName>
+    <ProjectGuid>99067222-32d5-49e3-b4f8-5aba0f7722b7</ProjectGuid>
+    <avrdevice>ATmega328P</avrdevice>
+    <avrdeviceseries>none</avrdeviceseries>
+    <OutputType>Executable</OutputType>
+    <Language>CPP</Language>
+    <OutputFileName>$(MSBuildProjectName)</OutputFileName>
+    <OutputFileExtension>.elf</OutputFileExtension>
+    <OutputDirectory>$(MSBuildProjectDirectory)\$(Configuration)</OutputDirectory>
+    <AssemblyName>pureboot</AssemblyName>
+    <Name>pureboot</Name>
+    <RootNamespace>pureboot</RootNamespace>
+    <ToolchainFlavour>avr-g++-16.1.0</ToolchainFlavour>
+    <KeepTimersRunning>true</KeepTimersRunning>
+    <OverrideVtor>false</OverrideVtor>
+    <CacheFlash>true</CacheFlash>
+    <ProgFlashFromRam>true</ProgFlashFromRam>
+    <RamSnippetAddress>0x20000000</RamSnippetAddress>
+    <UncachedRange />
+    <preserveEEPROM>true</preserveEEPROM>
+    <OverrideVtorValue>exception_table</OverrideVtorValue>
+    <BootSegment>2</BootSegment>
+    <ResetRule>0</ResetRule>
+    <eraseonlaunchrule>0</eraseonlaunchrule>
+    <EraseKey />
+    <AsfFrameworkConfig>
+      <framework-data xmlns="">
+  <options />
+  <configurations />
+  <files />
+  <documentation help="" />
+  <offline-documentation help="" />
+  <dependencies>
+    <content-extension eid="atmel.asf" uuidref="Atmel.ASF" version="3.52.0" />
+  </dependencies>
+</framework-data>
+    </AsfFrameworkConfig>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
+    <ToolchainSettings>
+      <AvrGccCpp>
+        <avrgcc.common.Device>-mmcu=atmega328p</avrgcc.common.Device>
+        <avrgcc.common.outputfiles.hex>True</avrgcc.common.outputfiles.hex>
+        <avrgcc.common.outputfiles.lss>True</avrgcc.common.outputfiles.lss>
+        <avrgcc.common.outputfiles.eep>True</avrgcc.common.outputfiles.eep>
+        <avrgcc.common.outputfiles.srec>True</avrgcc.common.outputfiles.srec>
+        <avrgcc.common.outputfiles.usersignatures>False</avrgcc.common.outputfiles.usersignatures>
+        <avrgcccpp.compiler.symbols.DefSymbols>
+          <ListValues>
+            <Value>NDEBUG</Value>
+            <Value>PUREBOOT_CLOCK_HZ=16000000</Value>
+            <Value>PUREBOOT_BAUD=115200</Value>
+            <Value>PUREBOOT_TIMEOUT=8</Value>
+          </ListValues>
+        </avrgcccpp.compiler.symbols.DefSymbols>
+        <avrgcccpp.compiler.directories.IncludePaths>
+          <ListValues>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
+          </ListValues>
+        </avrgcccpp.compiler.directories.IncludePaths>
+        <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>
+        <avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>
+        <avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>
+        <avrgcccpp.compiler.warnings.AllWarnings>True</avrgcccpp.compiler.warnings.AllWarnings>
+        <avrgcccpp.compiler.miscellaneous.OtherFlags>-std=c++26 -Wextra -Werror -mrelax -fno-exceptions -fno-rtti -fno-threadsafe-statics -fno-ivopts -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types</avrgcccpp.compiler.miscellaneous.OtherFlags>
+        <avrgcccpp.linker.optimization.GarbageCollectUnusedSections>True</avrgcccpp.linker.optimization.GarbageCollectUnusedSections>
+        <avrgcccpp.linker.miscellaneous.LinkerFlags>-mrelax -nostartfiles -Wl,--section-start=.text=0x7e00 -Wl,--defsym=pureboot_app=0 -Wl,--pmem-wrap-around=32k</avrgcccpp.linker.miscellaneous.LinkerFlags>
+      </AvrGccCpp>
+    </ToolchainSettings>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
+    <ToolchainSettings>
+      <AvrGccCpp>
+        <avrgcc.common.Device>-mmcu=atmega328p</avrgcc.common.Device>
+        <avrgcc.common.outputfiles.hex>True</avrgcc.common.outputfiles.hex>
+        <avrgcc.common.outputfiles.lss>True</avrgcc.common.outputfiles.lss>
+        <avrgcc.common.outputfiles.eep>True</avrgcc.common.outputfiles.eep>
+        <avrgcc.common.outputfiles.srec>True</avrgcc.common.outputfiles.srec>
+        <avrgcc.common.outputfiles.usersignatures>False</avrgcc.common.outputfiles.usersignatures>
+        <avrgcccpp.compiler.symbols.DefSymbols>
+          <ListValues>
+            <Value>DEBUG</Value>
+            <Value>PUREBOOT_CLOCK_HZ=16000000</Value>
+            <Value>PUREBOOT_BAUD=115200</Value>
+            <Value>PUREBOOT_TIMEOUT=8</Value>
+          </ListValues>
+        </avrgcccpp.compiler.symbols.DefSymbols>
+        <avrgcccpp.compiler.directories.IncludePaths>
+          <ListValues>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
+          </ListValues>
+        </avrgcccpp.compiler.directories.IncludePaths>
+        <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>
+        <avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>
+        <avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>
+        <avrgcccpp.compiler.warnings.AllWarnings>True</avrgcccpp.compiler.warnings.AllWarnings>
+        <avrgcccpp.compiler.miscellaneous.OtherFlags>-std=c++26 -Wextra -Werror -mrelax -fno-exceptions -fno-rtti -fno-threadsafe-statics -fno-ivopts -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types -gdwarf-4</avrgcccpp.compiler.miscellaneous.OtherFlags>
+        <avrgcccpp.linker.optimization.GarbageCollectUnusedSections>True</avrgcccpp.linker.optimization.GarbageCollectUnusedSections>
+        <avrgcccpp.linker.miscellaneous.LinkerFlags>-mrelax -nostartfiles -Wl,--section-start=.text=0x7e00 -Wl,--defsym=pureboot_app=0 -Wl,--pmem-wrap-around=32k</avrgcccpp.linker.miscellaneous.LinkerFlags>
+      </AvrGccCpp>
+    </ToolchainSettings>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="..\..\pureboot\pureboot.cpp">
+      <SubType>compile</SubType>
+      <Link>pureboot\pureboot.cpp</Link>
+    </Compile>
+  </ItemGroup>
+  <ItemGroup>
+    <Folder Include="pureboot" />
+  </ItemGroup>
+  <Import Project="$(AVRSTUDIO_EXE_PATH)\Vs\Compiler.targets" />
+</Project>

ide/tsb_asm/tsb_asm.cppproj

@@ -0,0 +1,112 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="14.0">
+  <PropertyGroup>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectVersion>7.0</ProjectVersion>
+    <ToolchainName>com.Atmel.AVRGCC8.CPP</ToolchainName>
+    <ProjectGuid>6618d3be-7eb3-49a2-9113-f128e396ff06</ProjectGuid>
+    <avrdevice>ATmega328P</avrdevice>
+    <avrdeviceseries>none</avrdeviceseries>
+    <OutputType>Executable</OutputType>
+    <Language>CPP</Language>
+    <OutputFileName>$(MSBuildProjectName)</OutputFileName>
+    <OutputFileExtension>.elf</OutputFileExtension>
+    <OutputDirectory>$(MSBuildProjectDirectory)\$(Configuration)</OutputDirectory>
+    <AssemblyName>tsb_asm</AssemblyName>
+    <Name>tsb_asm</Name>
+    <RootNamespace>tsb_asm</RootNamespace>
+    <ToolchainFlavour>avr-g++-16.1.0</ToolchainFlavour>
+    <KeepTimersRunning>true</KeepTimersRunning>
+    <OverrideVtor>false</OverrideVtor>
+    <CacheFlash>true</CacheFlash>
+    <ProgFlashFromRam>true</ProgFlashFromRam>
+    <RamSnippetAddress>0x20000000</RamSnippetAddress>
+    <UncachedRange />
+    <preserveEEPROM>true</preserveEEPROM>
+    <OverrideVtorValue>exception_table</OverrideVtorValue>
+    <BootSegment>2</BootSegment>
+    <ResetRule>0</ResetRule>
+    <eraseonlaunchrule>0</eraseonlaunchrule>
+    <EraseKey />
+    <AsfFrameworkConfig>
+      <framework-data xmlns="">
+  <options />
+  <configurations />
+  <files />
+  <documentation help="" />
+  <offline-documentation help="" />
+  <dependencies>
+    <content-extension eid="atmel.asf" uuidref="Atmel.ASF" version="3.52.0" />
+  </dependencies>
+</framework-data>
+    </AsfFrameworkConfig>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
+    <ToolchainSettings>
+      <AvrGccCpp>
+        <avrgcc.common.Device>-mmcu=atmega328p</avrgcc.common.Device>
+        <avrgcc.common.outputfiles.hex>True</avrgcc.common.outputfiles.hex>
+        <avrgcc.common.outputfiles.lss>True</avrgcc.common.outputfiles.lss>
+        <avrgcc.common.outputfiles.eep>True</avrgcc.common.outputfiles.eep>
+        <avrgcc.common.outputfiles.srec>True</avrgcc.common.outputfiles.srec>
+        <avrgcc.common.outputfiles.usersignatures>False</avrgcc.common.outputfiles.usersignatures>
+        <avrgcccpp.compiler.symbols.DefSymbols>
+          <ListValues>
+            <Value>NDEBUG</Value>
+          </ListValues>
+        </avrgcccpp.compiler.symbols.DefSymbols>
+        <avrgcccpp.compiler.directories.IncludePaths>
+          <ListValues>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
+          </ListValues>
+        </avrgcccpp.compiler.directories.IncludePaths>
+        <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>
+        <avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>
+        <avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>
+        <avrgcccpp.compiler.warnings.AllWarnings>True</avrgcccpp.compiler.warnings.AllWarnings>
+        <avrgcccpp.compiler.miscellaneous.OtherFlags>-std=c++26 -Wextra -Werror -mrelax -fno-exceptions -fno-rtti -fno-threadsafe-statics</avrgcccpp.compiler.miscellaneous.OtherFlags>
+        <avrgcccpp.linker.optimization.GarbageCollectUnusedSections>True</avrgcccpp.linker.optimization.GarbageCollectUnusedSections>
+        <avrgcccpp.linker.miscellaneous.LinkerFlags>-mrelax -nostartfiles -Wl,--section-start=.text=0x7e00 -Wl,--defsym=tsb_app=0 -Wl,--pmem-wrap-around=32k</avrgcccpp.linker.miscellaneous.LinkerFlags>
+      </AvrGccCpp>
+    </ToolchainSettings>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
+    <ToolchainSettings>
+      <AvrGccCpp>
+        <avrgcc.common.Device>-mmcu=atmega328p</avrgcc.common.Device>
+        <avrgcc.common.outputfiles.hex>True</avrgcc.common.outputfiles.hex>
+        <avrgcc.common.outputfiles.lss>True</avrgcc.common.outputfiles.lss>
+        <avrgcc.common.outputfiles.eep>True</avrgcc.common.outputfiles.eep>
+        <avrgcc.common.outputfiles.srec>True</avrgcc.common.outputfiles.srec>
+        <avrgcc.common.outputfiles.usersignatures>False</avrgcc.common.outputfiles.usersignatures>
+        <avrgcccpp.compiler.symbols.DefSymbols>
+          <ListValues>
+            <Value>DEBUG</Value>
+          </ListValues>
+        </avrgcccpp.compiler.symbols.DefSymbols>
+        <avrgcccpp.compiler.directories.IncludePaths>
+          <ListValues>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
+          </ListValues>
+        </avrgcccpp.compiler.directories.IncludePaths>
+        <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>
+        <avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareFunctionsForGarbageCollection>
+        <avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>True</avrgcccpp.compiler.optimization.PrepareDataForGarbageCollection>
+        <avrgcccpp.compiler.warnings.AllWarnings>True</avrgcccpp.compiler.warnings.AllWarnings>
+        <avrgcccpp.compiler.miscellaneous.OtherFlags>-std=c++26 -Wextra -Werror -mrelax -fno-exceptions -fno-rtti -fno-threadsafe-statics -gdwarf-4</avrgcccpp.compiler.miscellaneous.OtherFlags>
+        <avrgcccpp.linker.optimization.GarbageCollectUnusedSections>True</avrgcccpp.linker.optimization.GarbageCollectUnusedSections>
+        <avrgcccpp.linker.miscellaneous.LinkerFlags>-mrelax -nostartfiles -Wl,--section-start=.text=0x7e00 -Wl,--defsym=tsb_app=0 -Wl,--pmem-wrap-around=32k</avrgcccpp.linker.miscellaneous.LinkerFlags>
+      </AvrGccCpp>
+    </ToolchainSettings>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="..\..\tsb\tsb_asm.cpp">
+      <SubType>compile</SubType>
+      <Link>tsb\tsb_asm.cpp</Link>
+    </Compile>
+  </ItemGroup>
+  <ItemGroup>
+    <Folder Include="tsb" />
+  </ItemGroup>
+  <Import Project="$(AVRSTUDIO_EXE_PATH)\Vs\Compiler.targets" />
+</Project>

oracle/README.md

@@ -38,11 +38,23 @@ avra -I /usr/share/avra tsb-fixedbaud.asm   # after uncommenting .include "m328P
 ```
 
 **500 bytes with every feature** — the proof that ≤512 B and full feature parity
-are simultaneously reachable. The port's `tsb_asm` tier meets the same bar at
-510 B in the same 512 B section, written in C++ on libavr except the two
-routines whose remaining cost is the calling convention itself (the bounded rx
-and the page-store loop); `tsb_tricks` needs no assembly at all at 526 B, and
-`tsb_pure` stays fully idiomatic at 836 B, both in the 1 KB section.
+are simultaneously reachable. The port's four tiers reach it from the other
+side, and the gradient between them is the cost of the mechanisms each is
+allowed:
+
+| tier | bytes | section | what it is allowed |
+|---|---|---|---|
+| oracle | 500 | 512 B | hand-written assembly, the reference |
+| `tsb_asm` | 512 | 512 B | C++ on libavr, two routines in asm |
+| `tsb_tricks` | 528 | 1 KB | no asm; global register variables |
+| `tsb_policy` | 630 | 1 KB | pureboot's rules: no asm, no register variables |
+| `tsb_pure` | 776 | 1 KB | idiomatic libavr throughout |
+
+The two routines `tsb_asm` keeps are the ones whose remaining cost is the
+calling convention itself: the bounded rx and the page-store loop. It fills
+its section exactly, with the same one-bit-time turn-around guard the oracle
+spends six bytes on - every tier implements the whole feature set, which is
+what makes the column a gradient rather than four different loaders.
 
 The oracle targets 20 MHz / 33333 baud; the port targets 16 MHz / 115200 baud
 (what the simavr protocol test drives). Baud and geometry differ, code size and

pureboot/CMakeLists.txt

@@ -1,5 +1,5 @@
 # pureboot as a consumable CMake unit: the per-chip geometry, the default baud
-# ladder, and pureboot_add_loader() — the one way a loader target is created.
+# ladder, and pureboot_add_loader() - the one way a loader target is created.
 # A downstream project brings its usual libavr setup (the `libavr` target and
 # the LIBAVR_MCU toolchain preset), adds this directory, and states its
 # deployment; every argument is optional (README.md):
@@ -9,7 +9,7 @@
 
 # Per-family geometry, deployment defaults, and the linker wrap the PC modulo
 # needs. The slot is 512 bytes on every chip. The USART flags mirror the
-# hardware inventory the loader's own static asserts check — the plain 644 is
+# hardware inventory the loader's own static asserts check - the plain 644 is
 # the x4 family's one single-USART die (Atmel-2593).
 set(_pb_has_usart 1)
 set(_pb_has_usart1 0)
@@ -107,7 +107,7 @@ set(_pb_slot 512)
 math(EXPR _pb_base "${_pb_flash} - ${_pb_slot}")
 math(EXPR _pb_base_hex "${_pb_base}" OUTPUT_FORMAT HEXADECIMAL)
 # Patched-vector chips hand over through the trampoline word below the slot,
-# which is also the slot's own last word — their budget is slot − 2.
+# which is also the slot's own last word - their budget is slot - 2.
 if(LIBAVR_MCU MATCHES "^atmega" AND NOT LIBAVR_MCU MATCHES "^atmega48")
 	set(_pb_app 0)
 	set(_pb_limit ${_pb_slot})
@@ -116,6 +116,17 @@ else()
 	math(EXPR _pb_limit "${_pb_slot} - 2")
 endif()
 
+# The pins each USART owns. A bit-banged link deployed on them has to release
+# that USART before it can drive the line, and those instructions are the one
+# way the choice of pins moves the image - so a size matrix needs them as an
+# axis even though pins are otherwise immediate operands. Uniform across every
+# mega libavr covers: USART0 (the classics' un-numbered USART included) on
+# PD0/PD1, USART1 on PD2/PD3.
+set(_pb_usart0_rx pd0)
+set(_pb_usart0_tx pd1)
+set(_pb_usart1_rx pd2)
+set(_pb_usart1_tx pd3)
+
 # simavr names its cores after the base dies; the A revisions run on them
 # (the 644PA on the 644P core).
 set(_pb_sim_mcu ${LIBAVR_MCU})
@@ -125,6 +136,28 @@ elseif(LIBAVR_MCU STREQUAL "atmega644pa")
 	set(_pb_sim_mcu atmega644p)
 endif()
 
+# Where SRAM begins: the classic megas keep it right after the plain I/O
+# registers, the x8/x4 generations push it past their extended I/O file, and
+# the tinies match the classics. An autobaud loader keeps its measured unit
+# in GPIOR2:GPIOR1 wherever the chip has the pair (data 0x32 on the
+# t25/45/85, 0x4A from the x8 generation on) and as the first RAM object at
+# SRAM start where it does not (the t13s and classic megas). The host reads
+# whichever home applies (pureboot.py's geometry), and the unit-position
+# test holds the image to the same split.
+if(LIBAVR_MCU MATCHES "^atmega(8|16|32)a?$")
+	set(_pb_ram 0x60)
+	set(_pb_unit_gpior "")
+elseif(LIBAVR_MCU MATCHES "^atmega")
+	set(_pb_ram 0x100)
+	set(_pb_unit_gpior 0x4A)
+elseif(LIBAVR_MCU MATCHES "^attiny13")
+	set(_pb_ram 0x60)
+	set(_pb_unit_gpior "")
+else()
+	set(_pb_ram 0x60)
+	set(_pb_unit_gpior 0x32)
+endif()
+
 # The function runs in its caller's scope, so everything it needs crosses
 # scopes as global properties.
 set_property(GLOBAL PROPERTY PUREBOOT_BASE_HEX ${_pb_base_hex})
@@ -133,6 +166,10 @@ set_property(GLOBAL PROPERTY PUREBOOT_WRAP "${_pb_wrap}")
 set_property(GLOBAL PROPERTY PUREBOOT_DEFAULT_HZ ${_pb_hz})
 set_property(GLOBAL PROPERTY PUREBOOT_HAS_USART ${_pb_has_usart})
 set_property(GLOBAL PROPERTY PUREBOOT_HAS_USART1 ${_pb_has_usart1})
+set_property(GLOBAL PROPERTY PUREBOOT_USART0_RX ${_pb_usart0_rx})
+set_property(GLOBAL PROPERTY PUREBOOT_USART0_TX ${_pb_usart0_tx})
+set_property(GLOBAL PROPERTY PUREBOOT_USART1_RX ${_pb_usart1_rx})
+set_property(GLOBAL PROPERTY PUREBOOT_USART1_TX ${_pb_usart1_tx})
 
 # The port's own build (tests, the size matrix) reads the geometry from the
 # parent scope; a downstream consumer gets the same variables for free.
@@ -142,9 +179,15 @@ set(PUREBOOT_SLOT ${_pb_slot} PARENT_SCOPE)
 set(PUREBOOT_LIMIT ${_pb_limit} PARENT_SCOPE)
 set(PUREBOOT_EEPROM ${_pb_eeprom} PARENT_SCOPE)
 set(PUREBOOT_DEFAULT_HZ ${_pb_hz} PARENT_SCOPE)
+set(PUREBOOT_RAM_START ${_pb_ram} PARENT_SCOPE)
+set(PUREBOOT_UNIT_GPIOR "${_pb_unit_gpior}" PARENT_SCOPE)
 set(PUREBOOT_HAS_USART ${_pb_has_usart} PARENT_SCOPE)
 set(PUREBOOT_HAS_USART1 ${_pb_has_usart1} PARENT_SCOPE)
 set(PUREBOOT_SIM_MCU ${_pb_sim_mcu} PARENT_SCOPE)
+set(PUREBOOT_USART0_RX ${_pb_usart0_rx} PARENT_SCOPE)
+set(PUREBOOT_USART0_TX ${_pb_usart0_tx} PARENT_SCOPE)
+set(PUREBOOT_USART1_RX ${_pb_usart1_rx} PARENT_SCOPE)
+set(PUREBOOT_USART1_TX ${_pb_usart1_tx} PARENT_SCOPE)
 
 # The rates a default may pick, fastest first.
 set_property(GLOBAL PROPERTY PUREBOOT_BAUD_LADDER 115200 57600 38400 19200 9600)
@@ -190,19 +233,39 @@ function(pureboot_default_baud clock software outvar)
 		endif()
 	endforeach()
 	message(FATAL_ERROR "pureboot: no standard baud rate fits a ${clock} Hz clock within 2.5 % "
-	        "— pass BAUD <rate> to deploy a non-standard one")
+	        " - pass BAUD <rate> to deploy a non-standard one")
 endfunction()
 
 # pureboot_add_loader(<name> [CLOCK <hz>] [BAUD <bd>]
-#                     [SERIAL auto|hardware|software] [USART <n>]
-#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>])
+#                     [SERIAL auto|hardware|software|autobaud] [USART <n>]
+#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>] [OSCCAL <byte>]
+#                     [HALF_DUPLEX])
 #
 # The loader target plus its flashable images (<name>.hex for a programmer,
 # <name>.bin for --update-loader). The resolved deployment is stamped on the
 # target as PUREBOOT_HZ / PUREBOOT_BAUD / PUREBOOT_LINK (the link spelled
-# usart0, usart1 or sw:<RX>,<TX>) — what a test harness speaks to it with.
+# usart0, usart1, or sw:<RX>,<TX> with a trailing @<n> where those pins are a
+# USART's own) - what a test harness speaks to it with.
+#
+# HALF_DUPLEX is the one-wire deployment, per backend: on the hardware USART
+# it enables the library's .half_duplex turn-around (RXD and TXD tied
+# together off-chip); on a software or autobaud link it puts both directions
+# on the RX pin - the same thing RX == TX spells directly.
+#
+# SERIAL autobaud measures the host's bit timing at run time, so the image
+# carries no clock and no baud: CLOCK and BAUD are not build parameters there,
+# and one binary per chip serves every F_CPU and every rate. The stamped
+# PUREBOOT_HZ/PUREBOOT_BAUD then record what a harness should *drive* it at,
+# not what it was built for.
+#
+# OSCCAL bakes a measured oscillator trim into the loader (README.md: the
+# RC-oscillator deployment answer): the byte is written at the top of run(),
+# so every reset path - the watchdog hand-over included - runs on the
+# corrected clock. Orthogonal to the backend: an autobaud build may carry it
+# purely for the application's benefit, its own link being clock-free. No
+# value, no code.
 function(pureboot_add_loader name)
-	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT" "" ${ARGN})
+	cmake_parse_arguments(PB "HALF_DUPLEX" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT;OSCCAL" "" ${ARGN})
 	if(PB_UNPARSED_ARGUMENTS)
 		message(FATAL_ERROR "pureboot_add_loader(${name}): unknown arguments ${PB_UNPARSED_ARGUMENTS}")
 	endif()
@@ -222,8 +285,8 @@ function(pureboot_add_loader name)
 	if(NOT PB_SERIAL)
 		set(PB_SERIAL auto)
 	endif()
-	if(DEFINED PB_USART AND PB_SERIAL STREQUAL "software")
-		message(FATAL_ERROR "pureboot_add_loader(${name}): USART ${PB_USART} contradicts SERIAL software")
+	if(DEFINED PB_USART AND NOT PB_SERIAL MATCHES "^(auto|hardware)$")
+		message(FATAL_ERROR "pureboot_add_loader(${name}): USART ${PB_USART} contradicts SERIAL ${PB_SERIAL}")
 	endif()
 	if(DEFINED PB_USART)
 		set(PB_SERIAL hardware)
@@ -239,23 +302,38 @@ function(pureboot_add_loader name)
 			message(FATAL_ERROR "pureboot_add_loader(${name}): ${LIBAVR_MCU} has no hardware USART")
 		endif()
 		set(_serial_defines PUREBOOT_USART=${PB_USART})
+		if(PB_HALF_DUPLEX)
+			list(APPEND _serial_defines PUREBOOT_HALF_DUPLEX)
+		endif()
 		set(_link usart${PB_USART})
 	else()
 		if(PB_SERIAL STREQUAL "auto")
 			if(_usart AND (PB_RX OR PB_TX))
 				message(WARNING "pureboot_add_loader(${name}): RX/TX apply to the software UART, "
-				        "which auto does not pick on ${LIBAVR_MCU} — SERIAL software to force it")
+				        "which auto does not pick on ${LIBAVR_MCU} - SERIAL software to force it")
 			endif()
 			if(_usart)
 				set(_link usart0)
+				if(PB_HALF_DUPLEX)
+					set(_serial_defines PUREBOOT_HALF_DUPLEX)
+				endif()
 			else()
 				set(PB_SERIAL software)
 			endif()
 		endif()
-		if(PB_SERIAL STREQUAL "software")
+		if(PB_SERIAL MATCHES "^(software|autobaud)$")
 			if(NOT PB_RX)
 				set(PB_RX pb0)
 			endif()
+			if(PB_HALF_DUPLEX)
+				# One-wire: both directions on the RX pin. RX == TX spells
+				# the same deployment directly.
+				if(PB_TX AND NOT PB_TX STREQUAL PB_RX)
+					message(FATAL_ERROR "pureboot_add_loader(${name}): HALF_DUPLEX puts both "
+					        "directions on RX (${PB_RX}); TX ${PB_TX} contradicts it")
+				endif()
+				set(PB_TX ${PB_RX})
+			endif()
 			if(NOT PB_TX)
 				set(PB_TX pb1)
 			endif()
@@ -264,12 +342,35 @@ function(pureboot_add_loader name)
 					message(FATAL_ERROR "pureboot_add_loader(${name}): pin '${_pin}' is not of the form pb1")
 				endif()
 			endforeach()
-			set(_serial_defines PUREBOOT_SOFT_SERIAL PUREBOOT_RX=${PB_RX} PUREBOOT_TX=${PB_TX})
-			# sw:<RX>,<TX> as port letter and bit, upcased.
+			if(PB_SERIAL STREQUAL "autobaud")
+				set(_serial_defines PUREBOOT_AUTOBAUD PUREBOOT_RX=${PB_RX} PUREBOOT_TX=${PB_TX})
+			else()
+				set(_serial_defines PUREBOOT_SOFT_SERIAL PUREBOOT_RX=${PB_RX} PUREBOOT_TX=${PB_TX})
+			endif()
+			# sw:<RX>,<TX> as port letter and bit, upcased - with @<n> where
+			# the TX pin is a USART's own TXD, since a harness driving that
+			# link has to know the USART owns the pin until the loader
+			# releases it.
 			string(SUBSTRING ${PB_RX} 1 2 _rx_pin)
 			string(SUBSTRING ${PB_TX} 1 2 _tx_pin)
 			string(TOUPPER "sw:${_rx_pin},${_tx_pin}" _link)
 			string(REPLACE "SW" "sw" _link ${_link})
+			get_property(_tx0 GLOBAL PROPERTY PUREBOOT_USART0_TX)
+			get_property(_tx1 GLOBAL PROPERTY PUREBOOT_USART1_TX)
+			get_property(_rx0 GLOBAL PROPERTY PUREBOOT_USART0_RX)
+			get_property(_rx1 GLOBAL PROPERTY PUREBOOT_USART1_RX)
+			if(_usart AND PB_TX STREQUAL _tx0)
+				set(_link "${_link}@0")
+			elseif(_usart1 AND PB_TX STREQUAL _tx1)
+				set(_link "${_link}@1")
+			elseif(PB_TX STREQUAL PB_RX AND _usart AND PB_RX STREQUAL _rx0)
+				# One-wire on a USART's RXD: RXEN forces that pin's direction
+				# (section 20.7.3), so the driven shared pin is held exactly like a
+				# TXD - the harness models the hold either way.
+				set(_link "${_link}@0")
+			elseif(PB_TX STREQUAL PB_RX AND _usart1 AND PB_RX STREQUAL _rx1)
+				set(_link "${_link}@1")
+			endif()
 		endif()
 	endif()
 	if(NOT PB_BAUD)
@@ -280,23 +381,45 @@ function(pureboot_add_loader name)
 		endif()
 	endif()
 
-	set(_defines PUREBOOT_CLOCK_HZ=${PB_CLOCK} PUREBOOT_BAUD=${PB_BAUD} PUREBOOT_TIMEOUT=${PB_TIMEOUT}
-	    ${_serial_defines})
+	if(PB_SERIAL STREQUAL "autobaud")
+		# No clock and no baud reach the image; the window is a poll budget.
+		set(_defines ${_serial_defines})
+	else()
+		set(_defines PUREBOOT_CLOCK_HZ=${PB_CLOCK} PUREBOOT_BAUD=${PB_BAUD} PUREBOOT_TIMEOUT=${PB_TIMEOUT}
+		    ${_serial_defines})
+	endif()
+	if(DEFINED PB_OSCCAL)
+		math(EXPR _osccal "${PB_OSCCAL}" OUTPUT_FORMAT DECIMAL)
+		if(_osccal LESS 0 OR _osccal GREATER 255)
+			message(FATAL_ERROR "pureboot_add_loader(${name}): OSCCAL ${PB_OSCCAL} is not one byte")
+		endif()
+		list(APPEND _defines PUREBOOT_OSCCAL=${_osccal})
+	endif()
 
 	add_executable(${name} ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/pureboot.cpp)
 	target_link_libraries(${name} PRIVATE libavr)
 	target_compile_definitions(${name} PRIVATE ${_defines})
-	# Codegen shaping for the loader TU only, worth 14–36 B depending on the
-	# chip. At -Os GCC otherwise rewrites the byte-stream loops' counters into
-	# end-pointer forms that cost registers (-fno-ivopts,
-	# -fno-split-wide-types), leaves register pressure on the table with the
-	# default allocator (-fira-algorithm=priority), and keeps loop-invariant
-	# immediates and expression temporaries in registers
-	# (-fno-move-loop-invariants, -fno-tree-ter) — but every loop body here
-	# contains a call, so a register held across it costs more than the
-	# load-immediate it saves.
-	target_compile_options(${name} PRIVATE
-	    -fno-ivopts -fira-algorithm=priority -fno-move-loop-invariants -fno-tree-ter -fno-split-wide-types)
+	# Codegen shaping for the loader TU only. At -Os GCC otherwise rewrites the
+	# byte-stream loops' counters into end-pointer forms that cost registers
+	# (-fno-ivopts, -fno-split-wide-types), leaves register pressure on the
+	# table with the default allocator (-fira-algorithm=priority), and keeps
+	# expression temporaries in registers (-fno-tree-ter) - but every loop body
+	# here contains a call, so a register held across it costs more than the
+	# load-immediate it saves. The set is fitted to the loader's body and has to
+	# be re-measured when that body changes: -fno-move-loop-invariants belonged
+	# here while the command loop carried four transfer bodies and costs bytes
+	# now that it carries one, and -fno-ivopts is fitted per backend - an
+	# autobaud body needs ivopts to keep the calibration countdown a single
+	# induction variable (without it the counter is duplicated and the
+	# measurement loop runs 9 cycles instead of its contracted 7), while the
+	# fixed-baud bodies still measure smaller with it off.
+	if(PB_SERIAL STREQUAL "autobaud")
+		target_compile_options(${name} PRIVATE
+		    -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types)
+	else()
+		target_compile_options(${name} PRIVATE
+		    -fno-ivopts -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types)
+	endif()
 	target_link_options(${name} PRIVATE -nostartfiles -Wl,--section-start=.text=${_base_hex}
 	                    -Wl,--defsym=pureboot_app=${_app} ${_wrap})
 	add_custom_command(TARGET ${name} POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:${name}>)
@@ -311,37 +434,3 @@ function(pureboot_add_loader name)
 	                      PUREBOOT_LINK ${_link})
 endfunction()
 
-# pureboot_add_autobaud(<name> <source> [RX <pin>] [TX <pin>])
-#
-# An autobaud software-serial loader from <source> (pureboot_autobaud_*.cpp).
-# Autobaud measures the host's bit timing at runtime, so the image carries no
-# clock and no baud — one binary per chip runs at any F_CPU. Same per-chip
-# geometry, link and codegen flags as pureboot_add_loader(); only the clock and
-# baud axes fall away. Two source files are under review (autobaud.md):
-# pureboot_autobaud_pure.cpp and pureboot_autobaud_reg.cpp.
-function(pureboot_add_autobaud name source)
-	cmake_parse_arguments(PB "" "RX;TX" "" ${ARGN})
-	if(NOT PB_RX)
-		set(PB_RX pb0)
-	endif()
-	if(NOT PB_TX)
-		set(PB_TX pb1)
-	endif()
-	foreach(_pin ${PB_RX} ${PB_TX})
-		if(NOT _pin MATCHES "^p[a-h][0-7]$")
-			message(FATAL_ERROR "pureboot_add_autobaud(${name}): pin '${_pin}' is not of the form pb1")
-		endif()
-	endforeach()
-	get_property(_base_hex GLOBAL PROPERTY PUREBOOT_BASE_HEX)
-	get_property(_app GLOBAL PROPERTY PUREBOOT_APP)
-	get_property(_wrap GLOBAL PROPERTY PUREBOOT_WRAP)
-
-	add_executable(${name} ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${source})
-	target_link_libraries(${name} PRIVATE libavr)
-	target_compile_definitions(${name} PRIVATE PUREBOOT_RX=${PB_RX} PUREBOOT_TX=${PB_TX})
-	target_compile_options(${name} PRIVATE
-	    -fno-ivopts -fira-algorithm=priority -fno-move-loop-invariants -fno-tree-ter -fno-split-wide-types)
-	target_link_options(${name} PRIVATE -nostartfiles -Wl,--section-start=.text=${_base_hex}
-	                    -Wl,--defsym=pureboot_app=${_app} ${_wrap})
-	add_custom_command(TARGET ${name} POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:${name}>)
-endfunction()

pureboot/README.md

@@ -2,52 +2,67 @@
 
 A serial bootloader on [libavr](https://git.blackmark.me/avr/libavr), pure by
 constraint: one C++ source, no inline assembly, no global register variables
-(attributes and compiler flags allowed), **512 bytes on every chip libavr
-targets — all 37**. The device speaks primitives; every composite — verify,
+(attributes and compiler flags allowed), **a 512-byte slot on every chip
+libavr targets — all 37**. The device speaks primitives; every composite — verify,
 erase, reset-vector surgery, updating the loader itself — lives in the host
 tool (`pureboot.py`).
 
 The image is **position-independent**: control flow is PC-relative, the
-read/write paths take wire addresses, the write guard protects the slot the
-code is *running* in (from the runtime return address), the info block is
-addressed from that same anchor, and the application jump is an indirect call
-to an absolute entry. The identical binary therefore runs from any slot with
-every command intact, which makes pureboot **its own staging loader**: the
-host installs the same binary one slot below the resident, jumps into it, and
-lets it rewrite the resident.
+transfer paths take wire addresses, nothing is flash-resident to address at
+all, and the application jump is an indirect call to an absolute entry. It does
+not know which slot it occupies and does not need to. The identical binary
+therefore runs from any slot with
+every command intact, which makes pureboot **its own staging loader**: the host
+installs the same binary one slot below the resident, jumps into it, and lets
+it rewrite the resident. The lint holds it to that literally — the image must
+come out byte-identical linked at a different base.
 
 ## Chips
 
-Sizes are the default configuration: the hardware USART0 at 115200 8N1 on a
-16 MHz crystal, or the software UART on RX = PB0 / TX = PB1 at 57600 8N1 on
-the tinies' RC oscillator (9.6 MHz on the t13s, 8 MHz above). Every axis moves
-per build — see *Configuration*; the largest image any of them produces is a
-software UART at a slow baud, which on the 1284s is 494 B, the tightest fit in
-the whole matrix at 18 B spare.
+The Stock column is the default configuration: the hardware USART0 at 115200
+8N1 on a 16 MHz crystal, or the software UART on RX = PB0 / TX = PB1 at
+57600 8N1 on the tinies' RC oscillator (9.6 MHz on the t13s, 8 MHz above).
+Every axis moves per build — see *Configuration*. The Autobaud column is the
+worst configuration the space produces for the chip: the clock-free build —
+it alone carries the calibration machinery — with the `OSCCAL` trim baked
+and, where the chip has a USART, the link deployed on that USART's own pins,
+which the loader then has to release (*Pin ownership*). Folding the same
+build onto a single pin (*One-wire*) measures identically on every chip, so
+the column covers that twin too. On default pins without the trim the same
+loaders run 4–10 B smaller.
 
-| Chip | Flash | Loader at | Link | Size |
-|---|---|---|---|---|
-| ATtiny13, ATtiny13A † | 1 KiB | 0x0200 | software | 416 B |
-| ATtiny25 † | 2 KiB | 0x0600 | software | 420 B |
-| ATtiny45 † | 4 KiB | 0x0e00 | software | 424 B |
-| ATtiny85 † | 8 KiB | 0x1e00 | software | 424 B |
-| ATmega8, 8A | 8 KiB | 0x1e00 | USART0 | 396 B |
-| ATmega16, 16A | 16 KiB | 0x3e00 | USART0 | 400 B |
-| ATmega32, 32A | 32 KiB | 0x7e00 | USART0 | 400 B |
-| ATmega48, 48A, 48P, 48PA † | 4 KiB | 0x0e00 | USART0 | 414 B |
-| ATmega88, 88A, 88P, 88PA | 8 KiB | 0x1e00 | USART0 | 434 B |
-| ATmega168, 168A, 168P, 168PA | 16 KiB | 0x3e00 | USART0 | 438 B |
-| ATmega328, 328P | 32 KiB | 0x7e00 | USART0 | 438 B |
-| ATmega164A, 164P, 164PA | 16 KiB | 0x3e00 | USART0 | 438 B |
-| ATmega324A, 324P, 324PA | 32 KiB | 0x7e00 | USART0 | 438 B |
-| ATmega644, 644A, 644P, 644PA | 64 KiB | 0xfe00 | USART0 | 432 B |
-| ATmega1284, 1284P | 128 KiB | 0x1fe00 | USART0 | 478 B |
+| Chip | Flash | Loader at | Link | Stock | Autobaud |
+|---|---|---|---|---|---|
+| ATtiny13, ATtiny13A † | 1 KiB | 0x0200 | software | 372 B | 460 B |
+| ATtiny25 † | 2 KiB | 0x0600 | software | 376 B | 452 B |
+| ATtiny45 † | 4 KiB | 0x0e00 | software | 376 B | 452 B |
+| ATtiny85 † | 8 KiB | 0x1e00 | software | 376 B | 452 B |
+| ATmega8, 8A | 8 KiB | 0x1e00 | USART0 | 350 B | 480 B |
+| ATmega16, 16A | 16 KiB | 0x3e00 | USART0 | 352 B | 484 B |
+| ATmega32, 32A | 32 KiB | 0x7e00 | USART0 | 352 B | 484 B |
+| ATmega48, 48A, 48P, 48PA † | 4 KiB | 0x0e00 | USART0 | 366 B | 454 B |
+| ATmega88, 88A, 88P, 88PA | 8 KiB | 0x1e00 | USART0 | 376 B | 464 B |
+| ATmega168, 168A, 168P, 168PA | 16 KiB | 0x3e00 | USART0 | 378 B | 468 B |
+| ATmega328, 328P | 32 KiB | 0x7e00 | USART0 | 378 B | 468 B |
+| ATmega164A, 164P, 164PA | 16 KiB | 0x3e00 | USART0 | 378 B | 468 B |
+| ATmega324A, 324P, 324PA | 32 KiB | 0x7e00 | USART0 | 378 B | 468 B |
+| ATmega644, 644A, 644P, 644PA | 64 KiB | 0xfe00 | USART0 | 372 B | 462 B |
+| ATmega1284, 1284P | 128 KiB | 0x1fe00 | USART0 | 390 B | 480 B |
 
-† No hardware boot section: the host patches the reset vector, and the budget
-is 510 bytes, since the slot's last word is the trampoline.
+† No hardware boot section: the host patches the reset vector, and an image's
+budget is 510 bytes. The last word of the **lower** slot belongs to the host —
+it is the trampoline holding the application's relocated reset vector, and it
+is a staging copy's own last word during a self-update — so an image must fit
+below it. The resident loader has all 512 bytes of the slot it runs in; 510 is
+what an image must fit so that a copy of it staged one slot down leaves that
+word alone.
 
-The 1284s are the heaviest because they alone carry the far-flash machinery —
-ELPM reads, RAMPZ page commands, a word-addressed wire.
+The tightest fit in the whole space is therefore the 1284s' 480 of their
+512: they alone carry the far-flash machinery (ELPM reads, RAMPZ page
+commands) on top of everything the column already stacks. The flash bank
+riding in a transfer's selector byte keeps even those chips' addressing the
+same 16-bit form every other chip uses, which is why they are no longer the
+outlier they were.
 
 The software UART enables the RX pull-up; TX idles high. All multi-byte wire
 quantities are little-endian.
@@ -62,10 +77,12 @@ repo's build and by a downstream project alike:
 |---|---|---|
 | `CLOCK <hz>` | the clock the board runs | 16 MHz megas, 8 MHz t25/45/85, 9.6 MHz t13s |
 | `BAUD <bd>` | the wire rate | the ladder below |
-| `SERIAL auto\|hardware\|software` | the link backend | `auto`: the hardware USART where the chip has one |
+| `SERIAL auto\|hardware\|software\|autobaud` | the link backend | `auto`: the hardware USART where the chip has one |
 | `USART <n>` | the USART instance (x4 megas carry two) | 0 |
 | `RX <pin>`, `TX <pin>` | software-UART pins | `pb0`, `pb1` |
 | `TIMEOUT <s>` | the activation window | 8 |
+| `OSCCAL <byte>` | a measured oscillator trim, applied before anything runs | none — no value, no code |
+| `HALF_DUPLEX` | one-wire: both directions on one line (*One-wire* below) | off |
 
 The default baud is the fastest of 115200/57600/38400/19200/9600 the clock
 reaches within 2.5 % — the same U2X-included divisor search libavr's baud
@@ -74,15 +91,96 @@ receiver's 100-cycles-a-bit floor. Whatever is picked or overridden is
 re-checked in the compile: an infeasible combination, or a USART the chip does
 not have, fails with a named static assert.
 
+Putting a bit-banged link on a USART's own pins is a supported deployment, and
+the usual one where a board's USB bridge is wired to RXD/TXD: the link's `init`
+clears that USART's `UCSRnB` first, because while its `TXEN` is set the USART —
+not the port register — owns the TX pin, and a loader entered from an
+application that left it enabled would receive and obey while answering nothing
+(§20.6.3). It costs one store — four bytes on the extended-I/O chips, two on
+the classic megas — and only on those pins.
+
+`SERIAL autobaud` takes neither: the loader **measures** the host's bit timing
+at run time, so `CLOCK` and `BAUD` are not build parameters there and one
+binary per chip serves every clock and every rate. It is for the deployments
+whose clock is not known at build time and does not hold still — the internal
+RC oscillator, ±10 % from the factory and moving with supply and temperature —
+where a fixed-baud software build has to be rebuilt per clock and still drifts
+out of tolerance. The cost is that it is software-serial only (a hardware USART
+needs its divisor programmed) and that activation counts poll iterations rather
+than seconds, since there is no clock to convert them against
+(`PUREBOOT_AUTOBAUD_POLLS`, default 4,000,000). The wait spends nine cycles a
+poll (measured, and held by the `pureboot.window.autobaud` gate), so the
+default window is 36 M cycles: 4.5 s at 8 MHz, 3.75 s at 9.6 MHz, 36 s at
+1 MHz.
+
+**Pick the rate by cycles a bit, and leave the oscillator room.** What the
+calibration can measure is bounded by how many clock cycles one bit lasts, so a
+rate is only ever sensible relative to the clock. Two different floors matter:
+
+| | cycles a bit |
+|---|---|
+| the logic's floor — exact clock, simulated | solid to ~36, fails outright by ~31 (`pureboot.autobaud` gates a point here) |
+| a factory-trimmed internal RC, measured on an ATtiny13A | reliable at ~118; already locking 1 attempt in 5 by ~59 |
+
+The gap is the oscillator's own jitter, and no exact-clock simulation shows it.
+So on an RC part, **budget about 100 cycles a bit** — the same order as the
+fixed-baud software receiver's floor — rather than the logic's ~36. Measured
+envelope on that ATtiny13A, with an application resident: 9.6 and 4.8 MHz reach
+115200, 1.2 MHz reaches 9600, 600 kHz reaches 4800, 128 kHz reaches 2400.
+
+One trap in testing this: on a patched-vector chip an **erased** application
+region walks straight back up into the loader, so every expired window opens
+another one and the host's retries eventually catch the pulse. That reads as far
+more reliable than the same part with an application resident, which gets one
+window per reset. Measure with an application in place.
+
+## One-wire
+
+`HALF_DUPLEX` puts both directions on one line — the deployment for a board
+with a single spare pin, or a native-UART bootloader's shared-line wiring.
+Each backend has its shape:
+
+- **Software and autobaud links** fold onto the RX pin (`RX == TX` spells
+  the same deployment directly). The pin idles as the receiver's pull-up
+  input; each transmitted frame takes the pin's direction and hands it back
+  with the stop bit's level already on the pull-up, so neither flip makes
+  an edge. This costs nothing: the frame's direction wrap is exactly what
+  the dropped second-pin init paid, and the tightest image in the space —
+  the 1284s' autobaud + `OSCCAL` on their USART's RXD — measures the same
+  480 bytes one-wire as two-wire. On a USART's own pin the release applies
+  as ever, RXD included: `RXEN` forces that pin's direction (§20.7.3),
+  which a receive-only link could live with and a driven shared pin cannot.
+- **The hardware USART** (`SERIAL hardware`/`auto` + `HALF_DUPLEX`) uses
+  libavr's `.half_duplex` turn-around — exactly one direction enabled at a
+  time, each written byte held to transmit-complete before the line can be
+  released — and needs RXD and TXD tied together off-chip. It costs
+  +42…50 B over the stock loader (m8 392, m328P 428, 1284P 440 — all far
+  inside the slot); the activation window is unchanged, its poll merely
+  runs through the release-line test (18 cycles a poll in bit-addressable
+  I/O, 22 in extended — measured, and held per chip by
+  `pureboot.window.halfduplex`).
+
+Host wiring, for an FTDI-style adapter: **adapter TX through ~1 kΩ to the
+line, adapter RX and the MCU pin directly on it.** The resistor lets the MCU
+win the line while it answers; the price is that the adapter reads back every
+byte it transmits. `pureboot.py --one-wire` consumes that echo byte for byte
+— a missing echo is reported as the wiring fault it is, and a device reply
+that lands between the echoes of the knock (a loader already in session
+re-prompts mid-knock) is held for the reader. The knock is the protocol's
+one blind multi-byte write, so on real wiring its second byte can be lost to
+that collision outright; the tool's knock retries absorb it. Everything else
+is ack-paced and cannot collide.
+
 A downstream project brings its usual libavr setup (the `libavr` target, the
 chip via the `LIBAVR_MCU` toolchain preset), consumes this directory, and
 states its deployment — an ATmega328P on its shipped 1 MHz fuses with the
-software UART on hand-picked pins, say:
+software UART on hand-picked pins, say. A submodule pins the loader version
+(the tags name them; this repo pins its own libavr the same way), where
+FetchContent tracks whatever `main` is:
 
 ```cmake
-FetchContent_Declare(bootloader GIT_REPOSITORY git@git.blackmark.me:avr/bootloader.git GIT_TAG main)
-FetchContent_MakeAvailable(bootloader)
-add_subdirectory(${bootloader_SOURCE_DIR}/pureboot pureboot)
+# git submodule add <forge>/avr/bootloader.git bootloader   — or FetchContent
+add_subdirectory(bootloader/pureboot pureboot)
 
 pureboot_add_loader(myboot CLOCK 1000000 SERIAL software TX pb1 RX pb5)
 ```
@@ -113,87 +211,261 @@ window; any other byte is discarded and awaited again, so line noise can delay
 the loader but never lock it. A window expiring on an idle line boots the
 application.
 
+An autobaud build opens differently, because it has to learn the rate before it
+can read a byte at all: the host sends the **calibration byte 0xC0** — a start
+bit plus six zero data bits form one low pulse of seven bit-times — and the
+loader times that pulse into its bit period. A single `p` then activates; the
+pulse has already proven a host is present, which the two-byte knock exists to
+establish elsewhere. Both waits are bounded, so a stray low pulse with no host
+behind it costs one window and then boots the application rather than holding
+the loader.
+
 The window is a compile-time constant (`TIMEOUT`, 8 s by default), so the whole
 EEPROM belongs to the application — pureboot keeps no state of its own.
-Re-timing a deployed loader is a self-update with a re-timed build.
+Re-timing a deployed loader is a self-update with a re-timed build. An autobaud
+build counts poll iterations instead (`PUREBOOT_AUTOBAUD_POLLS`), there being
+no clock to turn into seconds.
+
+### Entering from a running application
+
+A window opens at a reset and nowhere else, which assumes a reset edge exists —
+a button, or DTR wired to it. On a board with neither, the way in has to come
+from the application, by jumping to the loader base.
+
+Deciding *when* to jump is the application's business and deliberately not
+pureboot's: a console command, a held pin, a magic byte, an idle timeout — each
+board's answer differs, and none of them belongs in a 512-byte loader. pureboot
+offers no knock detector and no consumable header for this. What follows is the
+handful of non-obvious mechanics.
+
+Give the linker the base as a symbol rather than casting a literal to a
+function pointer, so the address is stated once and in the same units the
+loader is linked at:
+
+```cmake
+target_link_options(app PRIVATE "LINKER:--defsym=pureboot_loader=0x7e00")
+```
+
+```cpp
+extern "C" [[noreturn]] void pureboot_loader();
+
+// ... then, with interrupts off and the watchdog disabled:
+pureboot_loader();
+```
+
+That is the whole jump: an ordinary call to an absolute symbol, which the
+linker resolves and `-mrelax` shortens where it can — `call 0x7e00` in four
+bytes, or an `rjmp` on a part small enough for one.
+
+It is worth saying what *not* to copy here, because pureboot's own hand-over
+(`run_app()`) looks different: it launders its target through a
+`[[gnu::noipa]]` indirect call. That is a position-independence measure, and it
+belongs to the loader alone — the same image runs from either slot, so it must
+never bake an absolute address. An application is linked at a fixed base and
+has no such problem; using the indirect form costs two `ldi`s and a helper call
+to reach the same place a plain call reaches in four bytes.
+
+Three things must be true before the jump:
+
+- **Interrupts off and the watchdog disabled.** The loader is polled and
+  vector-less; an ISR landing in it vectors into the application's table.
+- **WDRF clear.** pureboot hands straight back to the application on a watchdog
+  reset (above), and it only *peeks* MCUSR — so a jump arriving with WDRF still
+  set opens no window at all.
+- **Release any peripheral holding the link.** A loader entered by a jump
+  inherits the application's registers rather than reset values: with `TXEN0`
+  still set the USART owns TxD, and a bit-banging loader then receives
+  perfectly and answers into a pin it does not control. Clearing `UCSR0B`
+  before jumping is the whole fix, and the symptom without it is a loader that
+  is mute rather than deaf, which reads as a dead board.
 
 ## Session
 
-After the knock the loader stays in its command loop until `J` jumps away or
-the chip resets. Before reading each command it waits for any pending EEPROM
+After the knock the loader stays in its command loop until a jump takes it away
+or the chip resets. Before reading each command it waits for any pending EEPROM
 write and sends the prompt `+` (0x2b), which is therefore also the previous
 command's completion ack. A session is: await `+`, send a command, read its
-reply, repeat.
+verdict, then its reply, repeat.
 
-On chips whose flash exceeds 64 KiB (the 1284s — info-block flag bit 1) the
-`R`/`W` flash addresses are **word** addresses; everywhere else they are byte
-addresses (the 644s' 64 KiB is exactly the 16-bit byte space). EEPROM
-addresses and all counts are bytes.
+There is no invalid opcode: every byte begins a command, and it is the seal
+rather than a table of known letters that rejects noise. The knock is the one
+thing that must survive being sent into a loader already in session, which is
+why both its bytes — `p` and `b` — carry the identify bit: they answer the
+identity and consume nothing else.
+
+Addresses are **byte addresses within a 64 KiB bank**, and the bank rides in
+the command's selector byte, so no command has to speak word addresses. The jump
+is the exception: its address is a word address, because that is what the
+hardware's own jump takes — it still carries a selector byte (reserved,
+ignored) so its decode is the same three reads as every other command's.
+EEPROM and data-space addresses and all counts are bytes.
 
 The loader trusts the host to keep addresses in range: it does not bound them
-against the info block. **Gotcha:** a `w` (or `r`) that runs past `E2END` wraps
-— EEAR is only as wide as the array, so an address past the end truncates onto
-low EEPROM and the write silently overwrites it. Keeping writes within the
-advertised sizes is the host's job (the shipped tool does); the flash budget
-is better spent on features than on re-checking a bound the host already holds.
+against the chip. **Gotcha:** a write (or read) that runs past `E2END` wraps —
+EEAR is only as wide as the array, so an address past the end truncates onto
+low EEPROM and the write silently overwrites it. Keeping transfers within the
+real sizes is the host's job (the shipped tool does); the flash budget is
+better spent on features than on re-checking a bound the host already holds.
 
-| Cmd | Arguments | Reply |
+Every command but the identity has **one header shape** — six bytes, the last
+of them a seal over the other five:
+
+```text
+    op8   sel8   addr_lo8   addr_hi8   n8   seal8
+    seal = op ^ sel ^ addr_lo ^ addr_hi ^ n ^ 0x5A
+```
+
+The loader folds those fields and compares before it decodes what the command
+is, then **answers the seal**: `+` accepts, `0xD4` refuses and nothing has
+happened. The verdict comes before any payload, which is what keeps a refusal
+local — a fill or a write burst sends its data only once the header is in, so
+a rejected header never leaves the host pushing bytes into a loader that has
+gone back to reading commands.
+
+The opcode is bits, not a letter. A transfer is the absence of the other three,
+and its direction is the low bit.
+
+| Opcode | | Arguments after the header | Reply |
+|---|---|---|---|
+| 0x00 | read | none | verdict, then n bytes from the selected space (n = 0 means 256), then `+` |
+| 0x01 | write | n data bytes | verdict, then `+` per byte once its write has begun, then `+` |
+| 0x04 | fill | one page of data | verdict, then `+` when the page is in |
+| 0x08 | jump | none | verdict, then execution continues at the word address |
+| 0x20 | identify | *unsealed, one byte on its own* | 4 bytes: the version, then the three signature bytes, then `+` |
+
+The **selector** byte's low nibble names the space and its high nibble carries
+the flash bank.
+
+| Space | | |
 |---|---|---|
-| `b` | — | the 12-byte info block |
-| `R` | addr16, n8 | n flash bytes (n = 0 means 256) |
-| `W` | addr16 (any address in the page), then one page of data | — (completion = next prompt) |
-| `r` | addr16, n8 | n EEPROM bytes (n = 0 means 256) |
-| `w` | addr16, n8, then n data bytes | `+` per byte, sent once its write has begun |
-| `F` | — | 4 bytes: low fuse, lock, extended fuse, high fuse |
-| `J` | word address (16-bit) | `+`, then execution continues there |
-| other | — | ignored; the loop re-prompts (send a junk byte, await `+`, to resync) |
+| 0 | flash | read-only here; it is written through the fill and the SPM command |
+| 1 | EEPROM | |
+| 2 | data | SRAM — and with it the register file and every I/O register, which share the data address space on AVR |
+| 3 | fuse and lock | index 0..3 in the hardware's own Z order: low, lock, extended, high |
+| 4 | SPM | write-only: the byte goes to SPMCSR and fires the instruction at the address |
 
-`W` streams exactly one SPM page (size from the info block) into the buffer,
-then erases and programs — except pages inside the 512-byte slot
-the loader is *running* in, which are drained and left alone, so a broken host
-cannot brick the running copy and a staged copy may rewrite the resident.
+The data space is worth more than it looks. pureboot keeps **zero static RAM**
+and pushes no register, so at loader entry an application's SRAM is still
+whatever the application left there, bar the handful of bytes of return-address
+stack — which makes a read over space 2 a post-mortem of a running application,
+not just a poke hole. The same address space carries the register file and the
+I/O registers, so peripheral state is readable too; reading some of those has
+side effects (reading UDR clears its flags), which is the host's business to
+know.
 
-The loader never clears the SPM buffer before a fill, so **one `W` may program
+Programming a page is therefore a fill to load the buffer, then an SPM command
+for the erase, another for the write, and on a boot-sectioned chip a third to
+re-enable the RWW section — `0x03`, `0x05` and `0x11`, the SPMCSR encodings
+every part pureboot targets shares. The loader carries no page-commit logic of
+its own, and the same primitive reaches every other SPM operation, lock bits
+included.
+
+An SPM command has **no data phase**: its SPMCSR byte rides the header's count
+field, where the seal covers it. That is the whole reason the field is
+overloaded — a byte arriving behind the header would arrive after the seal had
+been checked, and the one command that cannot be taken back is exactly the one
+that must not be decided by an unchecked byte. Setting the lock bits is
+therefore an ordinary sealed command (`0x09`) rather than something the loader
+refuses: deliberate is expressible, accidental is not reachable.
+
+The SPM store and the SPM instruction must issue within four cycles of each
+other (§26.2), which no host can hit across a serial link — so this one
+primitive is *fused* rather than being a poke of SPMCSR followed by a poke of
+something else. That four-cycle window is the floor on how low-level a
+bootloader's primitives can go; it is not a byte-count decision.
+
+Nothing refuses an address, the loader's own slot included. Through pureboot 8
+a running-slot write was dropped; the seal replaced that guard, because what
+the guard defended against was a wire fault naming an address, and a wire fault
+can no longer name one. What it costs is that a host bug aimed at the running
+slot now lands. What it buys is that a resident copy can write its own slot —
+which is the only route a self-update has on a chip whose boot section *is* the
+512-byte slot, where no staged copy can run SPM at all: the resident plants a
+primitive in its own spare space and an application-side installer drives it.
+A copy that erases the page it is executing from does not come back, so which
+page matters; erasing any other page of its own slot it survives. That needs a
+page the image does not reach into, which the stock builds have and the biggest
+do not: a 378 B loader on a 128-byte-page mega leaves 384..511 entirely free,
+while the 480 B autobaud build reaches into it and has none.
+
+The loader never clears the SPM buffer before a fill, so **one fill may program
 the wrong bytes, and the host is what fixes it**. The buffer is write-once per
 word until cleared, and two things leave words in it: a refused page, and —
 where SPM runs from anywhere, the tinies and the m48s — an application that
-self-programmed before entering. The next `W` takes those stale words and
-clears them, since a page write auto-erases the buffer (§26.2.1; §19.2 on the
-tinies), so repeating it programs correctly. The host therefore verifies every
-page it writes and rewrites what comes back wrong (three retries, then it
+self-programmed before entering. The next page write takes those stale words
+and clears them, since a page write auto-erases the buffer (§26.2.1; §19.2 on
+the tinies), so repeating it programs correctly. The host therefore verifies
+every page it writes and rewrites what comes back wrong (three retries, then it
 stops).
 
-`w` is host-paced: send the next byte only after the previous byte's `+`. `F`
-returns the bytes in the hardware's Z order; on a chip without an extended
-fuse byte that slot carries no meaning. Fuse *writing* does not exist — SPM
-reaches flash and boot lock bits only.
+A write is host-paced: send the next byte only after the previous byte's `+`. Fuse
+*writing* does not exist — SPM reaches flash and boot lock bits only.
 
-`J` is the one control-transfer primitive: it runs the application (word 0 or
-the trampoline word, both known from the info block) and moves between loader
+The jump is the one control-transfer primitive: it runs the application (word 0 or
+the trampoline word, both derived from the chip) and moves between loader
 copies during a self-update. A jump to a slot's base re-enters that copy's own
 startup, which must then be knocked afresh.
 
-The info block (`b`):
+Identify answers with the loader's identity — its version and the chip's signature —
+and nothing else. Everything else the host needs (page size, loader base,
+EEPROM size, whether the reset vector must be patched, how many flash banks)
+follows from the signature, and the host holds that table; the loader derived
+the same facts from its own chip database at build time, so nothing is guessed,
+it is simply not sent twice.
 
-| Offset | Content |
-|---|---|
-| 0–2 | `'P'`, `'B'`, pureboot version (3) |
-| 3–5 | device signature |
-| 6 | SPM page size in bytes (0 means 256) |
-| 7–8 | loader base — application flash ends here (a word address when bit 1 is set) |
-| 9–10 | EEPROM size |
-| 11 | bit 0: host must patch the reset vector (no hardware boot section); bit 1: flash wire addresses are word addresses |
+An update image, though, is a bare 512-byte slot with no device to ask, and
+installing one built for another chip bricks the target. Every loader image
+therefore carries a six-byte **stamp** — `'P'`, `'B'`, the version, the three
+signature bytes — which the loader itself never reads and the host tool refuses
+to install a mismatch against.
 
 ## Version
 
-The info block's third byte is the **pureboot version** — the loader's one
-identity number, and the only way to tell what a deployed loader is. Nothing
-else is numbered: the wire protocol has no version, a pureboot version implies
-it, and the host tool holds that map. The tool states the window of loader
-versions it speaks (`OLDEST_LOADER`/`NEWEST_LOADER` in `pureboot.py`), and a
-version that changes the protocol becomes the new floor there. None has so
-far: 1 through 3 speak the identical session. A loader newer than the tool is
-refused by name rather than decoded on the assumption that nothing moved.
+The identity's first byte is the **pureboot version** — the loader's one identity
+number, and the only way to tell what a deployed loader is. Nothing else is
+numbered: the wire protocol has no version, a pureboot version implies it, and
+the host tool holds that map. The tool states the window of loader versions it
+speaks (`OLDEST_LOADER`/`NEWEST_LOADER` in `pureboot.py`), and a version that
+changes the protocol becomes the new floor there. A loader newer than the tool
+is refused by name rather than decoded on the assumption that nothing moved.
+
+Two generations exist. **1 through 4** speak one session — a 12-byte info block
+from `b`, and a command per memory (`R`/`W` flash, `r`/`w` EEPROM, `F` fuses).
+**5** replaced those with the single `G`/`g` pair over selector-named spaces
+above; the shipped tool speaks both, choosing on the version it reads, so a
+deployed pureboot 4 stays drivable and self-updatable to 5. **6** changes
+nothing on the wire: it marks the builds that may carry a baked `OSCCAL` trim
+(Configuration), so a tool driving an update knows such images exist. **7**
+moves `J` onto the unified decode — it gains the selector byte the table
+shows, which older loaders do not read, so the tool sends each form to the
+version that speaks it — and re-homes the autobaud unit into the GPIOR pair
+on the chips that have one (Session: what must not be written), which is
+where `--info`'s measured clock now reads it on those parts. **8** changes
+nothing on the wire either: it marks the builds whose deployment may be
+one-wire (*One-wire* above) — the hardware USART's half-duplex turn-around,
+or a software link folded onto a single pin. The host-side trace is
+`--one-wire`, the echo discard a shared line requires of any tool driving
+it. **9** is the third wire change and the largest: bit opcodes in place of
+the letters, one sealed header shape for every command, and a verdict on that
+seal before the command runs (*Session* above). It also drops the
+running-slot write guard, which the seal makes redundant and which was the
+only thing standing between a resident copy and its own slot. Identify is
+answered by both knock bytes so version discovery works before the version is
+known, which is what keeps a deployed pureboot 8 drivable and self-updatable
+to 9.
+
+Every closed generation is tagged in this repo at its era's last commit — the
+commit just before the next version bump, so a tag holds everything its
+version ever gained — and each tag carries the `libavr/` submodule pinned to
+the libavr that loader was built against, as the whole libavr era does commit
+by commit. `git checkout v3 && git submodule update --init libavr` followed by
+the usual preset build therefore reproduces the v3 loader exactly; the open
+generation is `main`.
+
+Collapsing four command bodies into one transfer loop is what paid for the
+version: the data space, the host-issued SPM operations and the fuses now share
+the loop, the cursor and the argument decode that `R`/`r`/`w` each carried a
+copy of. The loader shrank while gaining all three.
 
 The tool carries its own version, free to drift; `--version` prints it and the
 window.
@@ -223,7 +495,7 @@ ATmega328P profiles (addresses for its 32 KiB):
 | BOOTSZ | BOOTRST | Behavior |
 |---|---|---|
 | 256 words (512 B) | programmed | *Standalone*: reset always enters the loader; **self-update impossible** (the staging slot lies outside the boot section, where SPM is disabled). |
-| 512 words (1 KB) | unprogrammed | *Self-update, app-first*: reset always boots the application, which owns all 31.5 KB and must offer its own jump to 0x7e00 to reach the loader (a virgin chip reaches it by reset across erased flash). Updates are power-fail-safe except mid-rewrite of the resident slot itself (no reset path leads to the staging copy then). |
+| 512 words (1 KB) | unprogrammed | *Self-update, app-first*: reset always boots the application, which owns all 31.5 KB and must offer its own jump to 0x7e00 to reach the loader (a virgin chip reaches it by reset across erased flash; *Entering from a running application* under Activation is how that jump is written). Updates are power-fail-safe except mid-rewrite of the resident slot itself (no reset path leads to the staging copy then). |
 | 512 words (1 KB) | programmed | *Self-update, loader-first*: reset lands at 0x7c00 — the staging slot, normally erased, so execution walks up into the loader; during an update it is the staging copy itself, so a mid-rewrite power loss recovers by reset. The loss windows move to the staging install/retire page writes instead (page-write scale). The host keeps `[0x7c00, 0x7e00)` clear of application data (`--force` overrides). |
 
 Applications are flashed unmodified here — word 0 stays the application's own
@@ -245,6 +517,18 @@ mega (SPM only executes from the boot section — reflash the .hex), but *runs*
 on a patched-vector chip, and the ordinary `--update-loader` flow re-homes it
 into the top slot from there (`pureboot.rehome`).
 
+**Fixed-baud on an internal RC oscillator is a deployment risk the build
+cannot see.** The factory trim is ±10 % where an 8N1 frame survives about
+±4: a part at the edge answers nothing at the built rate, and the symptom —
+silence — reads as a wiring fault (a real ATtiny13A measured −5.5 %, outside
+every standard rate at its own documented default). The **autobaud build is
+the deployment-proof backend**: it has no rate to miss. Where fixed-baud on
+RC is wanted anyway, measure first and bake the trim: an autobaud session's
+`--info` prints the part's true clock from the loader's own measured bit
+period, OSCCAL moves the oscillator about 1 % per step, and `OSCCAL <byte>`
+builds the correction in — one build–measure iteration converges. A loader
+already deployed and silent is diagnosed with `--scan` (Host tool).
+
 ## Updating the loader
 
 `pureboot.py --update-loader new_pureboot.bin` replaces the resident loader
@@ -252,11 +536,32 @@ with any pureboot build — a re-timed window, a newer version — using the
 loader itself as its own staging loader. The image is the loader's own 512
 bytes as a raw binary, or the Intel HEX the build emits beside it.
 
-The preflight refuses an image built for another chip: the info block embedded
-in every pureboot binary (signature, page size, loader base, EEPROM size,
-flags) must match the device's own, and the error names both. Die revisions
-share their base signature and geometry, so their images are interchangeable —
-as the silicon is.
+One thing the image cannot tell the host: **which link it speaks.** The update
+works by entering copies of the *new* image (steps 3 and 4 below), so a build
+made for another baud or another backend answers on that one and not on the
+session's — and 512 bytes of position-independent code carry no header to read
+it from. Where the new image's link differs, name it:
+
+```sh
+# a 57600 fixed-baud resident, replaced by an autobaud build
+pureboot.py --port … --baud 57600 --update-loader ab.bin --staged-autobaud
+# …or by a 38400 build of the same backend
+pureboot.py --port … --baud 57600 --update-loader sw38400.bin --staged-baud 38400
+```
+
+The host retunes on the open port, so no DTR pulse resets the copy it is talking
+to. Omit them against a changed link and the update stops after installing the
+staging copy, saying so and naming this as the cause.
+
+An `OSCCAL`-baked image is a link change in effect even at an unchanged rate
+on paper: the staging copy shifts the physical clock the moment its `run()`
+starts, and from then on speaks exactly what it was built for. Declare it
+like any other link change — `--staged-baud` with the new build's rate.
+
+The preflight refuses an image built for another chip: the stamp every pureboot
+binary carries must resolve to the device's own geometry, and the error names
+both. Die revisions share their base signature and geometry, so their images
+are interchangeable — as the silicon is.
 
 1. The staging slot `[base−512, base)` is saved to a host-side state file (on
    the 1 KB tiny13s that is the whole application, vectors included).
@@ -264,7 +569,7 @@ as the silicon is.
    the host composes the slot's last word as a jump to the resident base, so
    even an abandoned staging copy times out into a loader. A loader already
    sitting whole in the staging slot is left as the staging copy instead —
-   rewriting it would only meet its own running-slot guard.
+   rewriting it in place would be a copy overwriting itself as it runs.
 3. `J` enters the staging copy, which rewrites the resident slot. Where a
    patched reset vector routes through the resident, the host first re-aims
    word 0 at the staging copy, so a power loss mid-rewrite still resets into a
@@ -273,10 +578,15 @@ as the silicon is.
    content, and the state file is discarded.
 
 Every phase is idempotent and keyed off the actual flash state, so re-running
-the same command after any interruption resumes and completes. The state file
-carries the only bytes not recoverable from the device; losing it mid-update
-still completes the update, and the staging region comes back by reflashing
-the application. A boot-sectioned mega needs its fuses for the preflight — read
+the same command after any interruption resumes and completes — with one
+qualification, which is the link again: from step 2 on, the copy the re-run has
+to reach is the *new* image, so a resumed run needs the same `--staged-*` as the
+first one. On a patched-vector part step 3 also re-aims word 0 at the staging
+copy, so after that point a reset reaches the new image's link and **only** that
+one; a re-run on the resident's link finds nothing at all. The state file carries
+the only bytes not recoverable from the device; losing it mid-update still
+completes the update, and the staging region comes back by reflashing the
+application. A boot-sectioned mega needs its fuses for the preflight — read
 from the device, or supplied with `--assume-fuses` where reading is impossible
 (simulators).
 
@@ -293,25 +603,73 @@ to reset gets its reset pulse and opens the activation window by itself.
         --info --fuses --flash app.hex
 
 Operations run in a fixed order within one session: info, fuses, loader
-update, flash (erase / program / read / verify), EEPROM (the same) — then the
-loader hands over to the application. `--stay` keeps the session alive
+update, flash (erase / program / read / verify), EEPROM (the same), then
+`--poke` and `--peek` in that order, so one invocation writes and reads the
+write back — then the loader hands over to the application. `--stay` keeps the session alive
 instead, and a later invocation reconnects into it. `--flash` and `--eeprom`
 verify by read-back unless `--no-verify`, and a flash page that reads back
-wrong is rewritten up to three times before the run stops (see `W` above).
+wrong is rewritten up to three times before the run stops (see the fill above).
 `--verify-flash` only reports. Images are raw binary, or Intel HEX by
 extension. `--force` overrides the refusable safety checks — today, flashing
 application data into a mega's reset walk region.
 
-Readouts come one fact per line: `--info` decodes the info block field by
-field, `--fuses` each fuse byte plus, on a boot-sectioned mega, its decoded
-meaning. Transfers that take wire time draw a transient progress bar on stderr
+`--autobaud` opens with the calibration pulse instead of the plain knock, for a
+loader built `SERIAL autobaud`; the rest of the session is identical, at
+whatever `--baud` the host chose. Its `--info` adds the **measured clock** —
+the loader's bit-period unit, decoded and multiplied by the session rate —
+which is the number an `OSCCAL` bake or a fixed-baud build for the part is
+held against; `--clock <hz>` states the drift against a nominal.
+
+`--one-wire` marks the link as a shared line (*One-wire* above): the tool
+reads back and verifies its own echoed bytes, whatever the backend.
+It combines with everything, `--scan` included — undiscarded echoes would
+answer every rate a scan probes.
+
+`--scan` is the diagnosis once a fixed-baud loader has gone silent: it walks
+±10 % around `--baud` in 2 % steps, nearest first, one probe per activation
+window — reset the target as each probe announces itself (a board with DTR
+wired to reset is pulsed by the probe's own port-open). A loader
+off-frequency answers at its oscillator's ratio, and the report gives the
+found rate as the session workaround, the offset, the OSCCAL correction's
+direction at ~1 % per step, and the autobaud way out. Standalone — no other
+operation combines with it.
+
+`--poke ADDR:HEX` and `--peek ADDR[:N]` reach the data space (pureboot 5) —
+SRAM, and through the same address space the register file and every I/O
+register. Reading an I/O register can have side effects (reading UDR clears its
+flags), which is the caller's business to know.
+
+Reads are safe anywhere; **two small regions cannot be written without ending the
+session,** because they are what the loader is standing on:
+
+- the **top of SRAM**, where its stack lives — a handful of bytes below RAMEND;
+- on an **autobaud** build, the **measured bit period**: two bytes in
+  GPIOR2:GPIOR1 where the chip has the pair (data `0x32..0x33` on the
+  t25/45/85, `0x4A..0x4B` from the x8 generation on — such a loader has *no*
+  static RAM at all), and the two bytes at RAMSTART on the chips without one
+  (the t13s and classic megas), where they are the whole of the loader's
+  static RAM. Overwrite either home and the next reply is timed against
+  garbage — the symptom is a mangled prompt byte rather than any error; the
+  loader is fine, it simply is no longer speaking the agreed rate.
+
+Both are self-inflicted rather than defects, and a reset clears them. Note also
+that `--poke` can write OSCCAL, which does take effect — but a session can only
+survive a step or two of it before the clock walks the link out of the rate
+autobaud locked to, and OSCCAL reverts on reset regardless.
+
+Readouts come one fact per line: `--info` prints the device's version and
+signature and the geometry that follows from them, `--fuses` each fuse byte
+plus, on a boot-sectioned mega, its decoded meaning. Transfers that take wire time draw a transient progress bar on stderr
 when it is a tty. `-v`/`--verbose` adds the decisions as they happen: knock
 counts, the programming plan, update state handling and per-phase page counts.
 
 ## Tests
 
-`tools/check.sh` runs every chip's workflow (`--full` adds the reflect-mode
-builds of libavr's spot set; `tools/make_presets.py` regenerates the presets).
+libavr rides as the `libavr/` submodule (`git submodule update --init libavr`);
+`LIBAVR_ROOT` (cache or environment) overrides it for tandem development
+against a working tree. `tools/check.sh` runs every chip's workflow (`--full`
+adds the reflect-mode builds of libavr's spot set; `tools/make_presets.py`
+regenerates the presets).
 Per chip preset, `ctest` runs:
 
 - `pureboot.size` — the 510-byte (patched-vector) / 512-byte budget;
@@ -321,23 +679,50 @@ Per chip preset, `ctest` runs:
   the fastest clock — where a software UART's per-bit spin outgrows its
   one-register delay loop and takes the 16-bit one. That is the largest image
   the configuration space produces, and a shape the ladder default (always the
-  *fastest* rate a clock reaches) never picks. Pins are immediate operands and
-  the timeout is a constant: neither is an axis;
-- `pbm_*.size` — under `--full`, the exhaustive cross product replacing that
-  compact matrix: every plausible oscillator (the internal ones, the CKDIV8
-  floor, the plain and the UART crystals) × every rate reachable from it ×
-  every backend, unreachable combinations dropping out rather than aborting
-  the configure. Bounded to one chip per size-bearing class — flash
-  addressing, hand-over shape, page size, USART inventory — since everything
-  else in the image is chip-independent code;
-- `pureboot.pi` — the position-independence lint: no absolute `jmp`/`call`, the
-  info block within the image's first 256 bytes;
+  *fastest* rate a clock reaches) never picks. Pins are an axis for one reason
+  only, and it is enough: a bit-banged link on a USART's own pins has to
+  release that USART, so `pureboot_{sw,autobaud}_on_usart{0,1}` build there
+  too. The timeout is a constant and is no axis;
+- `pureboot_autobaud.size` — the clock-free build, which has no clock or baud
+  axis of its own: one binary per chip has to serve every point the matrix
+  below sweeps. `pureboot*osccal*.size` add the `OSCCAL` trim on the stock
+  shape and on the tightest image in the space (autobaud on a USART's own
+  pins), holding both of the trim write's addressing encodings to the budget;
+- `pureboot_autobaud.unit` — the measured bit period sits where `--info`
+  reads it (wire contract, not layout accident): in the GPIOR pair, with no
+  RAM object at all, on the chips that have one; as the loader's only RAM
+  object at exactly ram_start elsewhere;
+- `pbm_*.size` — with `PUREBOOT_FULL_MATRIX=1`, the exhaustive cross product
+  replacing that compact matrix, on **every** chip: every plausible oscillator
+  (the internal ones, the CKDIV8 floor, the plain and the UART crystals) ×
+  every rate reachable from it × every backend, unreachable combinations
+  dropping out rather than aborting the configure. Thousands of points per
+  chip, and cheap enough to run rather than reason about;
+- `pureboot.pi` — the position-independence lint: no absolute `jmp`/`call`, no
+  flash-resident section but `.text`, and the image byte-identical when linked
+  at a different base — which is position independence itself rather than a
+  proxy for it;
+- `pureboot.handshake` — the host tool's activation must not hang on a target
+  that never falls quiet: the drain after a prompt is bounded by the handshake
+  deadline, and a well-behaved loader still connects;
+- `pureboot.updatelink` — an update whose image changes the baud or the backend
+  must follow the staging copy onto *its* link, since that copy is the new image;
+  and where nothing was declared, the failure must name the link rather than
+  report a bare activation timeout, because by then the staging slot is written
+  and on a 1 KiB tiny that was the application;
 - `pureboot.planner` — the host tool's pure logic: programming orders and their
   recovery properties, the surgery, the staging composition, the boot-fuse
   decode, the update preflight over synthetic fuse bytes, and the repairing
   verify against a fake device;
+- `pureboot.scan` — `--scan`'s walk and report logic: the probe order, the
+  rate arithmetic, and the trim advice's direction. A pty carries bytes at
+  any termios rate, so the rate physics itself belongs to the hardware
+  harness, and what the wire would arbitrate is pinned as logic;
+- `presets.generated` — CMakePresets.json matches its generator
+  (`tools/make_presets.py --check`), so a hand edit or a generator change
+  cannot drift the pair apart;
 - `pureboot.protocol` — end to end against a simavr device
-  (`test/pureboot_device.c`: a hardware USART as a pty, or a cycle-timed
+  (`test/pureboot_device.cpp`: a hardware USART as a pty, or a cycle-timed
   GPIO⇄pty bridge for a software-UART build, plus the SPM/NVM module simavr's
   tiny cores lack) driven by the real host tool through knock-from-reset,
   program + verify of both memories, session reconnect, an external reset
@@ -354,6 +739,12 @@ Per chip preset, `ctest` runs:
 - `pureboot.usart1` (644A) — the same suite over the second hardware USART:
   instance selection is compile-checked everywhere, but only a live session
   proves the loader polls the USART it claims;
+- `pureboot.mute` (328P) — a software link on USART0's own pins, entered from an
+  application that handed over with that USART still enabled: the loader must
+  still answer, which it does only because it releases it. The pin ownership is
+  the runner's, not simavr's — simavr wires a USART through IRQs and never takes
+  the pin from the port, so without that model the state under test could not
+  arise at all;
 - `pureboot.dirty` (328P) — entering the loader from a running application over
   an SPM buffer it deliberately dirtied, the case the loader declines to guard:
   a bare verify must see the corruption and the repairing verify must fix it in
@@ -361,7 +752,59 @@ Per chip preset, `ctest` runs:
   anywhere, which is what makes the path constructible;
 - `pureboot.update` — the full `--update-loader` flow, then every power-fail
   phase: the device is killed mid-write, restarted from its flash dump, and a
-  re-run must complete the update with the application intact.
+  re-run must complete the update with the application intact;
+- `pureboot.osccal` (328P, t85) — a loader built with the `OSCCAL` axis holds
+  the trim register at the built byte from its first prompt, observed through
+  the wire on one chip per addressing encoding (`sts` and low-I/O `out`);
+- `pureboot.autobaud` (328P, 1284P) — the clock-free build over the GPIO⇄pty
+  bridge: the calibration handshake, a flash + EEPROM + fuse round trip against
+  the simulator's own memory, a data-space round trip, the hand-over — then the
+  same binary again at double the clock, which is the property the backend
+  exists for. The measured clock `--info` prints is asserted against the
+  simulator's exact clock, inside the unit encoding's own envelope, at both
+  points. A lone calibration pulse with no knock behind it must still let
+  the application boot, so no wait in activation can be unbounded.
 
-`size`, `pi` and `planner` are host logic and run anywhere; the
-simulator-driven targets need simavr and a pty, so they are POSIX-only.
+`size`, `unit`, `pi`, `planner`, `scan` and `handshake` are host logic and run
+anywhere; the simulator-driven targets need simavr and a pty, so they are
+POSIX-only.
+
+## Hardware
+
+The suite above proves the protocol on every chip; it cannot prove a *board*.
+Two things live only on silicon: an RC oscillator that is not on its nominal, and
+a reset edge that has to come from somewhere. `tools/pbrig.py` and
+`tools/pbhw.py` cover that, and know nothing per-board — every deployment fact
+is a flag or a `PUREBOOT_*` environment variable.
+
+```sh
+export PUREBOOT_PROGRAMMER=atmelice_isp PUREBOOT_PART=t13 PUREBOOT_PORT=COM6
+tools/pbrig.py backup rig-backup/          # verified, before anything is written
+tools/pbhw.py --autobaud --loader build/ab.bin --app build/pbapp.hex --marker APP
+```
+
+`pbrig.py` is the primitives — `signature`, `reset`, `flash`, `fuses`, `backup`,
+`rate` — and the module `pbhw.py` builds on. Two rig facts are encoded in it
+because neither is guessable: an **ISP access is the reset edge** (the part runs
+the moment the programmer releases it, which is the only edge available when the
+adapter's DTR is not wired to reset, so a session begins with an ISP touch and
+knocks immediately after), and **avrdude splits `-U` on colons**, so a Windows
+path's drive letter breaks the spec and every file is passed as a bare name with
+avrdude run in its own directory.
+
+`pbrig.py rate` is the one that turns "the loader is silent, so the wiring must
+be wrong" into a number. Against a fixture built with `PUREBOOT_HEARTBEAT` — a
+*fixed* cycles-per-bit transmitter — it sweeps the host rate, and the band where
+the marker still decodes brackets the part's true bit rate; with the clock the
+image was built for, that is the clock the part is really running at. No
+instrument beyond the adapter already attached. An ATtiny13A measured this way
+came out at 9.072 MHz against its 9.6 MHz nominal, −5.5 % — inside the
+datasheet's ±10 % and outside what an 8N1 frame survives, which is the whole
+case for the autobaud backend on such a part.
+
+`pbhw.py` takes its bounds from the info block the loader reports, so one run
+covers a 1 KiB tiny and a 128 KiB mega alike: identity, the EEPROM round trip
+and erase, an application flashed and verified and then *seen running*, the
+application region read and erased, the loader slot proven intact across that
+erase by an independent ISP read, and an oversized image refused. It overwrites
+the application flash and EEPROM, which is why `backup` comes first.

pureboot/autobaud.md

@@ -1,251 +0,0 @@
-# pureboot autobaud — findings, and how the version question settled itself
-
-The autobaud loader measures the host's bit timing **at runtime** from a
-calibration pulse, so the image carries no clock: one clock-agnostic binary per
-chip runs at any F_CPU and locks onto whatever baud the host sends. It exists
-for the software-serial deployments — the RC-oscillator parts (the tinies,
-internal-oscillator megas) whose exact clock is uncertain and drifts, so today
-each needs a per-clock build. Autobaud erases that axis. That is the win —
-deployment, not bytes.
-
-This document records how the fit was established, why the two versions that
-were under review are both dead, and what the loader looks like now.
-
-## The decision, settled by measurement
-
-Two autobaud loaders were built for review, differing in one tradeoff — the
-running-slot write guard against strict purity. `pureboot_autobaud_pure.cpp`
-landed at **508 B** on the 1284P (4 B spare) and `pureboot_autobaud_reg.cpp` at
-**512** (zero spare).
-
-Then hardware testing found a defect that neither could absorb.
-
-**A single spurious calibration pulse wedged the loader.** `run()` budgeted only
-the start-edge wait inside `measure()`; the `rx()` that read the knock behind it
-was unbudgeted and blocked forever. One stray low pulse on an unattended device
-— EMI, or a host that opens the port and never knocks — held the loader in its
-activation loop and the application never ran. On a field device that is a hang,
-not a hiccup, and it is exactly the deployment autobaud is for.
-
-The fix is to bound the whole activation: an expired knock budget returns a byte
-that cannot be the knock, so control falls back into the budgeted `measure()`,
-and a line that stays idle boots the application there. It costs about 22 bytes.
-
-| 1284P, with the activation fix | size | 512 B budget |
-|---|---|---|
-| `pureboot_autobaud_pure.cpp` | 530 | **over by 18** |
-| `pureboot_autobaud_reg.cpp` | 534 | **over by 22** |
-| `pureboot_autobaud_uni.cpp` | **464** | **48 B spare** |
-
-Both candidates were unshippable, and the margin they were competing over was
-never real — it was the space the missing fix should have occupied. So the
-choice is not between them. It is the third loader below, which fits with room
-to spare *and* carries features neither had. The two sources stay in the tree
-for the record; only the unified one is built.
-
-## The unified loader
-
-The insight that paid was the one that had already paid once: **merging command
-bodies removes cost that moving them around only redistributes.** Folding `R`,
-`r` and `w` into a single address-and-count path had been worth 14 B earlier.
-Pushed further — one read command and one write command over *named spaces* —
-it is worth far more, because four transfer loops collapse into one.
-
-`pureboot_autobaud_uni.cpp` is pureboot 5. It is strictly pure: no inline
-assembly, no global register variable, and **no GPIOR either** — the measured
-unit lives in a plain static, so the loader claims no chip resource an
-application might want, and the GPIOR-versus-static question disappears along
-with the chips that have no GPIOR.
-
-### The protocol
-
-| command | arguments | |
-|---|---|---|
-| `b` | — | version, then the three signature bytes |
-| `J` | addr16 | ack, then jump (word address) |
-| `W` | sel8, addr16, page bytes | fill the flash page buffer |
-| `G` | sel8, addr16, n8 | read n bytes (0 means 256) |
-| `g` | sel8, addr16, n8, then n bytes | write, each byte acked |
-
-`sel` is `space | bank << 4`. The low nibble names the space; the high nibble is
-flash's third address byte, so every transfer speaks a **byte** address inside a
-64 KiB bank and no command has to carry word addresses. The host must not span a
-bank boundary in one transfer — it already chunks by page, so nothing it does
-comes close.
-
-| space | | |
-|---|---|---|
-| 0 | flash | `lpm`/`elpm` |
-| 1 | EEPROM | |
-| 2 | data | SRAM — and with it the register file and every I/O register, which share the data address space on AVR |
-| 3 | fuse and lock | |
-| 4 | SPM | write-only: the data byte goes to SPMCSR and fires the instruction at the selected address |
-
-Three things follow from that table that the loader never had:
-
-- **RAM read and write**, the missing feature. In a per-command design it would
-  have cost a fresh dispatch arm and a fresh loop, ~30 B, on a loader with 4 B
-  spare. As one more space on a shared loop it is a single `ld`/`st`. It also
-  hands the host arbitrary **I/O register access** for free, because AVR maps
-  the peripherals into the same address space.
-- **Host-driven SPM.** `W` used to end with a hardcoded erase, write and RWW
-  re-enable — 42 B. Those are now three writes to the SPM space, reusing the
-  store path's own address, data byte and ack. The host pays three extra
-  round-trips per page (18 wire bytes against 256 of data) and gains the ability
-  to issue *any* SPM operation, lock bits included.
-- **`W` on the same footing as everything else.** It takes the same selector and
-  the same byte address instead of a word address of its own, which made flash
-  addressing uniform across the protocol *and* was 20 B cheaper than keeping its
-  private convention.
-
-Read and write are `G` and `g` — the same letter, one bit apart — so the
-transfer loop picks its direction with a one-word skip rather than a compare.
-
-### Why the SPM space has to be one primitive
-
-It is tempting to go further and expose a generic "poke this I/O register", from
-which the host could drive SPM itself. The hardware forbids it: `out SPMCSR, x`
-and the `spm` that follows must issue within four cycles, and EEPROM's
-EEMPE→EEPE window is the same shape. A host cannot hit a four-cycle window
-across a serial link. **Atomicity is the floor, and the atomic unit must be
-resident.** That is the real limit on how low-level a bootloader's primitives
-can go — not the byte count.
-
-## Where the bytes went
-
-The starting point was the 508 B pure build, disassembled and attributed:
-
-| phase | bytes |
-|---|---|
-| `link::rx` + `link::tx` (bit-banged UART) | 102 |
-| autobaud measure + knock | 62 |
-| reset vector, pin init, WDRF check, jump, ack, EEPROM wait | 50 |
-| command loop head + dispatch tree | 42 |
-| `W` program flash page | 98 |
-| `R` / `r` / `w` / `F` bodies, plus the shared address decode | 122 |
-| `b` info, `J` jump | 32 |
-
-**208 B — 41% — is physical layer and activation**, which no protocol change can
-touch. The command bodies were the entire addressable surface, and they were
-four copies of one idea.
-
-The route from a first attempt to the final loader, all on the 1284P:
-
-| step | size |
-|---|---|
-| unified `G`/`P`, `load`/`store` outlined, `__uint24` cursor | 600 |
-| …`load`/`store` inlined; unit in `.noinit`, not `.bss` | 534 |
-| …16-bit cursor with the bank in the selector; direction as a command bit | 510 |
-| …erase/write/RWW moved out to the SPM space | 484 |
-| …`W` sharing the selector-and-address decode | **464** |
-
-Three of those steps are worth keeping as lessons:
-
-- **Outlining `load`/`store` cost more than the four bodies they replaced.** As
-  functions they were 110 B against the 106 B of inline bodies — the AVR ABI's
-  argument marshalling plus prologue ate the entire saving. Inlined into the one
-  shared loop they cost only their own instructions. The call-site lesson cuts
-  both ways: *merging* call sites pays, *creating* one does not.
-- **A `.bss` static drags in `__do_clear_bss`** — 18 B of startup code to zero a
-  variable that is always measured before it is read. `.noinit` is correct here
-  and free.
-- **A three-byte cursor taxes every space.** Widening the shared cursor so flash
-  could reach past 64 KiB put an extra increment on EEPROM and RAM reads that
-  never need it. Moving the bank into the selector byte kept the cursor at
-  sixteen bits and cost nothing on the wire.
-
-### What did not work
-
-- **Encoding the space in the command byte** (so dispatch becomes masking rather
-  than a compare tree) cannot carry the bank's four bits alongside a space. The
-  cheap half of the idea survived as the direction bit; the rest lost to the
-  selector byte, which is also more extensible.
-- **A generic primitive interpreter** — a loader with no logic at all, driven
-  entirely by the host — is not reachable on AVR. Harvard architecture means the
-  program counter cannot fetch from data space, so the classic "upload a flash
-  algorithm into RAM and jump to it" bootstrap is impossible, and on every
-  boot-sectioned part SPM only takes effect from the boot section anyway. What
-  remains is a fixed primitive set: still a protocol, still logic, only at a
-  different granularity. debugWIRE reaches that design point only because its
-  interpreter is *in silicon*; it costs the loader nothing because it is not in
-  the loader.
-- Below 512 B the saved bytes are largely unspendable on the boot-sectioned
-  chips: the 328P's smallest boot section is exactly 512 B, and the 1284P's is
-  1024 B, of which pureboot already occupies only the top half. The margin
-  matters as headroom for correctness fixes — as this defect showed — not as
-  flash returned to the application. On the patch-vector parts, which have no
-  boot section, it *is* returned: on the ATtiny13 the loader is 43% of a 1 KiB
-  part, and every byte is real.
-
-## The codegen coupling, still load-bearing
-
-`count >> 2` is exact only because the calibration pulse's bit-count (7, from
-the 0xC0 byte) equals the poll loop's cycles per iteration (7 — `sbis` 1,
-`rjmp` 2, `adiw` 2, `rjmp` 2). The loop shape survived every restructuring here,
-verified in the disassembly, but a toolchain bump that reshapes it would break
-the lock silently. `test/pbautobaud.py` is what pins it: a wrong unit fails the
-flash verify.
-
-## Sizes — every chip
-
-Budget 510 B on the patch-vector parts, 512 elsewhere. The 1284P is no longer
-the tight one: the bank nibble made far flash *cheaper* than the near-flash
-arithmetic it replaced.
-
-| size | chips | budget | spare |
-|---|---|---|---|
-| 444 | ATtiny13, 13A | 510 | 66 |
-| 448 | ATmega48, 48A, 48P, 48PA; ATtiny25 | 510 | 62 |
-| 452 | ATtiny45, 85 | 510 | 58 |
-| 460 | ATmega644, 644A, 644P, 644PA | 512 | 52 |
-| 464 | **ATmega1284, 1284P**; ATmega8, 8A, 88, 88A, 88P, 88PA | 512 | 48 |
-| 466 | ATmega16, 16A, 32, 32A; 164A/P/PA, 168/A/P/PA, 324A/P/PA, 328, 328P | 512 | 46 |
-
-All 37 chips build and size-test green, plus the 12-preset reflect spot set
-(guidance rule 4 — the reflect matrix is never run in full), which matches its
-generated counterpart byte for byte on every chip in the set. Worst case across
-the whole set is **466 B, 46 under budget**.
-
-## Host tool and simulation
-
-- **`pureboot.py`** speaks both generations. `Info.version >= 5` selects the
-  unified path; everything below it keeps the four-command protocol, so the
-  fixed-baud loader is untouched. `--autobaud` sends the 0xC0 pulse and one
-  knock, then derives full geometry from the signature. New: `--peek ADDR[:N]`
-  and `--poke ADDR:HEX` reach the data space.
-- **`test/pbautobaud.py`** drives the loader over the GPIO⇄pty software-UART
-  bridge through the calibration handshake, a flash + EEPROM + fuse round-trip
-  cross-checked against the simulator's own memory, a RAM read/write round-trip,
-  and a hand-over to the fixture application — then repeats at double the F_CPU
-  with the same binary, which is the clock-agnostic property autobaud exists
-  for. Run on the near-flash 328P and the word-addressed 1284P.
-- It also **pins the activation hang**: the test sends a lone calibration pulse
-  with no knock behind it and requires the application to boot. Against the
-  unfixed loader that assertion never returns.
-
-## What remains
-
-- **Real-hardware acceptance.** A cycle-exact simulator cannot produce what
-  autobaud exists for: a real RC oscillator at ±10% with drift and jitter.
-  simavr proves the arithmetic and the fit at exact clocks; only silicon proves
-  the feature. Drive an internal-oscillator ATtiny at a fixed host baud and
-  confirm lock plus a full flash and verify.
-- **A generic `spm::command()` in libavr.** The SPM space issues a runtime
-  command through `spm::detail::page_command` where RAMPZ exists, and falls back
-  to a dispatch over the known operations where it does not — the one
-  preprocessor branch in the file. A two-line library addition would make it
-  uniform and save a few bytes on the 36 non-RAMPZ chips, none of which are
-  tight.
-- **Retire or revive the two dead variants.** They are kept only as the record
-  of the measurement; nothing builds them.
-
-## Files
-
-- `pureboot_autobaud_uni.cpp` — the loader. pureboot 5.
-- `pureboot_autobaud_pure.cpp`, `pureboot_autobaud_reg.cpp` — superseded, not
-  built; 530 and 534 B on the 1284P once the activation hang is fixed.
-- `pureboot.py` — `--autobaud`, the unified transfer path, `--peek`/`--poke`.
-- `test/pbautobaud.py` — the end-to-end sim test and the hang regression.
-- `local/scratch/autobaud/floor_1284.S` (libavr checkout) — the hand-asm floor
-  probe at 506 B, off-tree and gitignored; a size reference only. The unified
-  loader is 42 B under it, with features the probe never had.

pureboot/pureboot.cpp

@@ -1,14 +1,17 @@
-// pureboot — a serial bootloader on libavr: one C++ source, no inline
-// assembly, no global register variables, 512 bytes on every chip libavr
-// targets. The device speaks primitives; every composite (verify, erase,
+// pureboot - a serial bootloader on libavr: one C++ source, no inline
+// assembly, no global register variables, a 512-byte slot on every chip
+// libavr targets. The device speaks primitives; every composite (verify, erase,
 // reset-vector surgery, self-update) lives in the host tool. Protocol,
 // deployment and configuration: README.md next to this file.
 //
-// The image is position-independent — PC-relative control flow, wire
-// addresses in, the write guard and the info block both anchored on the
-// runtime return address — so the identical binary runs from any slot. That
-// is what makes a copy one slot below able to rewrite the resident one, and
-// every change here has to keep it (test/check_pi.py).
+// The image is position-independent - PC-relative control flow and wire
+// addresses in, no absolute address formed anywhere - so the identical binary
+// runs from any slot. That is what makes a copy one slot below able to rewrite
+// the resident one, and every change here has to keep it (test/check_pi.py).
+// It does not need to know *which* slot it is in: nothing here refuses an
+// address, so there is no running-slot comparison to anchor.
+
+#include <chrono>
 
 #include <libavr/libavr.hpp>
 
@@ -22,85 +25,176 @@ namespace {
 // Purely polled: every interrupt guard folds to nothing.
 constexpr auto off = avr::irq::guard_policy::unused;
 
+// The EEPROM procedure's step 2 - wait until SPMEN clears - guards a flash
+// operation still in flight, and this loader never has one when it touches
+// the EEPROM: commit() waits every boot-sectioned page operation out before
+// the ack, and everywhere else the CPU halts through the operation itself.
+// The posture states the omission the datasheet grants for exactly that
+// (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
 constexpr std::uint8_t ack = '+';
 
+// The refusal, which is the ack inverted: on a link whose whole problem is
+// flipped bits, the byte saying "nothing happened" should be as far as a byte
+// can be from the one saying "it did", and the complement is all eight bits.
+// It is also the only spelling that needs no justifying - every other value
+// would be a choice.
+constexpr std::uint8_t nak = static_cast<std::uint8_t>(~ack);
+
+// What a command's fields must fold to. Any non-zero constant does: zero is
+// what a run of one repeated byte folds to, and a repeated byte is the shape of
+// both a line stuck at a level and a page of erased flash arriving where a
+// header belongs.
+constexpr std::uint8_t seal = 0x5a;
+
+// The opcode, as bits rather than letters. Each is a one-instruction skip,
+// where a set of arbitrary values costs a compare and a branch apiece - and
+// with the seal deciding what is a command at all, there is nothing left for a
+// readable spelling to buy. A transfer is the absence of the other three, and
+// its direction is the low bit.
+//
+// Identify is bit 5 for one reason: 'p' and 'b' both carry it, and those are
+// the knock. Two things follow that no other assignment gives. A host cannot
+// know which generation it is talking to until something has answered, so the
+// command reporting the version has to mean the same thing before the version
+// is known - 'b' still asks it. And a knock aimed at a loader that is
+// *already* in session has to stay harmless: with no opcode reserved as
+// invalid, every byte now starts a command, so a knock that meant nothing to
+// earlier generations would otherwise consume the five header bytes behind it
+// and put the stream out of step. Answering both knock bytes with the identity
+// keeps the reconnect exactly as cheap as it was.
+enum : std::uint8_t { op_write = 1, op_fill = 4, op_jump = 8, op_identify = 0x20 };
+
 // Deployment parameters come from the build (pureboot_add_loader()). The
 // signature is not one of them: the chip database is the only universal
-// source — a tiny13A cannot read its own signature row from code.
-#if !defined(PUREBOOT_CLOCK_HZ) || !defined(PUREBOOT_BAUD)
+// source - a tiny13A cannot read its own signature row from code. An autobaud
+// build carries no clock and no baud at all; it measures both.
+#if !defined(PUREBOOT_AUTOBAUD) && (!defined(PUREBOOT_CLOCK_HZ) || !defined(PUREBOOT_BAUD))
 #error \
-    "PUREBOOT_CLOCK_HZ and PUREBOOT_BAUD select this build's clock and baud — create loader targets with pureboot_add_loader() (README.md)"
+    "PUREBOOT_CLOCK_HZ and PUREBOOT_BAUD select this build's clock and baud - create loader targets with pureboot_add_loader(), or PUREBOOT_AUTOBAUD for a clock-free one (README.md)"
 #endif
 
+#if !defined(PUREBOOT_AUTOBAUD)
 using dev = avr::device<{.clock = avr::hertz_t{PUREBOOT_CLOCK_HZ}}>;
 constexpr avr::baud_t wire_baud{PUREBOOT_BAUD};
-
-// The watchdog reset flag's home: MCUSR, or the classic megas' MCUCSR.
-consteval std::int16_t wdrf_field()
-{
-	auto reg = std::string_view{avr::hw::db.regs[static_cast<std::size_t>(avr::power::detail::reset_reg())].name};
-	return avr::hw::db.field_index(reg, "WDRF");
-}
+#endif
 
 // The loader owns the top 512 bytes; a staging copy goes in the slot below.
-// Chips without a hardware boot section — the tinies and the m48s, whose SPM
-// runs from anywhere (Atmel-8271 §26) — keep the application's relocated
-// reset vector in the word under the slot.
-constexpr std::uint16_t slot_bytes = 512;
-constexpr std::uint32_t base = spm::flash_bytes - slot_bytes;
+// Chips without a hardware boot section - the tinies and the m48s, whose SPM
+// runs from anywhere (Atmel-8271 section 26) - keep the application's relocated
+// reset vector in the word under the slot. The size itself is the linker's and
+// the host's business: nothing in here needs to know where the slot ends.
 constexpr std::uint16_t page = spm::page_bytes;
 constexpr bool boot_section = avr::hw::curated::has_boot_section();
 
-// Past 64 KiB a byte address no longer fits the wire's 16 bits, so flash
-// addresses there are word addresses ('J' always was one). A slot is 256 of
-// those — one value of a wire address's high byte, where 512 bytes span two.
-constexpr bool word_flash = spm::flash_bytes > 65536;
-constexpr std::uint16_t wire_base =
-    word_flash ? static_cast<std::uint16_t>(base / 2) : static_cast<std::uint16_t>(base);
+// Past 64 KiB one bank of flash does not cover the chip, so a transfer's
+// selector byte carries the bank and the wire address stays a byte address
+// within it. 'J' is the exception: it is a word address everywhere, because
+// that is what the hardware's own jump takes.
+constexpr bool banked_flash = spm::flash_bytes > 65536;
 
 // A compile-time window, so the whole EEPROM belongs to the application;
-// re-timing a deployed loader is a self-update with a re-timed build.
+// re-timing a deployed loader is a self-update with a re-timed build. An
+// autobaud build has no clock to convert seconds against and counts polls.
 #if !defined(PUREBOOT_TIMEOUT)
 #define PUREBOOT_TIMEOUT 8
 #endif
 constexpr std::uint8_t timeout_seconds = PUREBOOT_TIMEOUT;
 
-// The loader's one identity number. The protocol carries none of its own —
-// a version implies it, and the host tool holds that map (README.md).
-constexpr std::uint8_t version = 4;
+#if !defined(PUREBOOT_AUTOBAUD_POLLS)
+#define PUREBOOT_AUTOBAUD_POLLS 4000000
+#endif
+constexpr avr::uint24_t autobaud_budget = PUREBOOT_AUTOBAUD_POLLS;
 
-// The 'b' reply, byte for byte (layout: README.md). Flash-resident because
-// no crt copies a .data image — and flash_table's storage carries the word
-// alignment 'b' needs to halve the address on the large chips.
-// One wire byte per line: this is the reply's layout, not a list.
+// A build may bake a measured oscillator trim (README.md: the RC-oscillator
+// deployment answer); the byte is applied at the top of run(). Orthogonal to
+// the serial backend - an autobaud build may carry it for the application's
+// benefit alone.
+#if defined(PUREBOOT_OSCCAL)
+static_assert(PUREBOOT_OSCCAL >= 0 && PUREBOOT_OSCCAL <= 0xff, "PUREBOOT_OSCCAL is one OSCCAL byte");
+#endif
+
+// The loader's one identity number. The protocol carries none of its own  -
+// a version implies it, and the host tool holds that map (README.md).
+constexpr std::uint8_t version = 9;
+
+// The image's identity stamp, for the host tool rather than for the wire: an
+// update image is a bare 512-byte slot, and without this nothing in it says
+// which chip it was built for. The tool refuses to install an image whose
+// stamp does not match the device - flashing a foreign loader bricks the
+// target, and the loader itself cannot check what has already replaced it.
+//
+// Never read from flash by the loader - 'b' answers out of this array, but at
+// constant indices, so those fold to immediates and no runtime address of it
+// is ever formed. That folding is a correctness property, not a size one: the
+// bytes live in program memory and a formed address would be dereferenced as
+// *data* space, which is why this is the raw array rule 36 otherwise bans - a
+// `std::array` here stops the read loop unrolling and emits exactly that
+// `ld` (measured: +8 B and a wrong answer on the wire). `used` keeps the
+// compiler from dropping the copy the host needs and `retain` keeps
+// --gc-sections from collecting it.
 // clang-format off
-inline constexpr avr::flash_table<std::array<std::uint8_t, 12>{
-    'P',
-    'B',
-    version,
+[[gnu::used, gnu::retain, gnu::section(".text.stamp")]]
+inline constexpr std::uint8_t identity_stamp[]{
+    'P', 'B',                      // the magic the host scans an image for
+    version,                       // and from here on, exactly what 'b' answers
     avr::hw::db.signature[0],
     avr::hw::db.signature[1],
     avr::hw::db.signature[2],
-    static_cast<std::uint8_t>(page),                                           // 0 means 256
-    wire_base & 0xff,
-    wire_base >> 8,
-    avr::hw::db.mem.eeprom_size & 0xff,
-    avr::hw::db.mem.eeprom_size >> 8,
-    static_cast<std::uint8_t>((boot_section ? 0 : 1) | (word_flash ? 2 : 0)),  // patch-vector, word-addressed
-}>
-    info_data;
+};
 // clang-format on
+// Where the identity proper starts: past the magic the host scans for.
+constexpr std::uint8_t stamp_identity = 2;
 
-// The serial link, per the build's PUREBOOT_USART / PUREBOOT_SOFT_SERIAL,
-// defaulting to the chip's USART0 where it has one. The software receiver is
-// the polled one: the vector table belongs to the application. Templates on
-// the clock, so only the selected backend instantiates. pending() is the
-// cheap line test the activation window polls; drain() holds until the last
-// frame is off the wire, so a hand-over cannot let the target's re-init clip
-// the ack.
+// The address spaces a transfer can name, in a selector byte's low nibble.
+// Flash is 0 so it is the cheapest to select.
+//
+// spm_ops is the one that is not memory: naming it hands the count field to
+// SPMCSR and fires the instruction at the transfer's address, which is how
+// page erase, page write and RWW re-enable reach the wire without the loader
+// carrying a command for each. The hardware's four-cycle store-to-SPM window
+// is why this is one fused primitive and not a poke of SPMCSR - no host can
+// hit that window across a serial link.
+//
+// It is the one space with no direction: the opcode's write bit is not
+// consulted, because a sealed command naming this space says what it means and
+// there is nothing for the other direction to denote. Testing the bit anyway
+// would cost six bytes to catch a host contradicting itself, which is the same
+// trade the running-slot guard lost.
+enum : std::uint8_t { sp_flash = 0, sp_eeprom = 1, sp_data = 2, sp_fuse = 3, sp_spm = 4 };
+
+// A selector's high nibble is the flash bank - the address bits above the
+// 16-bit wire address, RAMPZ on the chips that have one. Keeping it here
+// rather than widening the wire address is what lets one 16-bit cursor serve
+// every space: a 24-bit cursor would pay its extra byte on EEPROM and data
+// reads that can never need it.
+[[gnu::always_inline]] inline std::uint8_t space_of(std::uint8_t selector)
+{
+	return selector & 0x0f;
+}
+
+[[gnu::always_inline]] inline std::uint8_t bank_of(std::uint8_t selector)
+{
+	return static_cast<std::uint8_t>(selector >> 4);
+}
+
+// The serial link, per the build's PUREBOOT_USART / PUREBOOT_SOFT_SERIAL /
+// PUREBOOT_AUTOBAUD, defaulting to the chip's USART0 where it has one. The
+// software receiver is the polled one: the vector table belongs to the
+// application. Templates on the clock, so only the selected backend
+// instantiates. pending() is the cheap line test the activation window polls;
+// drain() holds until the last frame is off the wire, so a hand-over cannot
+// let the target's re-init clip the ack.
 #if defined(PUREBOOT_SOFT_SERIAL) && defined(PUREBOOT_USART)
 #error "PUREBOOT_SOFT_SERIAL and PUREBOOT_USART select opposing serial backends"
 #endif
+#if defined(PUREBOOT_AUTOBAUD) && defined(PUREBOOT_USART)
+#error "PUREBOOT_AUTOBAUD measures a software link; it cannot drive a hardware USART"
+#endif
+#if defined(PUREBOOT_HALF_DUPLEX) && (defined(PUREBOOT_SOFT_SERIAL) || defined(PUREBOOT_AUTOBAUD))
+#error "PUREBOOT_HALF_DUPLEX is the hardware USART's one-wire mode; a software link goes one-wire by RX == TX"
+#endif
 #if !defined(PUREBOOT_RX)
 #define PUREBOOT_RX pb0
 #endif
@@ -108,18 +202,61 @@ inline constexpr avr::flash_table<std::array<std::uint8_t, 12>{
 #define PUREBOOT_TX pb1
 #endif
 #if defined(PUREBOOT_USART)
-constexpr char usart_digit = '0' + PUREBOOT_USART;
+constexpr int usart_unit = PUREBOOT_USART;
 #else
-constexpr char usart_digit = '0';
+constexpr int usart_unit = 0;
 #endif
 
-template <avr::hertz_t C>
-struct hardware_link {
-	using uart = avr::uart::usart<usart_digit, C, {.baud = wire_baud, .max_baud_error = 2.5_pct}>;
+// One-wire on the hardware USART (PUREBOOT_HALF_DUPLEX): RXD and TXD tied
+// together off-chip, exactly one direction enabled at a time - the library's
+// .half_duplex turn-around. The activation window is unchanged; only its
+// poll grows the release-line test readable() carries in this mode.
+constexpr bool hw_half_duplex =
+#if defined(PUREBOOT_HALF_DUPLEX)
+    true;
+#else
+    false;
+#endif
 
-	// The compiled idle poll: lds UCSR0A (2), sbrc skipping the exit (2),
-	// sbiw + sbci + sbci + brne (6).
-	static constexpr std::uint8_t poll_cycles = 10;
+template <avr::hertz_t C, avr::baud_t B>
+struct hardware_link {
+	// The deployment envelope is the build's: pureboot_baud_feasible() holds
+	// every configured rate within 2.5 %, which the bench has proven across
+	// the fleet, and the datasheet's stricter per-frame tolerance table would
+	// refuse the stock 115200 at 16 MHz (+2.1 %) that every deployed board
+	// runs. .allow_baud_error states that this is meant.
+	using uart = avr::uart::usart<usart_unit, C,
+	                              {
+	                                  .baud = B,
+	                                  .allow_baud_error = true,
+	                                  .half_duplex = hw_half_duplex,
+	                              }>;
+
+	// The compiled idle poll around the window's narrow (uint24_t) countdown:
+	// the RXC test, then sbiw + sbci + brne (5). The test's cost follows the
+	// status register's home - a 2-cycle bit-skip where UCSRnA sits in
+	// bit-addressable I/O (the classic megas), lds + skip (4) in extended
+	// I/O. Half-duplex polls through readable()'s release-line test, which
+	// -Os outlines: the rcall (3), the UCSR#B read and not-taken skip with
+	// the jump over the write (I/O 3, extended 5), the ret (4) - and the
+	// call in the loop body pushes the countdown into call-saved registers,
+	// where the uint24_t step is ldi+sub+sbc+sbc (4) instead of sbiw+sbci
+	// (3). Measured off the built loops: 18 a poll in bit-addressable I/O,
+	// 22 in extended. A uint32_t countdown pays one more sbci  -
+	// window_polls() adds it where the count forces the wide type. Held per
+	// chip by the pureboot.window gates. The lookup rides the baud parameter
+	// so it stays dependent: the trait is an incomplete type on the
+	// USART-less chips, which parse this template without ever instantiating
+	// it.
+	template <avr::baud_t Baud, typename U = avr::hw::usart_of<usart_unit>>
+	static consteval std::uint8_t poll_cost()
+	{
+		if (hw_half_duplex) {
+			return U::ucsra::addr < 0x40 ? 18 : 22;
+		}
+		return U::ucsra::addr < 0x40 ? 7 : 9;
+	}
+	static constexpr std::uint8_t poll_cycles = poll_cost<B>();
 
 	static void init()
 	{
@@ -128,7 +265,7 @@ struct hardware_link {
 
 	static bool pending()
 	{
-		return uart::rx_ready();
+		return uart::readable();
 	}
 
 	static std::uint8_t rx()
@@ -143,18 +280,26 @@ struct hardware_link {
 
 	static void drain()
 	{
-		uart::drain();
+		// A drain here always follows this link's own write - the frame is
+		// in flight by construction, so the completion the wait needs is
+		// guaranteed and the bounded default's countdown would be dead bytes.
+		uart::drain_unbounded();
 	}
 };
 
-template <avr::hertz_t C>
+template <avr::hertz_t C, avr::baud_t B>
 struct software_link {
-	using rx_t = avr::uart::software_rx_polled<C, avr::PUREBOOT_RX, wire_baud>;
-	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, wire_baud>;
+	// RX == TX is the one-wire deployment: the transmitter becomes a guest
+	// on the receiver's pull-up line, taking the pin's direction for exactly
+	// one frame per byte.
+	using rx_t = avr::uart::software_rx_polled<C, avr::PUREBOOT_RX, B>;
+	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, B, avr::PUREBOOT_RX == avr::PUREBOOT_TX>;
 
-	// The compiled idle poll: sbis skipping the exit (2), sbiw + sbci +
-	// sbci + brne (6).
-	static constexpr std::uint8_t poll_cycles = 8;
+	// The compiled idle poll around the window's narrow (uint24_t) countdown:
+	// sbis skipping the exit (2), sbiw + sbci + brne (5). A uint32_t
+	// countdown pays one more sbci - window_polls() adds it where the count
+	// forces the wide type. Held by the pureboot.window gate.
+	static constexpr std::uint8_t poll_cycles = 7;
 
 	static void init()
 	{
@@ -182,18 +327,54 @@ struct software_link {
 	}
 };
 
-#if defined(PUREBOOT_USART)
-static_assert(avr::uart::has_usart<usart_digit>(), "PUREBOOT_USART selects a hardware USART this chip does not have");
-using link = hardware_link<dev::clock>;
+// The clock-free link: the bit period is measured from the host's calibration
+// pulse instead of derived from a clock, so one image serves every F_CPU and
+// every rate. Activation differs in kind from the other two - there is no
+// clock to time a window against - so this backend brings its own, below.
+struct autobaud_link {
+	// The unit in GPIOR2:GPIOR1 where the chip has them: the loader owns the
+	// whole chip while it runs, and the pair costs one word per access where
+	// the RAM word costs two - six words across the image.
+	using uart = avr::uart::software_autobaud<avr::PUREBOOT_RX, avr::PUREBOOT_TX, avr::uart::unit_home::gpior>;
+
+	static void init()
+	{
+		avr::init<uart>();
+	}
+
+	static std::uint8_t rx()
+	{
+		return uart::template read<off>();
+	}
+
+	static void tx(std::uint8_t byte)
+	{
+		uart::template write<off>(byte);
+	}
+
+	static void drain()
+	{
+		// A drain here always follows this link's own write - the frame is
+		// in flight by construction, so the completion the wait needs is
+		// guaranteed and the bounded default's countdown would be dead bytes.
+		uart::drain_unbounded();
+	}
+};
+
+#if defined(PUREBOOT_AUTOBAUD)
+using link = autobaud_link;
+#elif defined(PUREBOOT_USART)
+static_assert(avr::uart::has_usart<usart_unit>(), "PUREBOOT_USART selects a hardware USART this chip does not have");
+using link = hardware_link<dev::clock, wire_baud>;
 #elif defined(PUREBOOT_SOFT_SERIAL)
-using link = software_link<dev::clock>;
+using link = software_link<dev::clock, wire_baud>;
 #else
-using link =
-    std::conditional_t<avr::uart::has_usart<usart_digit>(), hardware_link<dev::clock>, software_link<dev::clock>>;
+using link = std::conditional_t<avr::uart::has_usart<usart_unit>(), hardware_link<dev::clock, wire_baud>,
+                                software_link<dev::clock, wire_baud>>;
 #endif
 
 // The application's entry, pinned by the linker (--defsym): word 0 on a
-// boot-sectioned mega, the trampoline at base − 2 elsewhere. Reaching it must
+// boot-sectioned mega, the trampoline at base - 2 elsewhere. Reaching it must
 // not depend on where this copy runs, so the jump goes through a pointer, and
 // [[gnu::noipa]] keeps the constant from folding back into a relative call.
 extern "C" [[noreturn]] void pureboot_app();
@@ -204,24 +385,71 @@ extern "C" [[noreturn]] void pureboot_app();
 	__builtin_unreachable();
 }
 
-[[gnu::noinline, noreturn]] void run_app()
+[[noreturn]] void run_app()
 {
 	jump(pureboot_app);
 }
 
-// The window as one 32-bit countdown, divided by the backend's counted
-// poll-loop cycles. Whole seconds is all it promises.
+// Activation: a bounded wait for the host, then the knock. Both forms boot the
+// application when the window closes on an idle line, and both bound *every*
+// wait - a knock awaited without a deadline would let one stray edge hold an
+// unattended device in the loader forever.
+#if defined(PUREBOOT_AUTOBAUD)
+// The window is a fixed poll budget: with no clock, whole seconds cannot be
+// timed. A uint24_t holds it - a fourth byte would cost two words at every
+// countdown step for range never used.
+void await_host()
+{
+	for (;;) {
+		if (!link::uart::calibrate(autobaud_budget)) {
+			run_app();
+		}
+		// The calibration pulse has already proven a host is there, so one
+		// byte activates. A knock that never arrives falls back to calibrate(),
+		// whose own budget then boots the application.
+		if (link::uart::template read<off>(autobaud_budget) == 'p') {
+			return;
+		}
+	}
+}
+#else
+// The window as one countdown, divided by the backend's counted poll-loop
+// cycles. Whole seconds is all it promises. The per-poll cost depends on the
+// countdown's own width (a uint32_t decrement chain is one sbci longer), and
+// the width depends on the poll count - solved narrow-first: a count that
+// fits 24 bits at the narrow cost keeps the narrow loop, anything else takes
+// the wide loop at its own cost. A count fitting 24 bits only at the wide
+// cost stays wide, so the choice cannot oscillate on the boundary.
+consteval std::uint32_t polls_at(std::uint32_t per_poll)
+{
+	// Whole-window cycles first, then the per-poll division: one truncation
+	// instead of one per second. Same instructions either way - only the
+	// countdown's immediate moves.
+	return static_cast<std::uint32_t>(dev::cycles_for<std::chrono::seconds{timeout_seconds}>() / per_poll);
+}
+
+consteval bool narrow_window()
+{
+	return polls_at(link::poll_cycles) <= 0xffffff;
+}
+
 consteval std::uint32_t window_polls()
 {
-	return timeout_seconds * static_cast<std::uint32_t>(dev::clock.hz / link::poll_cycles);
+	return polls_at(narrow_window() ? link::poll_cycles : link::poll_cycles + 1u);
 }
 
+// The countdown in the narrowest type that holds it: a fourth byte would
+// cost a wider decrement chain at every poll for range most windows never
+// use (the autobaud budget makes the same choice).
+using window_t = std::conditional_t<narrow_window(), avr::uint24_t, std::uint32_t>;
+
 bool pending_before_deadline()
 {
-	std::uint32_t polls = window_polls();
+	window_t polls = window_polls();
 	do {
-		if (link::pending())
+		if (link::pending()) {
 			return true;
+		}
 	} while (--polls);
 	return false;
 }
@@ -230,11 +458,20 @@ bool pending_before_deadline()
 // application runs.
 std::uint8_t rx_deadline()
 {
-	if (!pending_before_deadline())
+	if (!pending_before_deadline()) {
 		run_app();
+	}
 	return link::rx();
 }
 
+void await_host()
+{
+	// 'p' then 'b', each under a fresh window; anything else is line noise.
+	while (rx_deadline() != 'p' || rx_deadline() != 'b') {
+	}
+}
+#endif
+
 // Inlined: read across a call, the first byte strands in a call-saved
 // register the caller has to push and pop.
 [[gnu::always_inline]] inline std::uint16_t rx16()
@@ -243,7 +480,7 @@ std::uint8_t rx_deadline()
 	return static_cast<std::uint16_t>(low | (link::rx() << 8));
 }
 
-// The wire's byte pair as the word it is — AVR is little-endian too, so the
+// The wire's byte pair as the word it is - AVR is little-endian too, so the
 // cast is the identity a shift-and-or spelling makes the compiler rediscover.
 // Callers read into named variables first: the wire order is a sequence of
 // reads, not an argument order.
@@ -252,213 +489,227 @@ std::uint8_t rx_deadline()
 	return std::bit_cast<std::uint16_t>(pair);
 }
 
-// Counts arrive in the wire's 8-bit form: 0 means 256. Both streamers fold
-// into the one command that reads flash, which is what lets the far one's
-// 24-bit cursor sit in the command loop's own call-saved registers.
-[[maybe_unused, gnu::always_inline]] inline void send_flash_near(std::uint16_t address, std::uint8_t count)
-{
-	do
-		link::tx(avr::flash_load(reinterpret_cast<const std::uint8_t *>(address++)));
-	while (--count);
-}
-
-// The 24-bit cursor as the machine holds it — the RAMPZ byte and a 16-bit Z,
-// carried apart; the reassembled address folds away inside the far load.
-[[maybe_unused, gnu::always_inline]] inline void send_flash_far(std::uint16_t address, std::uint8_t count)
-{
-	std::uint8_t rampz = static_cast<std::uint8_t>(address >> 15);
-	std::uint16_t z = static_cast<std::uint16_t>(address << 1);
-	do {
-		link::tx(avr::flash_load_far<std::uint8_t>((static_cast<std::uint32_t>(rampz) << 16) | z));
-		// Carrying the wrap is smaller than the flat 32-bit cursor GCC
-		// builds without it.
-		if (++z == 0)
-			++rampz;
-	} while (--count);
-}
-
-[[gnu::always_inline]] inline void send_flash(std::uint16_t address, std::uint8_t count)
-{
-	if constexpr (word_flash)
-		send_flash_far(address, count);
-	else
-		send_flash_near(address, count);
-}
-
-// Out of line: three sites send it, and a call is shorter than three
-// load-immediates.
+// Out of line: several sites send it, and a call is shorter than a
+// load-immediate at each.
 [[gnu::noinline]] void tx_ack()
 {
 	link::tx(ack);
 }
 
-void send_eeprom(std::uint16_t address, std::uint8_t count)
+// A wire address and its selector's bank as the flash address they name.
+[[gnu::always_inline]] inline spm::flash_address_t flash_address([[maybe_unused]] std::uint8_t bank, std::uint16_t at)
 {
-	do
-		link::tx(ee::read(address++));
-	while (--count);
+	if constexpr (banked_flash) {
+		return (static_cast<spm::flash_address_t>(bank) << 16) | at;
+	} else {
+		return at;
+	}
 }
 
-// Host-paced: the ack goes out once the write has begun, so the next byte
-// arrives while it completes and nothing is missed without a buffer.
-void store_eeprom(std::uint16_t address, std::uint8_t count)
+// One byte out of any space. Every accessor shares the transfer's cursor, its
+// loop and its call site, so a space costs only its own instruction rather
+// than a body, a loop and a dispatch arm of its own.
+[[gnu::always_inline]] inline std::uint8_t load(std::uint8_t space, [[maybe_unused]] std::uint8_t bank,
+                                                std::uint16_t at)
 {
-	do {
-		ee::write<off>(address++, link::rx());
-		tx_ack();
-	} while (--count);
+	if (space == sp_eeprom) {
+		return ee::read<no_spm>(at);
+	}
+	if (space == sp_data) {
+		return *reinterpret_cast<volatile std::uint8_t *>(at);
+	}
+	if (space == sp_fuse) {
+		return spm::read_fuse<off>(static_cast<spm::fuse>(at));
+	}
+	if constexpr (banked_flash) {
+		return avr::flash_load_far<std::uint8_t>(flash_address(bank, at));
+	} else {
+		return avr::flash_load(reinterpret_cast<const std::uint8_t *>(at));
+	}
 }
 
-// One page into the SPM buffer, then erase and program — except the slot
-// this code is running in (`slot_high`, from run()), which is drained and
-// left alone. A broken host therefore cannot brick the running loader, and a
-// copy one slot lower may rewrite the resident one.
+// One byte into a writable space. Flash is not one of them - it arrives a
+// page at a time through 'W' and is committed by the sealed SPM command - and
+// the fuses are not writable at all: SPM reaches flash and boot lock bits only.
+[[gnu::always_inline]] inline void store(std::uint8_t space, std::uint16_t at, std::uint8_t value)
+{
+	if (space == sp_data) {
+		*reinterpret_cast<volatile std::uint8_t *>(at) = value;
+		return;
+	}
+	// Host-paced: the ack goes out once the write has begun, so the next byte
+	// arrives while it completes and nothing is missed without a buffer.
+	ee::write<off, no_spm>(at, value);
+}
+
+// The irreversible half of the protocol, and the whole of it: page erase, page
+// write and the lock bits are one SPM command each, and nothing else the loader
+// does outlasts being done again. Reached only from a sealed command (run()),
+// so both the byte handed to SPMCSR and the address it fires at are the ones
+// the host computed its seal over.
 //
-// Nothing discards the buffer first: it is write-once per word (§26.2.1), so
+// Nothing here refuses an address. A loader that will not write its own slot
+// cannot plant anything in it either, and a resident copy able to rewrite its
+// own trailing page is what lets a 512-byte boot section - where no staging
+// copy can run SPM at all - carry an SPM primitive for an application-side
+// installer to drive. The protection that made the guard look necessary is the
+// seal: a wire fault can no longer name an address, only a host can, and a host
+// that names this one means it.
+[[gnu::always_inline]] inline void commit(std::uint8_t bank, std::uint16_t at, std::uint8_t value)
+{
+	spm::command<off>(value, flash_address(bank, at));
+	// Only a boot-sectioned mega runs on while its RWW section programs;
+	// everywhere else the CPU halts through erase and write, so the wait
+	// is already over by the time it returns.
+	if constexpr (boot_section) {
+		spm::wait();
+	}
+}
+
+// One page into the SPM buffer, and only that: the erase and the write that
+// commit it are host-issued sp_spm stores, which reach the same fused
+// store-and-SPM pair through the transfer path's own address and data.
+//
+// Nothing discards the buffer first: it is write-once per word (section 26.2.1), so
 // filling over a refused page or an application's leavings programs stale
-// words — but a page write auto-erases it (§26.2.1; §19.2 on the tinies), so
+// words - but a page write auto-erases it (section 26.2.1; section 19.2 on the tinies), so
 // that write clears the condition and the host's read-back rewrites the page.
-void program_flash(std::uint16_t wire_address, std::uint8_t slot_high)
+void fill_page(std::uint8_t bank, std::uint16_t at)
 {
 	// The address names a page, so its in-page bits are dropped and the walk
-	// starts at the page base — one induction either way: a byte-addressed
-	// wire address walks the page itself (the offset bits wrap back to zero),
-	// while a word one becomes a byte cursor once. The slot index is the wire
-	// address's high byte — on byte-addressed chips the byte address's, with
-	// the low bit dropped, since a slot is two of those.
-	spm::flash_address_t address;
-	std::uint8_t page_high;
-	if constexpr (word_flash) {
-		// A page is aligned, so it never crosses 64 KiB: RAMPZ is a per-page
-		// constant and the 16-bit Z's low byte is the whole in-page offset.
-		const std::uint8_t rampz = static_cast<std::uint8_t>(wire_address >> 15);
-		const std::uint16_t z0 = static_cast<std::uint16_t>(wire_address << 1) & ~static_cast<std::uint16_t>(page - 1);
-		std::uint16_t z = z0;
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>((static_cast<spm::flash_address_t>(rampz) << 16) | z, word_of({low, high}));
-			z += 2;
-		} while (static_cast<std::uint8_t>(z));
-		address = (static_cast<spm::flash_address_t>(rampz) << 16) | z0;
-		page_high = static_cast<std::uint8_t>(wire_address >> 8);
-	} else {
-		address = static_cast<spm::flash_address_t>(wire_address & ~static_cast<std::uint16_t>(page - 1));
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>(address, word_of({low, high}));
-			address += 2;
-		} while (static_cast<std::uint8_t>(address) & (page - 1));
-		address -= 2; // back inside the page — erase and write ignore the word bits
-		page_high = static_cast<std::uint8_t>(address >> 8) & 0xfe;
-	}
-	if (page_high != slot_high) {
-		// Only a boot-sectioned mega runs on while its RWW section programs;
-		// everywhere else the CPU halts through erase and write.
-		spm::erase_page<off>(address);
-		if constexpr (boot_section)
-			spm::wait();
-		spm::write_page<off>(address);
-		if constexpr (boot_section)
-			spm::wait();
-	}
-	// Programming leaves the RWW section disabled; reads need it back on. The
-	// same store discards the buffer (§26.2.2), so a boot-sectioned mega never
-	// meets the stale-word case above.
-	if constexpr (boot_section)
-		spm::rww_enable<off>();
-}
-
-// The four fuse and lock bytes in the hardware's own Z order: low, lock,
-// extended, high.
-void send_fuses()
-{
-	std::uint8_t which = 0;
-	do
-		link::tx(spm::read_fuse<off>(static_cast<spm::fuse>(which)));
-	while (++which != 4);
+	// starts at the page base; the low byte of the cursor is the whole in-page
+	// offset, since a page is aligned and never crosses a bank.
+	std::uint16_t z = at & ~static_cast<std::uint16_t>(page - 1);
+	// The receipt is the erase-first contract's token and costs nothing here:
+	// the erase is the host's own sealed SPM command, before or after the fill
+	// as it chooses (the page write clears a stale buffer either way, above).
+	const auto open = spm::page::begin<spm::from::boot_section, off>(flash_address(bank, z));
+	do {
+		std::uint8_t low = link::rx();
+		std::uint8_t high = link::rx();
+		spm::fill<off>(open, flash_address(bank, z), word_of({low, high}));
+		z += 2;
+	} while (static_cast<std::uint8_t>(z) & (page - 1));
 }
 
 [[noreturn]] void run()
 {
+#if defined(PUREBOOT_OSCCAL)
+	// The build's oscillator trim, ahead of everything - the WDRF bail
+	// included - so every path out of reset, the watchdog hand-over to the
+	// application first among them, runs on the corrected clock.
+	avr::clock::calibrate(PUREBOOT_OSCCAL);
+#endif
 	// A watchdog reset belongs to the application, whose watchdog stays forced
-	// on until it clears WDRF — no activation window in its way.
-	if (avr::hw::field_impl<wdrf_field()>::test())
+	// on until it clears WDRF - no activation window in its way.
+	if (avr::power::peek_reset_cause().watchdog) {
 		run_app();
+	}
 
 	link::init();
-
-	// The high byte of the slot this copy runs at, which the write guard and
-	// the info block both follow: the return address is a word address, so its
-	// high byte is the 256-word slot index, doubled back into byte terms where
-	// the wire counts bytes. Taken as byteswap's low byte — the builtin already
-	// swaps the two stacked bytes, and the double swap folds away, where `>> 8`
-	// would leave the swap materialized.
-	const std::uint16_t ra_words = reinterpret_cast<std::uint16_t>(__builtin_return_address(0));
-	const std::uint8_t ra_high = static_cast<std::uint8_t>(std::byteswap(ra_words));
-	const std::uint8_t slot_high = word_flash ? ra_high : static_cast<std::uint8_t>(ra_high << 1);
-
-	// 'p' then 'b', each under a fresh window; anything else is line noise.
-	while (rx_deadline() != 'p' || rx_deadline() != 'b') {
-	}
+	await_host();
 
 	for (;;) {
 		// No prompt while an EEPROM write runs: it blocks SPM and fuse reads
-		// (§26.2.1), and the prompt is the previous command's completion ack.
-		ee::wait();
+		// (section 26.2.1), and the prompt is the previous command's completion ack.
+		ee::wait<no_spm>();
 		tx_ack();
 		const std::uint8_t command = link::rx();
-		switch (command) {
-		case 'J': { // jump to a wire word address: hand-over and staging transfer
-			auto target = reinterpret_cast<void (*)()>(rx16());
-			tx_ack();
-			link::drain();
-			jump(target);
-		}
-		case 'b':   // info block, read relative to the running slot
-		case 'R':   // read flash: addr16, n8 (0 = 256)
-		case 'r':   // read EEPROM: addr16, n8
-		case 'w': { // write EEPROM: addr16, n8, then n bytes each acked
-			// One address-and-count path for all four: 'b' is a flash read
-			// whose arguments the loader already knows, so it joins the
-			// wire-argument three rather than streaming from a call site of its
-			// own. That leaves one flash streamer in the image, and lets its
-			// cursor live in this never-returning loop's own call-saved
-			// registers instead of being saved and restored around a call.
-			std::uint16_t address;
-			std::uint8_t count;
-			if (command == 'b') {
-				// The block sits in the image's first 256 bytes (check_pi.py
-				// asserts it) and slots are 512-aligned, so the low byte of its
-				// link address is its offset in any slot — halved where wire
-				// units are words. The high byte is runtime data, so no
-				// absolute address is ever materialized.
-				const auto link_byte =
-				    static_cast<std::uint8_t>(reinterpret_cast<std::uint16_t>(info_data.storage.data()));
-				const std::uint8_t low = word_flash ? static_cast<std::uint8_t>(link_byte >> 1) : link_byte;
-				address = static_cast<std::uint16_t>(low | (slot_high << 8));
-				count = static_cast<std::uint8_t>(info_data.size());
-			} else {
-				address = rx16();
-				count = link::rx();
+		if (command & op_identify) {
+			// Identity: the version, then the three signature bytes. Straight
+			// out of the stamp, so the wire and the image can never disagree
+			// about what this loader is. The indices are constant and the array
+			// is constexpr, so these are immediates, not flash reads: nothing
+			// here needs the stamp's runtime address. Unsealed, because it
+			// takes no argument and changes nothing - and because a command
+			// that cannot be got wrong is what a lost host resynchronises on.
+			for (std::uint8_t at = stamp_identity; at != sizeof identity_stamp; ++at) {
+				link::tx(identity_stamp[at]);
+			}
+		} else {
+			// One decode, one cursor and one loop for every space, both
+			// directions and the jump: a command per memory would carry a copy
+			// of all three each. The jump - the hand-over and staging transfer
+			// - carries a selector it ignores so its address rides the same two
+			// reads as everything else; the page fill joins the same decode
+			// rather than keeping an address form of its own, so flash
+			// addressing is uniform across every command that names it. Both
+			// carry the count they do not use for the same reason: one header
+			// shape is one decode, and one seal covers a fixed set of bytes.
+			const std::uint8_t selector = link::rx();
+			const std::uint8_t space = space_of(selector);
+			const std::uint8_t bank = bank_of(selector);
+			std::uint16_t at = rx16();
+			std::uint8_t count = link::rx();
+			const std::uint8_t sealed = link::rx();
+
+			// The seal: every field that decides what this command does folded
+			// into one byte the host chose, tested before any of it happens.
+			//
+			// Checked here rather than acknowledged afterwards, which is the
+			// whole point. An ack reports a command that has already run, and
+			// for the one command that cannot be taken back a report is not a
+			// defence. Once the running-slot guard is gone the address is as
+			// fatal as the command byte - a wrong one reaches the loader's own
+			// page - so the seal covers the act and the place together, and a
+			// stream that lost or mangled either cannot produce it.
+			//
+			// Folded here, after the last read, and never accumulated across
+			// the reads: every field is still live at this point because the
+			// command needs it anyway, so the fold costs one xor each and no
+			// register. An accumulator would have to survive four calls, and
+			// paying for that in call-saved registers costs more than the whole
+			// check costs in arithmetic - measured at fourteen bytes, on a
+			// budget of ten.
+			std::uint8_t fold = command;
+			fold ^= selector;
+			fold ^= static_cast<std::uint8_t>(at);
+			fold ^= static_cast<std::uint8_t>(at >> 8);
+			fold ^= count;
+			fold ^= sealed;
+			// The verdict, and it is not a courtesy. Every command whose
+			// payload the host sends without waiting - a page fill, a write
+			// burst - would otherwise be handed to a loader that has already
+			// gone back to reading commands, so a *detected* error would
+			// become the desync the seal exists to prevent: a 128-byte page
+			// read as command headers is twenty-one more chances at the one in
+			// two hundred and fifty-six. Answering the seal before the payload
+			// is what keeps a refusal local to the command that earned it.
+			//
+			// An unknown opcode lands here too - every bit pattern is now some
+			// command, so it is the seal, not a table of valid letters, that
+			// rejects noise, and the host hears about it either way.
+			if (fold != seal) {
+				link::tx(nak);
+			} else {
+				tx_ack();
+				if (command & op_jump) {
+					link::drain();
+					jump(reinterpret_cast<void (*)()>(at));
+				} else if (command & op_fill) {
+					fill_page(bank, at);
+				} else if (space == sp_spm) {
+					// An SPM command is the whole of what this loader can do
+					// that doing again will not undo, and it is one byte - so
+					// it rides the count field, inside the seal, rather than
+					// arriving as data after the seal has been checked. Which
+					// is also what makes a deliberate lock-bit write
+					// expressible, where refusing it outright did not.
+					commit(bank, at, count);
+				} else {
+					do {
+						// Direction is one bit of the opcode, so the loop
+						// picks it with a one-word skip.
+						if (command & op_write) {
+							store(space, at, link::rx());
+							tx_ack();
+						} else {
+							link::tx(load(space, bank, at));
+						}
+						++at;
+					} while (--count);
+				}
 			}
-			if (command == 'r')
-				send_eeprom(address, count);
-			else if (command == 'w')
-				store_eeprom(address, count);
-			else
-				send_flash(address, count);
-			break;
-		}
-		case 'W': // program one flash page: addr16, page bytes
-			program_flash(rx16(), slot_high);
-			break;
-		case 'F': // fuse and lock bytes
-			send_fuses();
-			break;
-		default: // unknown bytes are ignored; the loop re-acks
-			break;
 		}
 	}
 }
@@ -466,4 +717,7 @@ void send_fuses()
 } // namespace
 } // namespace pureboot
 
-template struct avr::startup::entry<pureboot::run>;
+// stack::hardware: activation is reset-only, so the reset logic's own
+// SP = RAMEND stands wherever the datasheet guarantees it (the classic
+// megas still get the write); a 'J' entry runs on the caller's live stack.
+template struct avr::startup::entry<pureboot::run, avr::startup::stack::hardware>;

pureboot/pureboot_autobaud_pure.cpp

@@ -1,396 +0,0 @@
-// SUPERSEDED — kept for the record, not built. With the activation hang fixed
-// (a lone calibration pulse used to wedge the loader, see autobaud.md) this
-// version is 530 B on the 1284P against a 512 B slot. Its 4 B of margin was
-// never spare capacity; it was the space the missing fix should have occupied.
-// pureboot_autobaud_uni.cpp replaces it at 464 B with more features.
-//
-// pureboot autobaud — the software-serial variant that measures the host's bit
-// timing at runtime, so one binary runs at any F_CPU: the image carries no
-// clock. THE PURE VERSION — no inline assembly, no global register variables,
-// exactly the constraints the fixed-baud loader keeps.
-//
-// Two source files exist for review (autobaud.md):
-// this one, pure, and pureboot_autobaud_reg.cpp, which keeps the running-slot
-// write guard at the cost of one global register variable. They differ only in
-// where the measured unit lives and whether the guard is present.
-//
-// What this version trades to fit 512 B in pure C++ (the 1284 at 508), each
-// licensed by "the host guarantees safety" (README.md) and the owner's approval
-// to simplify the info block:
-//   - the measured per-bit unit lives in the two general-purpose I/O scratch
-//     registers (GPIOR) where the chip has them, in a static otherwise —
-//     reached through libavr's named register surface, so no asm and no global
-//     register variable; the loader stays pure;
-//   - the info block is slimmed to the version and the signature — the chip's
-//     identity — from which the host derives page size, loader base, EEPROM
-//     size and the addressing flags via its own chip database;
-//   - no running-slot write guard: the host never programs the loader's own
-//     slot, and a broken host bricking the target is the host's bug;
-//   - a single-byte activation knock: the calibration pulse already proves a
-//     host is present.
-//
-// Position independence is kept and is in fact total here: control flow is
-// PC-relative, the wire carries addresses, and with the slimmed info block and
-// no write guard nothing anchors on the runtime address at all.
-
-#include <libavr/libavr.hpp>
-
-#include <util/delay_basic.h>
-
-using namespace avr::literals;
-namespace spm = avr::spm;
-namespace ee = avr::eeprom;
-namespace hw = avr::hw;
-
-namespace pureboot {
-namespace {
-
-// Purely polled: every interrupt guard folds to nothing.
-constexpr auto off = avr::irq::guard_policy::unused;
-
-constexpr std::uint8_t ack = '+';
-
-// Autobaud carries no clock, so PUREBOOT_CLOCK_HZ / PUREBOOT_BAUD are not
-// consulted; only the software-UART pins are a deployment parameter.
-#if !defined(PUREBOOT_RX)
-#define PUREBOOT_RX pb0
-#endif
-#if !defined(PUREBOOT_TX)
-#define PUREBOOT_TX pb1
-#endif
-
-// The watchdog reset flag's home: MCUSR, or the classic megas' MCUCSR.
-consteval std::int16_t wdrf_field()
-{
-	auto reg = std::string_view{hw::db.regs[static_cast<std::size_t>(avr::power::detail::reset_reg())].name};
-	return hw::db.field_index(reg, "WDRF");
-}
-
-// The loader owns the top 512 bytes; a staging copy goes in the slot below.
-constexpr std::uint16_t slot_bytes = 512;
-constexpr std::uint32_t base = spm::flash_bytes - slot_bytes;
-constexpr std::uint16_t page = spm::page_bytes;
-constexpr bool boot_section = hw::curated::has_boot_section();
-
-// Past 64 KiB a byte address no longer fits the wire's 16 bits, so flash
-// addresses there are word addresses ('J' always was one).
-constexpr bool word_flash = spm::flash_bytes > 65536;
-
-// The loader's one identity number (README.md); the slimmed info block carries
-// it and the signature, and the host maps a version to its protocol.
-constexpr std::uint8_t version = 4;
-
-// The activation window as a fixed poll budget: with no clock, whole seconds
-// cannot be timed. A __uint24 (AVR's three-byte type) holds it — a fourth byte
-// would cost two words at each countdown step for range never used.
-#if !defined(PUREBOOT_AUTOBAUD_POLLS)
-#define PUREBOOT_AUTOBAUD_POLLS 4000000
-#endif
-constexpr __uint24 autobaud_budget = PUREBOOT_AUTOBAUD_POLLS;
-
-// The measured per-bit delay (in _delay_loop_2 four-cycle iterations). It lives
-// in the two adjacent general-purpose I/O scratch registers (GPIOR1:GPIOR2)
-// where the chip has them — in/out reach them in one word where a static's
-// lds/sts take two, and there is no .bss to clear — and in a plain static
-// otherwise (the t13, m8 and m16/32 have no GPIOR). Both are pure: the named
-// register surface, no inline asm, no global register variable.
-constexpr bool have_gpior = hw::db.reg_index("GPIOR1") >= 0 && hw::db.reg_index("GPIOR2") >= 0;
-
-std::uint16_t unit_backing;
-
-template <bool Gpior = have_gpior>
-[[gnu::always_inline]] inline std::uint16_t get_unit()
-{
-	if constexpr (Gpior)
-		return static_cast<std::uint16_t>(hw::reg_impl<hw::db.reg_index("GPIOR1")>::read() |
-		                                  (hw::reg_impl<hw::db.reg_index("GPIOR2")>::read() << 8));
-	else
-		return unit_backing;
-}
-
-template <bool Gpior = have_gpior>
-[[gnu::always_inline]] inline void put_unit(std::uint16_t u)
-{
-	if constexpr (Gpior) {
-		hw::reg_impl<hw::db.reg_index("GPIOR1")>::write(static_cast<std::uint8_t>(u));
-		hw::reg_impl<hw::db.reg_index("GPIOR2")>::write(static_cast<std::uint8_t>(u >> 8));
-	} else
-		unit_backing = u;
-}
-
-// The autobaud software link: bit-banged with cycle-counted delays like the
-// fixed-baud software backend, but the per-bit delay is the measured unit, not
-// a consteval constant. rx/tx load the unit once into a local, so each bit
-// spins from a register with no per-bit reload.
-struct link {
-	using in_t = avr::io::input<avr::PUREBOOT_RX, avr::io::pull::up>;
-	using out_t = avr::io::output<avr::PUREBOOT_TX>;
-
-	static void init()
-	{
-		avr::init<in_t, out_t>();
-		out_t::set(); // idle high
-	}
-
-	// Time the calibration pulse into the unit. The host sends 0xC0 — a start
-	// bit plus six zero data bits are one low pulse of seven bit-times — and the
-	// counted poll loop is seven cycles an iteration, so the count is the pulse
-	// length in cycles ÷ 7 × 7 = one bit period in cycles, and count >> 2 is that
-	// period in _delay_loop_2's four-cycle iterations. Waits for the start edge
-	// under the poll budget; 0 (returned, and stored) means the budget expired.
-	static std::uint16_t measure(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		std::uint16_t count = 0;
-		while (!in_t::read())
-			++count;
-		const std::uint16_t u = static_cast<std::uint16_t>(count >> 2);
-		put_unit(u);
-		return u;
-	}
-
-	// The eight data bits, entered just past the falling edge of the start bit.
-	// Split out of rx() so the activation path can wait for that edge under a
-	// budget while the command loop waits for it indefinitely.
-	static std::uint8_t sample()
-	{
-		const std::uint16_t unit = get_unit();
-		_delay_loop_2(static_cast<std::uint16_t>(unit + (unit >> 1))); // 1.5 bits to the LSB centre
-		std::uint8_t value = 0;
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			value >>= 1;
-			if (in_t::read())
-				value |= 0x80;
-			_delay_loop_2(unit);
-		}
-		return value;
-	}
-
-	static std::uint8_t rx()
-	{
-		while (in_t::read()) // await the start edge
-			;
-		return sample();
-	}
-
-	// A byte under the poll budget, for the activation knock. An expired budget
-	// returns 0, which is not the knock, so the caller falls back into the
-	// budgeted measure() — and a line that stays idle boots the application
-	// there. Without this the knock's edge wait was unbounded, so a single
-	// spurious calibration pulse (EMI, or a host that opens the port and never
-	// knocks) wedged the loader and the application never ran.
-	static std::uint8_t rx_bounded(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		return sample();
-	}
-
-	static void tx(std::uint8_t value)
-	{
-		const std::uint16_t unit = get_unit();
-		out_t::clear(); // start bit
-		_delay_loop_2(unit);
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			out_t::write(value & 1);
-			value >>= 1;
-			_delay_loop_2(unit);
-		}
-		out_t::set(); // stop bit
-		_delay_loop_2(unit);
-	}
-
-	static void drain()
-	{
-		// The software transmitter returns only after the stop bit.
-	}
-};
-
-extern "C" [[noreturn]] void pureboot_app();
-
-[[gnu::noipa, noreturn]] void jump(void (*target)())
-{
-	target();
-	__builtin_unreachable();
-}
-
-[[gnu::noinline, noreturn]] void run_app()
-{
-	jump(pureboot_app);
-}
-
-// Inlined: read across a call, the first byte strands in a call-saved register
-// the caller has to push and pop.
-[[gnu::always_inline]] inline std::uint16_t rx16()
-{
-	std::uint16_t low = link::rx();
-	return static_cast<std::uint16_t>(low | (link::rx() << 8));
-}
-
-[[gnu::always_inline]] inline std::uint16_t word_of(std::array<std::uint8_t, 2> pair)
-{
-	return std::bit_cast<std::uint16_t>(pair);
-}
-
-[[maybe_unused, gnu::always_inline]] inline void send_flash_near(std::uint16_t address, std::uint8_t count)
-{
-	do
-		link::tx(avr::flash_load(reinterpret_cast<const std::uint8_t *>(address++)));
-	while (--count);
-}
-
-[[maybe_unused, gnu::always_inline]] inline void send_flash_far(std::uint16_t address, std::uint8_t count)
-{
-	std::uint8_t rampz = static_cast<std::uint8_t>(address >> 15);
-	std::uint16_t z = static_cast<std::uint16_t>(address << 1);
-	do {
-		link::tx(avr::flash_load_far<std::uint8_t>((static_cast<std::uint32_t>(rampz) << 16) | z));
-		if (++z == 0)
-			++rampz;
-	} while (--count);
-}
-
-[[gnu::always_inline]] inline void send_flash(std::uint16_t address, std::uint8_t count)
-{
-	if constexpr (word_flash)
-		send_flash_far(address, count);
-	else
-		send_flash_near(address, count);
-}
-
-[[gnu::noinline]] void tx_ack()
-{
-	link::tx(ack);
-}
-
-void send_eeprom(std::uint16_t address, std::uint8_t count)
-{
-	do
-		link::tx(ee::read(address++));
-	while (--count);
-}
-
-void store_eeprom(std::uint16_t address, std::uint8_t count)
-{
-	do {
-		ee::write<off>(address++, link::rx());
-		tx_ack();
-	} while (--count);
-}
-
-// One page into the SPM buffer, then erase and program. No running-slot write
-// guard: the host guarantees it never targets the loader's own slot (the pure
-// version's one dropped safety net, licensed — README.md).
-void program_flash(std::uint16_t wire_address)
-{
-	spm::flash_address_t address;
-	if constexpr (word_flash) {
-		const std::uint8_t rampz = static_cast<std::uint8_t>(wire_address >> 15);
-		const std::uint16_t z0 = static_cast<std::uint16_t>(wire_address << 1) & ~static_cast<std::uint16_t>(page - 1);
-		std::uint16_t z = z0;
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>((static_cast<spm::flash_address_t>(rampz) << 16) | z, word_of({low, high}));
-			z += 2;
-		} while (static_cast<std::uint8_t>(z));
-		address = (static_cast<spm::flash_address_t>(rampz) << 16) | z0;
-	} else {
-		address = static_cast<spm::flash_address_t>(wire_address & ~static_cast<std::uint16_t>(page - 1));
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>(address, word_of({low, high}));
-			address += 2;
-		} while (static_cast<std::uint8_t>(address) & (page - 1));
-		address -= 2; // back inside the page — erase and write ignore the word bits
-	}
-	spm::erase_page<off>(address);
-	if constexpr (boot_section)
-		spm::wait();
-	spm::write_page<off>(address);
-	if constexpr (boot_section)
-		spm::wait();
-	if constexpr (boot_section)
-		spm::rww_enable<off>();
-}
-
-void send_fuses()
-{
-	std::uint8_t which = 0;
-	do
-		link::tx(spm::read_fuse<off>(static_cast<spm::fuse>(which)));
-	while (++which != 4);
-}
-
-[[noreturn]] void run()
-{
-	if (hw::field_impl<wdrf_field()>::test())
-		run_app();
-
-	link::init();
-
-	// Measure the calibration pulse into the unit, then take one 'p' knock. An
-	// expired budget (no host) boots the application; a pulse that decodes to
-	// anything but 'p' re-measures.
-	for (;;) {
-		if (link::measure(autobaud_budget) == 0)
-			run_app();
-		if (link::rx_bounded(autobaud_budget) == 'p')
-			break;
-	}
-
-	for (;;) {
-		ee::wait();
-		tx_ack();
-		const std::uint8_t command = link::rx();
-		switch (command) {
-		case 'J': { // jump to a wire word address: hand-over and staging transfer
-			auto target = reinterpret_cast<void (*)()>(rx16());
-			tx_ack();
-			link::drain();
-			jump(target);
-		}
-		case 'b': // chip identity: version then the three signature bytes
-			link::tx(version);
-			link::tx(hw::db.signature[0]);
-			link::tx(hw::db.signature[1]);
-			link::tx(hw::db.signature[2]);
-			break;
-		case 'R':   // read flash: addr16, n8 (0 = 256)
-		case 'r':   // read EEPROM: addr16, n8
-		case 'w': { // write EEPROM: addr16, n8, then n bytes each acked
-			// One address-and-count path for the three, so the flash streamer
-			// keeps a single call site and inlines into this never-returning
-			// loop — its cursor then lives in the loop's own call-saved
-			// registers instead of being saved and restored around a call
-			// (the call-site-count lesson, autobaud.md).
-			const std::uint16_t address = rx16();
-			const std::uint8_t count = link::rx();
-			if (command == 'r')
-				send_eeprom(address, count);
-			else if (command == 'w')
-				store_eeprom(address, count);
-			else
-				send_flash(address, count);
-			break;
-		}
-		case 'W': // program one flash page: addr16, page bytes
-			program_flash(rx16());
-			break;
-		case 'F': // fuse and lock bytes
-			send_fuses();
-			break;
-		default: // unknown bytes are ignored; the loop re-acks
-			break;
-		}
-	}
-}
-
-} // namespace
-} // namespace pureboot
-
-template struct avr::startup::entry<pureboot::run>;

pureboot/pureboot_autobaud_reg.cpp

@@ -1,371 +0,0 @@
-// SUPERSEDED — kept for the record, not built. With the activation hang fixed
-// (a lone calibration pulse used to wedge the loader, see autobaud.md) this
-// version is 534 B on the 1284P against a 512 B slot, so the global register
-// variable it broke purity for buys nothing. pureboot_autobaud_uni.cpp replaces
-// it at 464 B, strictly pure and with more features.
-//
-// pureboot autobaud — the software-serial variant that measures the host's bit
-// timing at runtime, so one binary runs at any F_CPU: the image carries no
-// clock. THE REGISTER VERSION — keeps the running-slot write guard, at the cost
-// of one global register variable (r4) holding the measured unit. That variable
-// is pureboot's single, deliberate break from its no-global-register-variable
-// rule, present only in this variant; everything else stays pure C++.
-//
-// Two source files exist for review (autobaud.md):
-// pureboot_autobaud_pure.cpp, fully pure but dropping the write guard, and this
-// one. They differ only in where the measured unit lives (a call-saved register
-// here, GPIOR/RAM there) and whether the guard is present.
-//
-// The register buys ~32 B over a RAM home — an outlined rx/tx reads it with one
-// move where a static costs an lds — and that is what lets the write guard stay
-// while the image still fits 512 B (the 1284 at 512, exactly). The unit is
-// written once through a noinline setter so the store lands immediately before a
-// ret: GCC otherwise deletes a global-register store whose only readers are
-// callees (autobaud.md, upstream bug 6).
-//
-// Simplifications shared with the pure version, each licensed: a slimmed info
-// block (version + signature; the host derives geometry from its chip database)
-// and a single-byte activation knock (the calibration pulse already proves a
-// host). Position independence is kept: control flow is PC-relative and the
-// write guard anchors on the runtime return address, as the fixed-baud loader.
-
-#include <libavr/libavr.hpp>
-
-#include <util/delay_basic.h>
-
-using namespace avr::literals;
-namespace spm = avr::spm;
-namespace ee = avr::eeprom;
-namespace hw = avr::hw;
-
-// The measured per-bit delay (in _delay_loop_2 four-cycle iterations) in a
-// call-saved register that the serial callees read directly, so no wire path
-// threads it and rx/tx reach it with a move, not a load. Written only through
-// set_unit() below.
-register std::uint16_t g_unit asm("r4");
-
-namespace pureboot {
-namespace {
-
-// Purely polled: every interrupt guard folds to nothing.
-constexpr auto off = avr::irq::guard_policy::unused;
-
-constexpr std::uint8_t ack = '+';
-
-// Autobaud carries no clock; only the software-UART pins are a parameter.
-#if !defined(PUREBOOT_RX)
-#define PUREBOOT_RX pb0
-#endif
-#if !defined(PUREBOOT_TX)
-#define PUREBOOT_TX pb1
-#endif
-
-consteval std::int16_t wdrf_field()
-{
-	auto reg = std::string_view{hw::db.regs[static_cast<std::size_t>(avr::power::detail::reset_reg())].name};
-	return hw::db.field_index(reg, "WDRF");
-}
-
-constexpr std::uint16_t slot_bytes = 512;
-constexpr std::uint32_t base = spm::flash_bytes - slot_bytes;
-constexpr std::uint16_t page = spm::page_bytes;
-constexpr bool boot_section = hw::curated::has_boot_section();
-constexpr bool word_flash = spm::flash_bytes > 65536;
-
-constexpr std::uint8_t version = 4;
-
-#if !defined(PUREBOOT_AUTOBAUD_POLLS)
-#define PUREBOOT_AUTOBAUD_POLLS 4000000
-#endif
-constexpr __uint24 autobaud_budget = PUREBOOT_AUTOBAUD_POLLS;
-
-// The one store into g_unit, isolated so it lands right before the ret: a
-// global-register store whose only later readers are callees is dropped
-// otherwise (autobaud.md, upstream bug 6).
-[[gnu::noinline]] void set_unit(std::uint16_t v)
-{
-	g_unit = v;
-}
-
-// The autobaud software link: bit-banged with cycle-counted delays, but the
-// per-bit delay is g_unit, measured from the host's calibration pulse.
-struct link {
-	using in_t = avr::io::input<avr::PUREBOOT_RX, avr::io::pull::up>;
-	using out_t = avr::io::output<avr::PUREBOOT_TX>;
-
-	static void init()
-	{
-		avr::init<in_t, out_t>();
-		out_t::set(); // idle high
-	}
-
-	// Time the calibration pulse into g_unit. The host sends 0xC0 — a start bit
-	// plus six zero data bits are one low pulse of seven bit-times — and the
-	// counted poll loop is seven cycles an iteration, so count >> 2 is the bit
-	// period in _delay_loop_2's four-cycle iterations. 0 means the budget
-	// expired.
-	static std::uint16_t measure(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		std::uint16_t count = 0;
-		while (!in_t::read())
-			++count;
-		const std::uint16_t u = static_cast<std::uint16_t>(count >> 2);
-		set_unit(u);
-		return u;
-	}
-
-	// The eight data bits, entered just past the falling edge of the start bit.
-	// Split out of rx() so the activation path can wait for that edge under a
-	// budget while the command loop waits for it indefinitely.
-	static std::uint8_t sample()
-	{
-		_delay_loop_2(static_cast<std::uint16_t>(g_unit + (g_unit >> 1))); // 1.5 bits to the LSB centre
-		std::uint8_t value = 0;
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			value >>= 1;
-			if (in_t::read())
-				value |= 0x80;
-			_delay_loop_2(g_unit);
-		}
-		return value;
-	}
-
-	static std::uint8_t rx()
-	{
-		while (in_t::read()) // await the start edge
-			;
-		return sample();
-	}
-
-	// A byte under the poll budget, for the activation knock. An expired budget
-	// returns 0, which is not the knock, so the caller falls back into the
-	// budgeted measure() — and a line that stays idle boots the application
-	// there. Without this the knock's edge wait was unbounded, so a single
-	// spurious calibration pulse (EMI, or a host that opens the port and never
-	// knocks) wedged the loader and the application never ran.
-	static std::uint8_t rx_bounded(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		return sample();
-	}
-
-	static void tx(std::uint8_t value)
-	{
-		out_t::clear(); // start bit
-		_delay_loop_2(g_unit);
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			out_t::write(value & 1);
-			value >>= 1;
-			_delay_loop_2(g_unit);
-		}
-		out_t::set(); // stop bit
-		_delay_loop_2(g_unit);
-	}
-
-	static void drain()
-	{
-		// The software transmitter returns only after the stop bit.
-	}
-};
-
-extern "C" [[noreturn]] void pureboot_app();
-
-[[gnu::noipa, noreturn]] void jump(void (*target)())
-{
-	target();
-	__builtin_unreachable();
-}
-
-[[gnu::noinline, noreturn]] void run_app()
-{
-	jump(pureboot_app);
-}
-
-[[gnu::always_inline]] inline std::uint16_t rx16()
-{
-	std::uint16_t low = link::rx();
-	return static_cast<std::uint16_t>(low | (link::rx() << 8));
-}
-
-[[gnu::always_inline]] inline std::uint16_t word_of(std::array<std::uint8_t, 2> pair)
-{
-	return std::bit_cast<std::uint16_t>(pair);
-}
-
-[[maybe_unused, gnu::always_inline]] inline void send_flash_near(std::uint16_t address, std::uint8_t count)
-{
-	do
-		link::tx(avr::flash_load(reinterpret_cast<const std::uint8_t *>(address++)));
-	while (--count);
-}
-
-[[maybe_unused, gnu::always_inline]] inline void send_flash_far(std::uint16_t address, std::uint8_t count)
-{
-	std::uint8_t rampz = static_cast<std::uint8_t>(address >> 15);
-	std::uint16_t z = static_cast<std::uint16_t>(address << 1);
-	do {
-		link::tx(avr::flash_load_far<std::uint8_t>((static_cast<std::uint32_t>(rampz) << 16) | z));
-		if (++z == 0)
-			++rampz;
-	} while (--count);
-}
-
-[[gnu::always_inline]] inline void send_flash(std::uint16_t address, std::uint8_t count)
-{
-	if constexpr (word_flash)
-		send_flash_far(address, count);
-	else
-		send_flash_near(address, count);
-}
-
-[[gnu::noinline]] void tx_ack()
-{
-	link::tx(ack);
-}
-
-void send_eeprom(std::uint16_t address, std::uint8_t count)
-{
-	do
-		link::tx(ee::read(address++));
-	while (--count);
-}
-
-void store_eeprom(std::uint16_t address, std::uint8_t count)
-{
-	do {
-		ee::write<off>(address++, link::rx());
-		tx_ack();
-	} while (--count);
-}
-
-// One page into the SPM buffer, then erase and program — except the slot this
-// code is running in (`slot_high`, from run()), which is drained and left
-// alone. A broken host therefore cannot brick the running loader, and a copy one
-// slot lower may rewrite the resident one.
-void program_flash(std::uint16_t wire_address, std::uint8_t slot_high)
-{
-	spm::flash_address_t address;
-	std::uint8_t page_high;
-	if constexpr (word_flash) {
-		const std::uint8_t rampz = static_cast<std::uint8_t>(wire_address >> 15);
-		const std::uint16_t z0 = static_cast<std::uint16_t>(wire_address << 1) & ~static_cast<std::uint16_t>(page - 1);
-		std::uint16_t z = z0;
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>((static_cast<spm::flash_address_t>(rampz) << 16) | z, word_of({low, high}));
-			z += 2;
-		} while (static_cast<std::uint8_t>(z));
-		address = (static_cast<spm::flash_address_t>(rampz) << 16) | z0;
-		page_high = static_cast<std::uint8_t>(wire_address >> 8);
-	} else {
-		address = static_cast<spm::flash_address_t>(wire_address & ~static_cast<std::uint16_t>(page - 1));
-		do {
-			std::uint8_t low = link::rx();
-			std::uint8_t high = link::rx();
-			spm::fill<off>(address, word_of({low, high}));
-			address += 2;
-		} while (static_cast<std::uint8_t>(address) & (page - 1));
-		address -= 2; // back inside the page — erase and write ignore the word bits
-		page_high = static_cast<std::uint8_t>(address >> 8) & 0xfe;
-	}
-	if (page_high != slot_high) {
-		spm::erase_page<off>(address);
-		if constexpr (boot_section)
-			spm::wait();
-		spm::write_page<off>(address);
-		if constexpr (boot_section)
-			spm::wait();
-	}
-	if constexpr (boot_section)
-		spm::rww_enable<off>();
-}
-
-void send_fuses()
-{
-	std::uint8_t which = 0;
-	do
-		link::tx(spm::read_fuse<off>(static_cast<spm::fuse>(which)));
-	while (++which != 4);
-}
-
-[[noreturn]] void run()
-{
-	if (hw::field_impl<wdrf_field()>::test())
-		run_app();
-
-	link::init();
-
-	// The high byte of the slot this copy runs at, which the write guard
-	// follows: the return address is a word address, so its high byte is the
-	// 256-word slot index, doubled back into byte terms on a byte-addressed
-	// chip. Taken as byteswap's low byte — the builtin already swaps the two
-	// stacked bytes, and the double swap folds away.
-	const std::uint16_t ra_words = reinterpret_cast<std::uint16_t>(__builtin_return_address(0));
-	const std::uint8_t ra_high = static_cast<std::uint8_t>(std::byteswap(ra_words));
-	const std::uint8_t slot_high = word_flash ? ra_high : static_cast<std::uint8_t>(ra_high << 1);
-
-	// Measure the calibration pulse into g_unit, then take one 'p' knock. An
-	// expired budget (no host) boots the application; a pulse that decodes to
-	// anything but 'p' re-measures.
-	for (;;) {
-		if (link::measure(autobaud_budget) == 0)
-			run_app();
-		if (link::rx_bounded(autobaud_budget) == 'p')
-			break;
-	}
-
-	for (;;) {
-		ee::wait();
-		tx_ack();
-		const std::uint8_t command = link::rx();
-		switch (command) {
-		case 'J': { // jump to a wire word address: hand-over and staging transfer
-			auto target = reinterpret_cast<void (*)()>(rx16());
-			tx_ack();
-			link::drain();
-			jump(target);
-		}
-		case 'b': // chip identity: version then the three signature bytes
-			link::tx(version);
-			link::tx(hw::db.signature[0]);
-			link::tx(hw::db.signature[1]);
-			link::tx(hw::db.signature[2]);
-			break;
-		case 'R':   // read flash: addr16, n8 (0 = 256)
-		case 'r':   // read EEPROM: addr16, n8
-		case 'w': { // write EEPROM: addr16, n8, then n bytes each acked
-			// One address-and-count path for the three, so the flash streamer
-			// keeps a single call site and inlines into this never-returning
-			// loop (autobaud.md).
-			const std::uint16_t address = rx16();
-			const std::uint8_t count = link::rx();
-			if (command == 'r')
-				send_eeprom(address, count);
-			else if (command == 'w')
-				store_eeprom(address, count);
-			else
-				send_flash(address, count);
-			break;
-		}
-		case 'W': // program one flash page: addr16, page bytes
-			program_flash(rx16(), slot_high);
-			break;
-		case 'F': // fuse and lock bytes
-			send_fuses();
-			break;
-		default: // unknown bytes are ignored; the loop re-acks
-			break;
-		}
-	}
-}
-
-} // namespace
-} // namespace pureboot
-
-template struct avr::startup::entry<pureboot::run>;

pureboot/pureboot_autobaud_uni.cpp

@@ -1,411 +0,0 @@
-// pureboot autobaud, the unified-primitive version — VARIANT A, an explicit
-// space byte. One read command and one write command carry a space selector, so
-// flash, EEPROM, RAM and the fuses share a single cursor, a single transfer loop
-// and a single argument decode instead of one command body each.
-//
-// Strictly pure: no inline assembly, no global register variables, and no GPIOR
-// either — the measured unit lives in a plain static, so the loader claims no
-// chip resource an application might want. (PUREBOOT_UNIT_GPIOR=1 puts it back
-// in the I/O scratch registers, kept only as a measurement axis.)
-//
-// Against pureboot_autobaud_pure.cpp this version:
-//   - adds RAM read and write, which the loader has never had. Because AVR maps
-//     the register file and the whole I/O space into the data address space,
-//     that one space also gives the host arbitrary peripheral access for free;
-//   - collapses 'R' (read flash), 'r' (read EEPROM), 'w' (write EEPROM) and 'F'
-//     (fuses) — four bodies, four loops — into 'G' and 'P' over four spaces;
-//   - fixes the activation hang: a lone calibration pulse used to leave the
-//     loader blocked forever in the knock's rx(), so a stray edge on an
-//     unattended device wedged it in the loader and the application never ran.
-//
-// Position independence is kept and is total: control flow is PC-relative, the
-// wire carries addresses, and nothing anchors on the runtime address.
-
-#include <libavr/libavr.hpp>
-
-#include <util/delay_basic.h>
-
-using namespace avr::literals;
-namespace spm = avr::spm;
-namespace ee = avr::eeprom;
-namespace hw = avr::hw;
-
-namespace pureboot {
-namespace {
-
-// Purely polled: every interrupt guard folds to nothing.
-constexpr auto off = avr::irq::guard_policy::unused;
-
-constexpr std::uint8_t ack = '+';
-
-// Autobaud carries no clock, so PUREBOOT_CLOCK_HZ / PUREBOOT_BAUD are not
-// consulted; only the software-UART pins are a deployment parameter.
-#if !defined(PUREBOOT_RX)
-#define PUREBOOT_RX pb0
-#endif
-#if !defined(PUREBOOT_TX)
-#define PUREBOOT_TX pb1
-#endif
-
-// The unit's home. A plain static by default — pureboot claims no GPIOR, so the
-// application keeps both scratch registers. The GPIOR spelling is retained
-// behind a macro purely so the two can be measured against each other.
-#if !defined(PUREBOOT_UNIT_GPIOR)
-#define PUREBOOT_UNIT_GPIOR 0
-#endif
-
-// The watchdog reset flag's home: MCUSR, or the classic megas' MCUCSR.
-consteval std::int16_t wdrf_field()
-{
-	auto reg = std::string_view{hw::db.regs[static_cast<std::size_t>(avr::power::detail::reset_reg())].name};
-	return hw::db.field_index(reg, "WDRF");
-}
-
-// The loader owns the top 512 bytes; a staging copy goes in the slot below.
-constexpr std::uint16_t slot_bytes = 512;
-constexpr std::uint32_t base = spm::flash_bytes - slot_bytes;
-constexpr std::uint16_t page = spm::page_bytes;
-constexpr bool boot_section = hw::curated::has_boot_section();
-
-// Past 64 KiB a byte address no longer fits the wire's 16 bits, so flash
-// addresses there are word addresses ('J' always was one).
-constexpr bool word_flash = spm::flash_bytes > 65536;
-
-// The loader's one identity number (README.md); the slimmed info block carries
-// it and the signature, and the host maps a version to its protocol.
-constexpr std::uint8_t version = 5;
-
-// The activation window as a fixed poll budget: with no clock, whole seconds
-// cannot be timed. A __uint24 (AVR's three-byte type) holds it — a fourth byte
-// would cost two words at each countdown step for range never used.
-#if !defined(PUREBOOT_AUTOBAUD_POLLS)
-#define PUREBOOT_AUTOBAUD_POLLS 4000000
-#endif
-constexpr __uint24 autobaud_budget = PUREBOOT_AUTOBAUD_POLLS;
-
-// The two SPM commands the loader still has to recognise by value, on the chips
-// where it cannot issue a runtime one generically. Taken from the chip's own
-// definitions rather than spelled 3 and 5 — though every part pureboot targets
-// agrees on those, which is what lets the host send the raw SPMCSR byte.
-constexpr std::uint8_t spm_erase = __BOOT_PAGE_ERASE;
-constexpr std::uint8_t spm_write = __BOOT_PAGE_WRITE;
-
-// The spaces a transfer can name. Flash is 0 so it is the cheap default.
-//
-// sp_spm is the one that is not memory: a write there hands its data byte to
-// SPMCSR and fires the instruction at the given flash address, so page erase,
-// page write and RWW re-enable become host-issued commands instead of a
-// hardcoded tail inside 'W'. The store side already owns an address, a data
-// byte and an ack, so the whole sequence costs only the fused out/spm pair. It
-// also lets the host reach every other SPM operation — lock bits included —
-// which the loader previously had no way to expose.
-enum : std::uint8_t { sp_flash = 0, sp_eeprom = 1, sp_ram = 2, sp_fuse = 3, sp_spm = 4 };
-
-// A transfer's selector byte is `space | bank << 4`: the low nibble names the
-// space, the high nibble carries flash's third address byte (RAMPZ) on the
-// chips that have one. Putting the bank here rather than widening the address
-// keeps the shared cursor sixteen bits for every space — a three-byte cursor
-// costs its extra increment on EEPROM and RAM reads too, which never need it.
-// The host must not span a bank boundary in one transfer; it already chunks by
-// page, so nothing it does today comes close.
-
-// .noinit, not .bss: the unit is always measured before it is read, so it needs
-// no zeroing — and a zeroed .bss would drag in __do_clear_bss, 18 bytes of
-// startup code for a variable that is written before its first use.
-[[gnu::section(".noinit")]] std::uint16_t unit_backing;
-
-[[gnu::always_inline]] inline std::uint16_t get_unit()
-{
-#if PUREBOOT_UNIT_GPIOR
-	return static_cast<std::uint16_t>(hw::reg_impl<hw::db.reg_index("GPIOR1")>::read() |
-	                                  (hw::reg_impl<hw::db.reg_index("GPIOR2")>::read() << 8));
-#else
-	return unit_backing;
-#endif
-}
-
-[[gnu::always_inline]] inline void put_unit(std::uint16_t u)
-{
-#if PUREBOOT_UNIT_GPIOR
-	hw::reg_impl<hw::db.reg_index("GPIOR1")>::write(static_cast<std::uint8_t>(u));
-	hw::reg_impl<hw::db.reg_index("GPIOR2")>::write(static_cast<std::uint8_t>(u >> 8));
-#else
-	unit_backing = u;
-#endif
-}
-
-// The autobaud software link: bit-banged with cycle-counted delays like the
-// fixed-baud software backend, but the per-bit delay is the measured unit, not
-// a consteval constant. rx/tx load the unit once into a local, so each bit
-// spins from a register with no per-bit reload.
-struct link {
-	using in_t = avr::io::input<avr::PUREBOOT_RX, avr::io::pull::up>;
-	using out_t = avr::io::output<avr::PUREBOOT_TX>;
-
-	static void init()
-	{
-		avr::init<in_t, out_t>();
-		out_t::set(); // idle high
-	}
-
-	// Time the calibration pulse into the unit. The host sends 0xC0 — a start
-	// bit plus six zero data bits are one low pulse of seven bit-times — and the
-	// counted poll loop is seven cycles an iteration, so the count is the pulse
-	// length in cycles ÷ 7 × 7 = one bit period in cycles, and count >> 2 is that
-	// period in _delay_loop_2's four-cycle iterations. Waits for the start edge
-	// under the poll budget; 0 (returned, and stored) means the budget expired.
-	//
-	// The shape of the counting loop is load-bearing, not incidental: the >> 2
-	// is exact only while the pulse's bit-count equals the loop's cycles per
-	// iteration. Both are 7 here (sbis 1 + rjmp 2 + adiw 2 + rjmp 2). Reshaping
-	// this loop silently changes the lock; test/pbautobaud.py is what pins it.
-	static std::uint16_t measure(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		std::uint16_t count = 0;
-		while (!in_t::read())
-			++count;
-		const std::uint16_t u = static_cast<std::uint16_t>(count >> 2);
-		put_unit(u);
-		return u;
-	}
-
-	// The eight data bits, entered just past the falling edge of the start bit.
-	// Split out of rx() so the activation path can wait for that edge under a
-	// budget while the command loop waits for it indefinitely.
-	static std::uint8_t sample()
-	{
-		const std::uint16_t unit = get_unit();
-		_delay_loop_2(static_cast<std::uint16_t>(unit + (unit >> 1))); // 1.5 bits to the LSB centre
-		std::uint8_t value = 0;
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			value >>= 1;
-			if (in_t::read())
-				value |= 0x80;
-			_delay_loop_2(unit);
-		}
-		return value;
-	}
-
-	static std::uint8_t rx()
-	{
-		while (in_t::read()) // await the start edge
-			;
-		return sample();
-	}
-
-	// A byte under the poll budget, for the activation knock. An expired budget
-	// returns 0, which is not the knock, so the caller falls back into the
-	// budgeted measure() — and a line that stays idle boots the application
-	// there. That is the whole hang fix: no wait during activation is unbounded,
-	// so a stray calibration pulse can no longer wedge the loader.
-	static std::uint8_t rx_bounded(__uint24 budget)
-	{
-		while (in_t::read())
-			if (!--budget)
-				return 0;
-		return sample();
-	}
-
-	static void tx(std::uint8_t value)
-	{
-		const std::uint16_t unit = get_unit();
-		out_t::clear(); // start bit
-		_delay_loop_2(unit);
-		for (std::uint8_t bit = 0; bit < 8; ++bit) {
-			out_t::write(value & 1);
-			value >>= 1;
-			_delay_loop_2(unit);
-		}
-		out_t::set(); // stop bit
-		_delay_loop_2(unit);
-	}
-
-	static void drain()
-	{
-		// The software transmitter returns only after the stop bit.
-	}
-};
-
-extern "C" [[noreturn]] void pureboot_app();
-
-[[gnu::noipa, noreturn]] void jump(void (*target)())
-{
-	target();
-	__builtin_unreachable();
-}
-
-[[gnu::noinline, noreturn]] void run_app()
-{
-	jump(pureboot_app);
-}
-
-// Inlined: read across a call, the first byte strands in a call-saved register
-// the caller has to push and pop.
-[[gnu::always_inline]] inline std::uint16_t rx16()
-{
-	std::uint16_t low = link::rx();
-	return static_cast<std::uint16_t>(low | (link::rx() << 8));
-}
-
-[[gnu::always_inline]] inline std::uint16_t word_of(std::array<std::uint8_t, 2> pair)
-{
-	return std::bit_cast<std::uint16_t>(pair);
-}
-
-[[gnu::noinline]] void tx_ack()
-{
-	link::tx(ack);
-}
-
-// One byte out of any space. The four accessors share the cursor, the loop and
-// the call site — the whole point of the unified commands — so each costs only
-// its own instruction rather than a body, a loop and a dispatch arm.
-[[gnu::always_inline]] inline std::uint8_t load(std::uint8_t space, std::uint8_t bank, std::uint16_t at)
-{
-	if (space == sp_eeprom)
-		return ee::read(at);
-	if (space == sp_ram)
-		return *reinterpret_cast<volatile std::uint8_t *>(at);
-	if (space == sp_fuse)
-		return spm::read_fuse<off>(static_cast<spm::fuse>(at));
-	if constexpr (word_flash)
-		return avr::flash_load_far<std::uint8_t>((static_cast<std::uint32_t>(bank) << 16) | at);
-	else
-		return avr::flash_load(reinterpret_cast<const std::uint8_t *>(at));
-}
-
-// One byte into a writable space. Flash is not one of them — it arrives a page
-// at a time through 'W' — and the fuses are not writable at all.
-[[gnu::always_inline]] inline void store(std::uint8_t space, [[maybe_unused]] std::uint8_t bank, std::uint16_t at,
-                                         std::uint8_t value)
-{
-	if (space == sp_ram) {
-		*reinterpret_cast<volatile std::uint8_t *>(at) = value;
-		return;
-	}
-	if (space == sp_spm) {
-		// The fused store-and-fire: SPMCSR takes the data byte and the SPM
-		// issues against Z in the same unscheduled pair the hardware's
-		// four-cycle window demands, which is exactly why this is one
-		// primitive and not a poke of SPMCSR followed by a poke of anything
-		// else. A host cannot hit that window across a serial link.
-		// The preprocessor rather than `if constexpr` only because
-		// spm::detail::page_command does not exist at all where there is no
-		// RAMPZ, and a discarded constexpr branch outside a template is still
-		// name-checked. A generic spm::command() in libavr would let the
-		// runtime command through on every chip and retire the dispatch below.
-#if defined(RAMPZ)
-		spm::detail::page_command(value, (static_cast<spm::flash_address_t>(bank) << 16) | at);
-#else
-		if (value == spm_erase)
-			spm::erase_page<off>(at);
-		else if (value == spm_write)
-			spm::write_page<off>(at);
-		else if constexpr (boot_section)
-			spm::rww_enable<off>();
-#endif
-		if constexpr (boot_section)
-			spm::wait();
-		return;
-	}
-	ee::write<off>(at, value);
-}
-
-// One page into the SPM buffer, and only that: the erase, the write and the RWW
-// re-enable that used to follow are now three host-issued writes to sp_spm,
-// which reach the same fused out/spm pair through the store path's own address
-// and data. No running-slot write guard: the host guarantees it never targets
-// the loader's own slot (licensed — README.md).
-void program_flash([[maybe_unused]] std::uint8_t bank, std::uint16_t at)
-{
-	std::uint16_t z = at & ~static_cast<std::uint16_t>(page - 1);
-	do {
-		std::uint8_t low = link::rx();
-		std::uint8_t high = link::rx();
-#if defined(RAMPZ)
-		spm::fill<off>((static_cast<spm::flash_address_t>(bank) << 16) | z, word_of({low, high}));
-#else
-		spm::fill<off>(z, word_of({low, high}));
-#endif
-		z += 2;
-	} while (static_cast<std::uint8_t>(z) & (page - 1));
-}
-
-[[noreturn]] void run()
-{
-	if (hw::field_impl<wdrf_field()>::test())
-		run_app();
-
-	link::init();
-
-	// Measure the calibration pulse into the unit, then take one 'p' knock —
-	// both under the poll budget. An expired budget (no host) boots the
-	// application; anything but 'p', including the knock timing out, re-measures
-	// and so returns to the budgeted wait that boots it.
-	for (;;) {
-		if (link::measure(autobaud_budget) == 0)
-			run_app();
-		if (link::rx_bounded(autobaud_budget) == 'p')
-			break;
-	}
-
-	for (;;) {
-		ee::wait();
-		tx_ack();
-		const std::uint8_t command = link::rx();
-		switch (command) {
-		case 'J': { // jump to a wire word address: hand-over and staging transfer
-			auto target = reinterpret_cast<void (*)()>(rx16());
-			tx_ack();
-			link::drain();
-			jump(target);
-		}
-		case 'b': // chip identity: version then the three signature bytes
-			link::tx(version);
-			link::tx(hw::db.signature[0]);
-			link::tx(hw::db.signature[1]);
-			link::tx(hw::db.signature[2]);
-			break;
-		case 'W':   // program one flash page: sel8, addr16, then page bytes
-		case 'G':   // read:  sel8, addr16, n8 (0 = 256)
-		case 'g': { // write: sel8, addr16, n8, then n bytes each acked
-			// The unified transfer. One decode, one cursor, one loop for every
-			// space and both directions — the four command bodies this replaces
-			// each carried their own copy of all three. Read and write are the
-			// same letter in the two cases, so the direction is bit 5 of the
-			// command and the loop tests it with a one-word skip. 'W' joins the
-			// same selector-and-address decode rather than keeping a word
-			// address of its own, which makes flash addressing uniform across
-			// every command that names it and costs nothing to share.
-			const std::uint8_t sel = link::rx();
-			const std::uint8_t space = sel & 0x0f;
-			const std::uint8_t bank = static_cast<std::uint8_t>(sel >> 4);
-			std::uint16_t at = rx16();
-			if (command == 'W') {
-				program_flash(bank, at);
-				break;
-			}
-			std::uint8_t count = link::rx();
-			do {
-				if (command & 0x20) {
-					store(space, bank, at, link::rx());
-					tx_ack();
-				} else
-					link::tx(load(space, bank, at));
-				++at;
-			} while (--count);
-			break;
-		}
-		default: // unknown bytes are ignored; the loop re-acks
-			break;
-		}
-	}
-}
-
-} // namespace
-} // namespace pureboot
-
-template struct avr::startup::entry<pureboot::run>;

test/check_pi.py

@@ -1,47 +1,78 @@
 #!/usr/bin/env python3
-"""Position-independence lint: the two link-time facts that let the identical
-image run from any slot, asserted from the built ELF.
+"""Position-independence lint: the property that lets the identical image run
+from any slot, asserted from the built ELF and its object.
 
-  1. No absolute jmp/call — -mrelax normally guarantees it, but a branch that
+  1. No absolute jmp/call - -mrelax normally guarantees it, but a branch that
      grows out of relaxation range would break it silently.
-  2. The info block within the image's first 256 bytes: 'b' rebuilds its
-     address as (running slot high byte : link address low byte).
+  2. Nothing flash-resident to address: the image is .text alone, so there is
+     no table whose runtime address has to be reconstructed.
+  3. The image is byte-identical when linked at a different base. This is
+     position independence itself rather than a proxy for it - an absolute
+     address anywhere in the image would move with the link and show up as a
+     differing byte.
 
-Usage: check_pi.py <objdump> <nm> <elf> <text_start_hex>
+Usage: check_pi.py <objdump> <objcopy> <cxx> <mcu> <elf> <object> <text_start_hex>
 """
 
+import os
 import re
 import subprocess
 import sys
+import tempfile
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
 
 
 def main():
-    objdump, nm, elf, text_start = sys.argv[1:]
+    objdump, objcopy, cxx, mcu, elf, obj, text_start = sys.argv[1:]
     text_start = int(text_start, 0)
 
     listing = subprocess.run([objdump, "-d", elf], capture_output=True, text=True, check=True).stdout
-    absolute = [
-        line
-        for line in listing.splitlines()
-        if re.search(r"\t(jmp|call)\t", line)
-    ]
+    absolute = [line for line in listing.splitlines() if re.search(r"\t(jmp|call)\t", line)]
     if absolute:
-        print("FAIL: absolute control flow in the image:")
-        print("\n".join(absolute))
-        sys.exit(1)
+        fail("absolute control flow in the image:\n" + "\n".join(absolute))
 
-    symbols = subprocess.run([nm, "-C", elf], capture_output=True, text=True, check=True).stdout
-    info = [line for line in symbols.splitlines() if "flash_table" in line and "::storage" in line]
-    if len(info) != 1:
-        print(f"FAIL: expected one info-block storage symbol, found {len(info)}")
-        sys.exit(1)
-    address = int(info[0].split()[0], 16)
-    offset = address - text_start
-    if not 0 <= offset < 256:
-        print(f"FAIL: info block at image offset {offset:#x}, must sit in the first 256 bytes")
-        sys.exit(1)
+    # Allocated flash beyond .text would be data the running copy has to find.
+    # Only ALLOC sections reach the device at all; .comment and the debug
+    # sections ride along in the ELF container and are never flashed. objdump
+    # prints each section's flags on the line following its header.
+    headers = subprocess.run([objdump, "-h", elf], capture_output=True, text=True, check=True).stdout.splitlines()
+    for index, line in enumerate(headers):
+        fields = line.split()
+        if len(fields) < 6 or not fields[0].isdigit():
+            continue
+        name, size = fields[1], int(fields[2], 16)
+        flags = headers[index + 1] if index + 1 < len(headers) else ""
+        if "ALLOC" not in flags or not size:
+            continue
+        if name not in (".text", ".noinit", ".bss"):
+            fail(f"flash-resident section {name} ({size} bytes): the image must be .text alone")
 
-    print(f"PI lint: control flow PC-relative, info block at offset {offset:#x}")
+    # Relink at a different base and compare the bytes.
+    with tempfile.TemporaryDirectory() as work:
+        elsewhere = text_start - 0x200 if text_start >= 0x200 else text_start + 0x200
+        images = []
+        for base, tag in ((text_start, "here"), (elsewhere, "there")):
+            relinked = os.path.join(work, f"{tag}.elf")
+            binary = os.path.join(work, f"{tag}.bin")
+            subprocess.run(
+                [cxx, f"-mmcu={mcu}", "-nostartfiles", f"-Wl,--section-start=.text={base:#x}",
+                 "-Wl,--defsym=pureboot_app=0", "-mrelax", obj, "-o", relinked],
+                check=True, capture_output=True)
+            subprocess.run([objcopy, "-O", "binary", relinked, binary], check=True)
+            images.append(open(binary, "rb").read())
+        if len(images[0]) != len(images[1]):
+            fail(f"the image is {len(images[0])} B linked at {text_start:#x} and "
+                 f"{len(images[1])} B at {elsewhere:#x} - relaxation followed the address")
+        if images[0] != images[1]:
+            differing = [i for i, (a, b) in enumerate(zip(*images)) if a != b]
+            fail(f"the image changes when linked at {elsewhere:#x} instead of {text_start:#x}: "
+                 f"{len(differing)} byte(s) differ, first at offset {differing[0]:#x}")
+
+    print(f"PI lint: control flow PC-relative, .text only, identical linked at {text_start:#x} and {elsewhere:#x}")
 
 
 if __name__ == "__main__":

test/check_size.cmake

@@ -4,6 +4,9 @@ if(NOT _res EQUAL 0)
 endif()
 # avr-size line 2 is "<text> <data> <bss> <dec> <hex> <file>".
 string(REGEX MATCH "\n[ \t]*([0-9]+)" _m "${_out}")
+if(NOT _m)
+	message(FATAL_ERROR "could not read a .text size out of ${SIZE_TOOL}'s output for ${ELF}:\n${_out}")
+endif()
 set(_text ${CMAKE_MATCH_1})
 if(_text GREATER LIMIT)
 	message(FATAL_ERROR ".text is ${_text} bytes, over the ${LIMIT}-byte boot section")

test/check_unit.cmake

@@ -0,0 +1,36 @@
+# Asserts the autobaud loader's measured unit sits where the host will read
+# it (--info's measured clock - the address is wire contract). Two homes: on
+# a chip with the GPIOR pair the unit lives there and the image must carry no
+# RAM word for it at all; elsewhere it is the first RAM object at SRAM start.
+# Run as
+#   cmake -DOBJDUMP=... -DELF=... -DRAM_START=<data address> [-DGPIOR=<data address>]
+#         -P check_unit.cmake
+
+execute_process(COMMAND ${OBJDUMP} -t ${ELF} OUTPUT_VARIABLE _syms RESULT_VARIABLE _res)
+if(NOT _res EQUAL 0)
+	message(FATAL_ERROR "${OBJDUMP} -t ${ELF} failed")
+endif()
+
+# The symbol line: "00800100 l     O .noinit  00000002 <mangled>6m_unitE".
+string(REGEX MATCH "\n0*([0-9a-f]+)[^\n]+[ \t][^ \t\n]*6m_unitE\n" _line "${_syms}")
+
+if(GPIOR)
+	if(_line)
+		message(FATAL_ERROR "m_unit RAM symbol present although the unit's home is GPIOR ${GPIOR} - "
+		                    "the host peeks the pair, and a RAM copy would be dead weight")
+	endif()
+	message(STATUS "no m_unit RAM object - the unit lives in the GPIOR pair at ${GPIOR}")
+	return()
+endif()
+
+if(NOT _line)
+	message(FATAL_ERROR "no m_unit symbol in ${ELF} - is this the autobaud loader?")
+endif()
+
+# AVR data-space symbols carry the 0x800000 VMA offset.
+math(EXPR _want "0x800000 + ${RAM_START}" OUTPUT_FORMAT HEXADECIMAL)
+math(EXPR _have "0x${CMAKE_MATCH_1}" OUTPUT_FORMAT HEXADECIMAL)
+if(NOT _have STREQUAL _want)
+	message(FATAL_ERROR "m_unit sits at ${_have}, ram_start is ${_want} - the host peeks ram_start")
+endif()
+message(STATUS "m_unit at ${_have} == ram_start")

test/device.cpp

@@ -7,103 +7,122 @@
 // SPM genuinely writes avr->flash on the mega cores, so on exit (or SIGTERM)
 // we dump the flash image to a file for a ground-truth cross-check against
 // what the client read back through the bootloader.
-#include <signal.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
+#include <array>
+#include <csignal>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <print>
+
 #include <unistd.h>
 
+// The parts headers (uart_pty.h) carry no C++ linkage guards of their own,
+// unlike simavr's core headers - the block covers both harmlessly.
+extern "C" {
 #include "avr_uart.h"
 #include "sim_avr.h"
 #include "sim_elf.h"
 #include "uart_pty.h"
+}
 
-static avr_t *avr;
-static uart_pty_t uart_pty;
-static const char *dump_path;
+namespace {
 
-static void finish(int sig)
+avr_t *avr;
+uart_pty_t uart_pty;
+const char *dump_path;
+
+[[noreturn]] void finish(int)
 {
-	(void)sig;
 	if (dump_path) {
-		FILE *f = fopen(dump_path, "wb");
+		std::FILE *f = std::fopen(dump_path, "wb");
 		if (f) {
-			fwrite(avr->flash, 1, avr->flashend + 1, f);
-			fclose(f);
+			std::fwrite(avr->flash, 1, avr->flashend + 1, f);
+			std::fclose(f);
 		}
 	}
 	uart_pty_stop(&uart_pty);
 	_exit(0);
 }
 
+} // namespace
+
 int main(int argc, char *argv[])
 {
 	if (argc < 3) {
-		fprintf(stderr, "usage: %s <tsb.elf> <boot_base_hex> [flash_dump.bin]\n", argv[0]);
+		std::println(stderr, "usage: {} <tsb.elf> <boot_base_hex> [flash_dump.bin]", argv[0]);
 		return 2;
 	}
-	uint32_t boot_base = (uint32_t)strtoul(argv[2], NULL, 0);
-	dump_path = argc >= 4 ? argv[3] : NULL;
+	auto boot_base = static_cast<std::uint32_t>(std::strtoul(argv[2], nullptr, 0));
+	dump_path = argc >= 4 ? argv[3] : nullptr;
 
 	avr = avr_make_mcu_by_name("atmega328p");
 	if (!avr) {
-		fprintf(stderr, "device: no ATmega328P core\n");
+		std::println(stderr, "device: no ATmega328P core");
 		return 1;
 	}
 	avr_init(avr);
 	avr->frequency = 16000000;
 	// Real flash powers up erased (0xff); the app region must look erased
 	// before the bootloader programs it.
-	memset(avr->flash, 0xff, avr->flashend + 1);
+	std::memset(avr->flash, 0xff, avr->flashend + 1);
 
 	// simavr's ELF loader flattens the flash base to 0 (it expects an app at
 	// 0x0), but it hands back the boot code in fw.flash; place it at the boot
 	// section base ourselves and enter there (BOOTRST is not modelled).
-	elf_firmware_t fw = {0};
+	elf_firmware_t fw{};
 	if (elf_read_firmware(argv[1], &fw) != 0) {
-		fprintf(stderr, "device: cannot read %s\n", argv[1]);
+		std::println(stderr, "device: cannot read {}", argv[1]);
 		return 1;
 	}
-	memcpy(avr->flash + boot_base, fw.flash, fw.flashsize);
+	// An image that runs past flash end cannot execute on hardware, and a
+	// naive copy of it would smash the heap beyond avr->flash - after which
+	// the simulation misbehaves in ways that point everywhere but here.
+	// Refuse it loudly instead.
+	if (boot_base + fw.flashsize > avr->flashend + 1) {
+		std::println(stderr, "device: {} B at {:#x} runs past flash end {:#x} - image does not fit its slot",
+		             fw.flashsize, boot_base, avr->flashend);
+		return 1;
+	}
+	std::memcpy(avr->flash + boot_base, fw.flash, fw.flashsize);
 	avr->pc = boot_base;
 	avr->codeend = avr->flashend;
 
 	// Optional: seed the config page (one page below the boot section) with a
 	// hex byte string, so the password gate and emergency erase can be tested.
 	// Layout: [appjump lo][appjump hi][timeout][password...][0xff].
-	const char *cfg = getenv("TSB_CONFIG");
+	const char *cfg = std::getenv("TSB_CONFIG");
 	if (cfg) {
-		uint32_t app_end = boot_base - 128; // config page sits directly below the boot code
+		std::uint32_t app_end = boot_base - 128; // config page sits directly below the boot code
 		for (int i = 0; cfg[i] && cfg[i + 1]; i += 2) {
-			char b[3] = {cfg[i], cfg[i + 1], 0};
-			avr->flash[app_end + i / 2] = (uint8_t)strtoul(b, NULL, 16);
+			const std::array pair{cfg[i], cfg[i + 1], '\0'};
+			avr->flash[app_end + i / 2] = static_cast<std::uint8_t>(std::strtoul(pair.data(), nullptr, 16));
 		}
 	}
 
 	// POLL_SLEEP makes simavr usleep(1) on every status-register read while the
-	// UART is idle — a host-CPU-saving hack that models no hardware and paces a
+	// UART is idle - a host-CPU-saving hack that models no hardware and paces a
 	// tight-polling loader (one that releases TX between bytes, as one-wire does)
 	// in real time, distorting protocol timing. Clear it so the loader runs at
 	// true cycle speed.
-	uint32_t uflags = 0;
+	std::uint32_t uflags = 0;
 	avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS('0'), &uflags);
 	uflags &= ~AVR_UART_FLAG_POLL_SLEEP;
 	avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS('0'), &uflags);
 
 	uart_pty_init(avr, &uart_pty);
 	uart_pty_connect(&uart_pty, '0');
-	printf("TSB_PTY %s\n", uart_pty.pty.slavename);
-	fflush(stdout);
+	std::println("TSB_PTY {}", uart_pty.pty.slavename);
+	std::fflush(stdout);
 
-	signal(SIGTERM, finish);
-	signal(SIGINT, finish);
+	std::signal(SIGTERM, finish);
+	std::signal(SIGINT, finish);
 
 	for (;;) {
 		int state = avr_run(avr);
-		if (state == cpu_Done || state == cpu_Crashed)
+		if (state == cpu_Done || state == cpu_Crashed) {
 			break;
+		}
 	}
 	finish(0);
-	return 0;
 }

test/pbapp.cpp

@@ -1,16 +1,20 @@
 // Test-fixture application for the pureboot protocol tests: prints "APP" on
-// the chip's serial link (the same link the loader uses) — the proof that
+// the chip's serial link (the same link the loader uses) - the proof that
 // the loader's hand-over, and on the tinies the host's reset-vector
 // surgery, actually launched it. Linked normally (crt, vectors at 0); on
 // the tinies its reset vector is the rjmp the host re-homes.
 //
 // On the hardware-USART link it then listens, and an 'L' makes it jump into
-// the resident loader — the application-owned loader entry a
+// the resident loader - the application-owned loader entry a
 // BOOTRST-unprogrammed mega relies on (reset always boots the application
 // there), exercised by the self-update tests. The software link idles:
 // reset reaches those loaders through the patched vector (or the runner
 // models BOOTRST), so the application owes them nothing.
 //
+// PUREBOOT_HANDOVER drops the listening and jumps straight in, leaving the
+// USART enabled behind it - the hand-over state a loader bit-banging on that
+// USART's own pins has to survive.
+//
 // The fixture speaks the deployment its loader was built for: the same
 // PUREBOOT_* defines configure it, and without them it assumes the stock
 // deployment (the crystal/RC clock table below, the chip's natural link).
@@ -26,10 +30,12 @@ consteval avr::hertz_t clock()
 	return avr::hertz_t{PUREBOOT_CLOCK_HZ};
 #else
 	auto name = std::string_view{avr::hw::db.name};
-	if (name.starts_with("ATtiny13"))
+	if (name.starts_with("ATtiny13")) {
 		return 9.6_MHz;
-	if (name.starts_with("ATtiny"))
+	}
+	if (name.starts_with("ATtiny")) {
 		return 8_MHz;
+	}
 	return 16_MHz;
 #endif
 }
@@ -37,6 +43,9 @@ consteval avr::hertz_t clock()
 #if !defined(PUREBOOT_TX)
 #define PUREBOOT_TX pb1
 #endif
+#if !defined(PUREBOOT_RX)
+#define PUREBOOT_RX pb0
+#endif
 #if !defined(PUREBOOT_USART)
 #define PUREBOOT_USART 0
 #endif
@@ -46,7 +55,7 @@ consteval bool use_hardware()
 #if defined(PUREBOOT_SOFT_SERIAL)
 	return false;
 #else
-	return avr::hw::db.has_instance("USART0") || avr::hw::db.has_instance("USART");
+	return avr::uart::has_usart<0>();
 #endif
 }
 
@@ -59,29 +68,57 @@ struct link {
 #else
 	static constexpr avr::baud_t baud{115200};
 #endif
-	using tx_t = avr::uart::usart<'0' + PUREBOOT_USART, C, {.baud = baud, .max_baud_error = 2.5_pct}>;
+	// The fixture speaks whatever rate the loader was built for, stock
+	// 115200 at 16 MHz included, which sits past the receiver-tolerance
+	// table's bound - the same deployment envelope the loader itself states.
+	using tx_t = avr::uart::usart<PUREBOOT_USART, C, {.baud = baud, .allow_baud_error = true}>;
+	static void init()
+	{
+		avr::init<tx_t>();
+	}
 	static void tx(char c)
 	{
 		tx_t::write(static_cast<std::uint8_t>(c));
 	}
+	// The loader sits in the top slot - 512 bytes on every chip. The jump
+	// takes a word address, which is what makes the >64 KiB chips' entry
+	// reachable through a 16-bit pointer at all.
+	static void enter_loader()
+	{
+		constexpr std::uint32_t slot = 512;
+		reinterpret_cast<void (*)()>(static_cast<std::uint16_t>((avr::hw::db.mem.flash_size - slot) / 2))();
+	}
+
 	[[noreturn]] static void idle()
 	{
-		// 'L' hands back to the loader in the top slot — 512 bytes on every
-		// chip. The jump takes a word address, which is what makes the
-		// >64 KiB chips' entry reachable through a 16-bit pointer at all.
-		constexpr std::uint32_t slot = 512;
+#if defined(PUREBOOT_HANDOVER)
+		// Hand back at once, with this USART still enabled - the state that
+		// leaves a bit-banged loader on its pins mute unless the loader
+		// releases it. Unconditional because there is no command wire to
+		// wait on: that loader's link is the pins, not this peripheral.
+		enter_loader();
+		__builtin_unreachable();
+#else
 		for (;;) {
 			auto command = tx_t::read_blocking();
-			if (command == 'L')
-				reinterpret_cast<void (*)()>(static_cast<std::uint16_t>((avr::hw::db.mem.flash_size - slot) / 2))();
+			if (command == 'L') {
+				enter_loader();
+			}
 			// 'D' leaves every word of the SPM page buffer dirty, so that a
 			// following 'L' enters the loader with the buffer it never clears.
+			// Hardware refuses application-section SPM on a boot-sectioned
+			// part; simavr dispatches it anyway, which is the whole reason the
+			// state is constructible - the stated section is the compilable
+			// fiction that matches what the simulator runs.
 			if (command == 'D') {
-				for (std::uint16_t at = 0; at < avr::spm::page_bytes; at += 2)
-					avr::spm::fill(at, 0xdead);
+				const auto open = avr::spm::page::begin<avr::spm::from::boot_section>(0);
+				for (std::uint16_t at = 0; at < avr::spm::page_bytes; at += 2) {
+					avr::spm::fill(open, at, 0xdead);
+				}
 				tx('D');
 			}
 		}
+#endif
 	}
 };
 
@@ -92,15 +129,49 @@ struct link<C, false> {
 #else
 	static constexpr avr::baud_t baud{57600};
 #endif
-	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, baud>;
+	// A shared-pin deployment (RX == TX) banners as a guest on its own line:
+	// the pull-up input is the released line, the transmitter takes the pin
+	// for exactly one frame per byte - the shape a real one-wire application
+	// beside this loader uses.
+	static constexpr bool one_wire = avr::PUREBOOT_RX == avr::PUREBOOT_TX;
+	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, baud, one_wire>;
+	static void init()
+	{
+		// The guest transmitter configures no pin; the released line - the
+		// pull-up input a receiver would own - is established here.
+		if constexpr (one_wire) {
+			avr::init<avr::io::input<avr::PUREBOOT_TX, avr::io::pull::up>, tx_t>();
+		} else {
+			avr::init<tx_t>();
+		}
+	}
 	static void tx(char c)
 	{
 		tx_t::write(static_cast<std::uint8_t>(c));
 	}
 	[[noreturn]] static void idle()
 	{
+#if defined(PUREBOOT_HEARTBEAT)
+		// Repeat the banner forever, which turns the fixture into a fixed
+		// cycles-per-bit transmitter: `tools/pbrig.py rate` sweeps the host rate
+		// against it to find the part's true bit rate, and from that the clock
+		// its RC oscillator is really running at. Only the *bit* timing carries
+		// the measurement - the delay merely spaces the lines out, so its own
+		// error does not matter. Software link only: the hardware-link idle owes
+		// the self-update tests a command loop, and a crystal deployment has
+		// nothing to measure.
+		while (true) {
+			tx('A');
+			tx('P');
+			tx('P');
+			tx('\r');
+			tx('\n');
+			dev::delay<50_ms>();
+		}
+#else
 		while (true) {
 		}
+#endif
 	}
 };
 
@@ -108,9 +179,14 @@ struct link<C, false> {
 
 int main()
 {
-	avr::init<typename link<dev::clock>::tx_t>();
+	link<dev::clock>::init();
+#if !defined(PUREBOOT_HANDOVER)
 	link<dev::clock>::tx('A');
 	link<dev::clock>::tx('P');
 	link<dev::clock>::tx('P');
+#endif
+	// The hand-over fixture stays silent: nothing is listening on the USART it
+	// brings up - the loader it hands to speaks those pins directly - so its
+	// banner would be a write into a peer that does not exist.
 	link<dev::clock>::idle();
 }

test/pbautobaud.py

@@ -1,21 +1,23 @@
 #!/usr/bin/env python3
 """End-to-end autobaud test: drive an autobaud loader in simavr through the
 calibration handshake and a flash + EEPROM + fuse round-trip, cross-checked
-against the simulator's ground-truth memory — then repeat at a second F_CPU with
+against the simulator's ground-truth memory - then repeat at a second F_CPU with
 the *same* loader binary, which is the property autobaud exists for: one
 clock-agnostic image that locks onto whatever rate the host sends.
 
 Usage: pbautobaud.py <device_bin> <loader_elf> <mcu> <base_hex> <page>
-                     <app_bin> <app_hz> <app_baud> <tool_py> <workdir>
+                     <app_bin> <app_hz> <app_baud> <tool_py> <workdir> [link]
 
-The loader is a software-serial build on PB0/PB1 (pureboot_add_autobaud's
-default), so the runner drives it over the GPIO⇄pty bridge (-l sw:B0,B1). The
-app fixture is built for (app_hz, app_baud); the hand-over is checked at that
-point, and a second point at half the clock proves the lock is measured, not
-baked in.
+The loader is a software-serial build, driven over the GPIO<->pty bridge; the
+optional link overrides the default -l sw:B0,B1 - RX == TX in it is the
+one-wire deployment, and every session then runs with the host's echo
+discard on. The app fixture is built for (app_hz, app_baud); the hand-over
+is checked at that point, and a second point at half the clock proves the
+lock is measured, not baked in.
 """
 
 import os
+import re
 import sys
 import time
 
@@ -26,8 +28,12 @@ def fail(message):
 
 
 def main():
-    (device_bin, elf, mcu, base_hex, page, app_bin, app_hz, app_baud, tool, workdir) = sys.argv[1:]
+    args = sys.argv[1:]
+    link = args.pop() if len(args) == 11 else "sw:B0,B1"
+    (device_bin, elf, mcu, base_hex, page, app_bin, app_hz, app_baud, tool, workdir) = args
     base, page, app_hz, app_baud = int(base_hex, 0), int(page), int(app_hz), int(app_baud)
+    one_wire = re.fullmatch(r"sw:([A-H][0-7]),\1(@[01])?", link) is not None
+    extra = ("--one-wire",) if one_wire else ()
     sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
     sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
     import pbsim
@@ -39,7 +45,7 @@ def main():
     open(ee_path, "wb").write(ee_image)
 
     # The geometry the surgery planner needs, from the chip class the runner is
-    # told — the same derivation pbtest.py makes: the boot-sectioned megas need
+    # told - the same derivation pbtest.py makes: the boot-sectioned megas need
     # no vector surgery, the tinies and the boot-section-less m48s do, and the
     # large chips speak word addresses.
     mega = mcu.startswith("atmega")
@@ -54,19 +60,32 @@ def main():
         """One clock point: reset, calibrate + knock, program, verify against the
         simulator's own flash, and (at the app's point) hand over to the fixture."""
         dump = os.path.join(workdir, f"flash_{label}.bin")
-        device = pbsim.Device(device_bin, elf, mcu, str(hz), base_hex, page, baud, dump, link="sw:B0,B1")
+        device = pbsim.Device(device_bin, elf, mcu, str(hz), base_hex, page, baud, dump, link=link)
         try:
             # The host tool, in autobaud mode, sends the 0xC0 calibration pulse
             # and a single knock at `baud`; the loader locks to it.
-            out = pbsim.run_tool(tool, device.pty, baud, "--autobaud", "--info", "--fuses",
-                                 "--flash", app_bin, "--eeprom", ee_path, "--stay")
+            out = pbsim.run_tool(tool, device.pty, baud, *extra, "--autobaud", "--info", "--clock", str(hz),
+                                 "--fuses", "--flash", app_bin, "--eeprom", ee_path, "--stay")
             for needed in ("version", "signature", "fuses", "verify:", "stays"):
                 if needed not in out:
                     fail(f"{label}: session output lacks {needed!r}\n{out}")
+            # The measured clock, decoded from the unit at whichever home this
+            # version keeps it in. The runner's clock is exact, so the figure
+            # must land inside the
+            # encoding's own envelope: the loader floors the bit period to
+            # 4-cycle spin granules after an 8-cycle discount, and the edge
+            # poll can shave a few cycles more - one granule of slack below
+            # the true clock, none above (in cycles per bit, times the rate).
+            measured = re.search(r"measured\s+(\d+) Hz", out)
+            if not measured:
+                fail(f"{label}: --info lacks the measured clock\n{out}")
+            measured = int(measured.group(1))
+            if not hz - 19 * baud <= measured <= hz + 4 * baud:
+                fail(f"{label}: measured clock {measured} Hz is {measured - hz:+d} off the true {hz}")
             # Read both memories back over the locked link and check them.
             read_flash = os.path.join(workdir, f"rf_{label}.bin")
             read_eeprom = os.path.join(workdir, f"re_{label}.bin")
-            out = pbsim.run_tool(tool, device.pty, baud, "--autobaud", "--verify-flash", app_bin,
+            out = pbsim.run_tool(tool, device.pty, baud, *extra, "--autobaud", "--verify-flash", app_bin,
                                  "--verify-eeprom", ee_path, "--read-flash", read_flash,
                                  "--read-eeprom", read_eeprom, "--stay")
             if out.count("verify:") != 2:
@@ -75,34 +94,37 @@ def main():
                 fail(f"{label}: EEPROM read-back mismatch")
 
             if hand_over:
-                # Regression: a calibration pulse with no knock behind it must
-                # not wedge the loader. The knock's edge wait used to be
-                # unbudgeted, so one stray low pulse — EMI, or a host that opens
-                # the port and never knocks — held the loader forever and the
-                # application never ran. The whole activation is bounded now, so
-                # the window closes and the app boots; the banner is the proof.
+                # A calibration pulse with no knock behind it must not wedge
+                # the loader: the whole activation is bounded, including the
+                # knock's edge wait, so one stray low pulse - EMI, or a host
+                # that opens the port and never knocks - closes the window and
+                # boots the application. The banner is the proof.
                 # (The pause lets the loader reach its measurement loop, so the
                 # pulse is genuinely seen and the test cannot pass vacuously.)
                 device.reset()
                 port = pb.Port(device.pty, baud)
+                if one_wire:
+                    port = pb.OneWirePort(port)
                 try:
                     time.sleep(0.2)
                     port.write(bytes((pb.CALIBRATE,)))
                     # Accumulate rather than match exactly: the reset leaves the
                     # idle line a framing artefact ahead of the banner, which is
-                    # noise here — the question is only whether the app ran.
+                    # noise here - the question is only whether the app ran.
                     seen = b""
                     deadline = time.monotonic() + 180.0
                     while b"APP" not in seen and time.monotonic() < deadline:
                         seen += port.read_available(1.0)
                     if b"APP" not in seen:
-                        fail(f"{label}: lone calibration pulse wedged the loader — app never bannered, saw {seen!r}")
+                        fail(f"{label}: lone calibration pulse wedged the loader - app never bannered, saw {seen!r}")
                     print(f"  {label}: lone calibration pulse does not wedge the loader")
                 finally:
                     port.close()
 
                 device.reset()
                 port = pb.Port(device.pty, baud)
+                if one_wire:
+                    port = pb.OneWirePort(port)
                 try:
                     loader = pb.Loader(port)
                     live = loader.connect_autobaud(15)
@@ -114,13 +136,13 @@ def main():
                         # the stack at the top. Reading it back over the same
                         # locked link proves both directions of the new space.
                         probe = bytes(range(0x30, 0x40))
-                        loader.write_ram(0x0200, probe)
-                        if loader.read_ram(0x0200, len(probe)) != probe:
+                        loader.write_data(0x0200, probe)
+                        if loader.read_data(0x0200, len(probe)) != probe:
                             fail(f"{label}: RAM round-trip mismatch")
                         # The register file and the I/O space share the data
                         # address space on AVR, so the same command reaches a
                         # peripheral register. SPMCSR reads back as idle here.
-                        verbose_ram = loader.read_ram(0x0200, 4)
+                        verbose_ram = loader.read_data(0x0200, 4)
                         print(f"  {label}: RAM read/write ok ({verbose_ram.hex()})")
                     loader.run_application()
                     banner = port.read_exact(3, 5.0)
@@ -141,13 +163,47 @@ def main():
         print(f"  {label}: locked at {hz} Hz / {baud} Bd, flash+EEPROM verified"
               + (", hand-over ok" if hand_over else ""))
 
+    def must_lock(hz, baud, label):
+        """The calibration alone, at a tight bit period. Nothing is programmed -
+        the question is only whether the loader can still measure the pulse."""
+        dump = os.path.join(workdir, f"flash_{label}.bin")
+        device = pbsim.Device(device_bin, elf, mcu, str(hz), base_hex, page, baud, dump,
+                              link=link)
+        try:
+            port = pb.Port(device.pty, baud)
+            if one_wire:
+                port = pb.OneWirePort(port)
+            try:
+                live = pb.Loader(port).connect_autobaud(15)
+                if live.version != pb.NEWEST_LOADER:
+                    fail(f"{label}: loader reports pureboot {live.version}")
+            finally:
+                port.close()
+        finally:
+            device.stop()
+        print(f"  {label}: locked at {hz} Hz / {baud} Bd ({hz / baud:.0f} cycles a bit)")
+
     # The app fixture is built for one clock; the hand-over banners there. A
     # second point at double that clock, same loader binary, proves the lock is
-    # measured, not baked in — the whole point of autobaud. (Doubling keeps the
+    # measured, not baked in - the whole point of autobaud. (Doubling keeps the
     # bit period healthy; halving would drop it below the software UART's floor.)
     round_trip(app_hz, app_baud, "clock-a", hand_over=True)
     round_trip(app_hz * 2, app_baud, "clock-b", hand_over=False)
-    print("pbautobaud: calibration lock and flash/EEPROM/fuse round-trip pass at both clocks")
+
+    # Both points above sit near 100 cycles a bit, which is comfortable. The
+    # calibration's real floor is far tighter, and it is worth a gate: measured
+    # here, the lock is solid down to ~36 cycles a bit and fails outright by ~31
+    # - a sharp edge, not a fraying one. This pins the tightest standard rate the
+    # fixture's clock reaches, so a change that raises the floor is caught.
+    #
+    # It does *not* bound what a real deployment can use. On silicon the
+    # oscillator's own jitter costs roughly a factor of two: an ATtiny13A on its
+    # factory RC trim was reliable at ~118 cycles a bit and already locking only
+    # 1 attempt in 5 by ~59, which no exact-clock simulation can show. The
+    # deployable envelope is a README matter; this is the logic's floor.
+    must_lock(app_hz, app_baud * 2, "tight-bit")
+    print("pbautobaud: calibration lock and flash/EEPROM/fuse round-trip pass at both clocks, "
+          "and the tight bit period still locks")
 
 
 if __name__ == "__main__":

test/pbdirty.py

@@ -1,12 +1,12 @@
 #!/usr/bin/env python3
 """Dirty-page-buffer acceptance test: with no discard in the loader, a page
 filled over words an earlier writer left takes those instead. The whole
-contract is asserted — a bare verify sees the corruption, the repairing
+contract is asserted - a bare verify sees the corruption, the repairing
 verify fixes it in one rewrite, and it stays fixed.
 
 The state is reached the one way the loader cannot prevent: an application
 dirties the buffer and jumps in with no reset between. Boot-sectioned megas
-forbid that outright (SPM runs only from the boot section, Atmel-8271 §26.2),
+forbid that outright (SPM runs only from the boot section, Atmel-8271 section 26.2),
 but simavr dispatches SPM from anywhere, which is what makes it constructible.
 
 Usage: pbdirty.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
@@ -66,7 +66,7 @@ def main():
                 fail(f"the read-back failed, but not at verify: {error}")
         else:
             # Either the fixture no longer dirties the buffer, or the loader
-            # clears it again — in which case this test's premise is gone.
+            # clears it again - in which case this test's premise is gone.
             fail("programming over a dirty page buffer came back clean")
 
         # What the programming path uses: one rewrite settles it, and it stays

test/pbglitch.py

@@ -0,0 +1,124 @@
+#!/usr/bin/env python3
+"""A link that damages bytes, deliberately and reproducibly, against the seal
+that exists for it.
+
+The board this was written for loses and mangles bytes on its own serial path,
+and the failure that made it matter - a page-fill byte lost, the stream one
+byte out, a page-address byte arriving where an SPMCSR value belongs - is not
+reachable by asking a healthy link nicely. So the damage is injected here, at
+a named byte index rather than a probability: a failing case is a case that
+fails again.
+
+Every check is a pair. The same bit flipped in the same field is applied on
+one side of the seal and then the other: *after* the host seals the header,
+which is a mangled command and must be refused, and *before*, which is a
+well-formed command for something else and must be obeyed. Only the pair
+proves anything - a test that showed the refusal alone would pass against a
+loader that had simply stopped doing SPM, and one that showed the corruption
+alone would not say what caught it.
+
+Usage: pbglitch.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                   <baud> <tool_py> <workdir>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def frame(pb, op, space, address, count, damage=None, before_seal=False):
+    """A sealed command header, optionally with one byte damaged.
+
+    `damage` is (index, mask). Applied before the seal is computed it produces
+    a valid command for whatever the damaged fields now say; applied after, a
+    command whose seal no longer matches its own body - which is the shape a
+    link fault actually has."""
+    head = bytearray((op, pb.selector(space, address), address & 0xFF,
+                      (address >> 8) & 0xFF, count & 0xFF))
+    if damage and before_seal:
+        head[damage[0]] ^= damage[1]
+    seal = pb.SEAL
+    for byte in head:
+        seal ^= byte
+    out = bytearray(head + bytes((seal,)))
+    if damage and not before_seal:
+        out[damage[0]] ^= damage[1]
+    return bytes(out)
+
+
+def main():
+    device_bin, elf, mcu, hz, base_hex, page, baud, tool, workdir = sys.argv[1:]
+    base, page, baud = int(base_hex, 0), int(page), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+    import pureboot as pb
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "dump.bin")
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump)
+    try:
+        port = pb.Port(device.pty, baud)
+        loader = pb.Loader(port)
+        info = loader.connect(25)
+        if info.version < pb.SEALED_LOADER:
+            fail(f"this test is for pureboot {pb.SEALED_LOADER} and later, not {info.version}")
+
+        # A page of known bytes to watch. Everything below aims at it, so
+        # "nothing happened" is a readable claim rather than an absence.
+        marker = bytes((0x40 + (i & 0x3F)) for i in range(page))
+        loader.write_page(0, marker)
+        if loader.read_flash(0, page) != marker:
+            fail("the marker page did not survive an undamaged write")
+
+        # Every field of the header, one bit each. A damaged seal must be
+        # refused, the loader must re-prompt, and the page must be untouched -
+        # and it is the erase being aimed at it, so a single escape is visible.
+        for index in range(6):
+            bad = frame(pb, pb.OP_WRITE, pb.SP_SPM, 0, pb.SPM_ERASE, damage=(index, 0x01))
+            port.write(bad)
+            answer = port.read_exact(1, 5.0)
+            if answer != pb.NAK:
+                fail(f"a header damaged in byte {index} was not refused (got {answer.hex()})")
+            if port.read_exact(1, 5.0) != pb.PROMPT:
+                fail(f"no prompt after refusing a header damaged in byte {index}")
+            if loader.read_flash(0, page) != marker:
+                fail(f"damage in byte {index} reached flash - the marker page changed")
+
+        # The fill, whose payload is the protocol's one unacked burst: a
+        # refused fill must be refused *before* the page is sent, or the host
+        # is left pushing 128 bytes into a loader reading commands. Nothing is
+        # sent after the verdict here, and the very next command must be
+        # understood - that is the whole claim.
+        bad = frame(pb, pb.OP_FILL, pb.SP_FLASH, 0, page, damage=(3, 0x80))
+        port.write(bad)
+        if port.read_exact(1, 5.0) != pb.NAK:
+            fail("a damaged fill header was not refused")
+        if port.read_exact(1, 5.0) != pb.PROMPT:
+            fail("no prompt after refusing a damaged fill header")
+        if loader.identity().raw != info.raw:
+            fail("the loader was out of step after refusing a fill")
+
+        # The pair's other half. The identical flip, applied before the seal:
+        # a well-formed erase of the page one bit away from the one intended.
+        # It must be obeyed - otherwise the refusals above prove nothing about
+        # the seal and only that this loader stopped erasing.
+        port.write(frame(pb, pb.OP_WRITE, pb.SP_SPM, 0, pb.SPM_ERASE,
+                         damage=(2, 0x01), before_seal=True))
+        if port.read_exact(1, 5.0) != pb.PROMPT:
+            fail("a correctly sealed erase was refused")
+        if port.read_exact(1, 5.0) != pb.PROMPT:
+            fail("no prompt after a correctly sealed erase")
+        if loader.read_flash(0, page) != b"\xff" * page:
+            fail("the sealed erase did not reach flash - the marker page is intact")
+        port.close()
+    finally:
+        device.stop()
+    print("pbglitch: damaged headers are refused, the identical damage sealed is obeyed")
+
+
+main()

test/pbmute.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+"""Hand-over with a USART left enabled on the loader's own pins.
+
+A software or autobaud link deployed on a USART's TxD is mute if an
+application hands over with that USART still enabled: TXEN keeps the USART
+owning the pin, so the bit-banged transmitter's port writes go nowhere and the
+loader receives and obeys while answering nothing. The link's init releases it.
+
+The state is reached the way silicon reaches it - an application that sets up
+its USART and jumps in with no reset between, so nothing clears UCSRnB for it.
+The pin ownership itself is modelled by the device runner: simavr wires a
+USART through IRQs alone and never takes the pin from the port, so without
+that the mute could not happen here at all (test/pureboot_device.cpp).
+
+Usage: pbmute.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                 <baud> <app_bin> <tool_py> <workdir> <link>
+"""
+
+import os
+import re
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    device_bin, elf, mcu, hz, base_hex, page, baud, app_bin, tool, workdir, link = sys.argv[1:]
+    page, baud = int(page), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+    import pureboot as pb
+
+    if "@" not in link:
+        fail(f"the link {link} names no owning USART - nothing would be under test")
+    # A shared line (RX == TX) echoes the host's own bytes; discard them the
+    # way the shipped --one-wire mode does.
+    one_wire = re.fullmatch(r"sw:([A-H][0-7]),\1@[01]", link) is not None
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "dump.bin")
+
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, link=link)
+    try:
+        port = pb.Port(device.pty, baud)
+        if one_wire:
+            port = pb.OneWirePort(port)
+        loader = pb.Loader(port)
+        loader.connect(25)
+        resident = loader.info.version
+
+        # Install the fixture and let it take over. It brings up the USART
+        # that owns these pins and jumps straight back in.
+        pb.op_flash(loader, app_bin, erase=False, verify=True)
+        loader.run_application()
+
+        # The loader is running again with that USART enabled behind it. Only
+        # the release makes it audible; without it the connect times out.
+        loader = pb.Loader(port)
+        try:
+            loader.connect(25)
+        except pb.Error as error:
+            fail(f"the loader never answered after the hand-over - the USART still owns its TX pin ({error})")
+        if loader.info.version != resident:
+            fail(f"identity changed across the hand-over: {resident} then {loader.info.version}")
+
+        # Answering is not enough: it has to still be a working loader.
+        pb.verify_pages(loader, pb.plan_flash(open(app_bin, "rb").read(), loader.info))
+        port.close()
+    finally:
+        device.stop()
+    print("pbmute: a loader on a USART's own pins answers after a hand-over that left it enabled")
+
+
+if __name__ == "__main__":
+    main()

test/pbosccal.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""The build-time OSCCAL trim, observed through the wire: a loader built with
+the OSCCAL axis holds the trim register at the built byte from its first
+prompt on - the write sits at the top of run(), ahead of the WDRF bail, so
+every path out of reset runs on the corrected clock. simavr's clock does not
+follow OSCCAL, which is what makes the value assertable at all: the register
+is plain state there, and the peek must return exactly what the build
+declared rather than whatever the oscillator needed.
+
+Usage: pbosccal.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                   <baud> <osccal_addr> <osccal_value> <tool_py> <workdir>
+                   [link]
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    args = sys.argv[1:]
+    link = args.pop() if len(args) == 12 else None
+    (device_bin, elf, mcu, hz, base_hex, page, baud, addr, value, tool, workdir) = args
+    addr, value, baud = int(addr, 0), int(value, 0), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "flash_dump.bin")
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, link=link)
+    try:
+        out = pbsim.run_tool(tool, device.pty, baud, "--peek", f"{addr:#x}:1")
+        want = f"{addr:#06x}  {value:02x}"
+        if want not in out:
+            fail(f"OSCCAL at {addr:#x} did not read back {value:#04x}:\n{out}")
+    finally:
+        device.stop()
+    print("OK")
+
+
+if __name__ == "__main__":
+    main()

test/pbrehome.py

@@ -3,11 +3,10 @@
 canonical slot must still be a working loader, and the ordinary
 --update-loader flow must put a build into the top slot from there.
 
-Two positions. Address 0, a raw .bin handed to a programmer: the staging
-install and the word-0 redirect run from copies outside page 0's slot, so the
-running-slot guard never blocks them. And the staging slot itself, where a
-loader already sitting there IS the staging copy — recognized by its embedded
-block and left in place, then streaming the new resident like any staged copy.
+Two positions. Address 0, a raw .bin handed to a programmer. And the staging
+slot itself, where a loader already sitting there IS the staging copy -
+recognized by its embedded block and left in place, then streaming the new
+resident like any staged copy.
 
 Usage: pbrehome.py <device_bin> <pureboot_elf> <update_bin> <mcu> <hz>
                    <base_hex> <page> <baud> <app_bin> <tool_py> <workdir>
@@ -22,7 +21,7 @@ def fail(message):
     sys.exit(1)
 
 
-def rehome_from(pbsim, pb, device_bin, elf, place_hex, guard_probe, update_bin, base, page, baud, app_bin, workdir,
+def rehome_from(pbsim, pb, device_bin, elf, place_hex, update_bin, base, page, baud, app_bin, workdir,
                 mcu, hz):
     """Place the loader at `place_hex`, heal through --update-loader, flash
     the application, expect the banner."""
@@ -36,15 +35,7 @@ def rehome_from(pbsim, pb, device_bin, elf, place_hex, guard_probe, update_bin,
         loader = pb.Loader(port)
         info = loader.connect(25)
         if info.base != base:
-            fail(f"the misplaced copy reports base {info.base:#06x} — the info block must stay canonical")
-
-        # The accidental slot still guards itself; re-homing rides on the
-        # canonical slots being writable from it.
-        probe = int(guard_probe, 0)
-        before = loader.read_flash(probe, info.page)
-        loader.write_page(probe, bytes(info.page))
-        if loader.read_flash(probe, info.page) != before:
-            fail("the misplaced copy's guard let its own slot change")
+            fail(f"the misplaced copy reports base {info.base:#06x} - the info block must stay canonical")
 
         # The ordinary update flow puts the build into the top slot.
         pb.op_update_loader(loader, 25, update_bin, state, None)
@@ -76,16 +67,15 @@ def main():
 
     os.makedirs(workdir, exist_ok=True)
 
-    # Address 0: the raw-.bin-to-a-programmer accident. The guard probe is
-    # the copy's own page 0.
-    rehome_from(pbsim, pb, device_bin, elf, "0x0", "0x0", update_bin, base, page, baud, app_bin, workdir, mcu, hz)
+    # Address 0: the raw-.bin-to-a-programmer accident.
+    rehome_from(pbsim, pb, device_bin, elf, "0x0", update_bin, base, page, baud, app_bin, workdir, mcu, hz)
     print("re-home from address 0: converged")
 
     # The staging slot: erased flash with the loader sitting exactly where
-    # a staging copy would — the tool must leave it in place and let it
+    # a staging copy would - the tool must leave it in place and let it
     # stream the (different) update build into the resident slot.
     stage = base - pb.SLOT
-    rehome_from(pbsim, pb, device_bin, elf, hex(stage), hex(stage), update_bin, base, page, baud, app_bin, workdir,
+    rehome_from(pbsim, pb, device_bin, elf, hex(stage), update_bin, base, page, baud, app_bin, workdir,
                 mcu, hz)
     print("re-home from the staging slot: converged")
 

test/pbreloc.py

@@ -1,9 +1,8 @@
 #!/usr/bin/env python3
 """Position-independence acceptance test: the identical binary, flashed one
 slot below the resident, must serve the complete command set from there. The
-info block must come back byte-identical, the write guard must refuse the
-staged copy's own slot and permit the resident's, and the staged copy must be
-able to rewrite the resident verbatim.
+info block must come back byte-identical, and the staged copy must be able to
+rewrite the resident verbatim - which is the whole of what relocation is for.
 
 Usage: pbreloc.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
                   <baud> <tool_py> <workdir>
@@ -64,20 +63,16 @@ def main():
         if loader.read_eeprom(0, len(pattern)) != pattern:
             fail("EEPROM round-trip through the staged copy")
 
-        # The guard, both ways: its own slot refused (drained, unchanged), the
-        # resident slot writable. The refusal leaves its drained words in the
-        # SPM buffer, so the write that follows may take them — and clears
-        # them by writing, so the retry must not.
-        before = loader.read_flash(stage, page)
-        loader.write_page(stage, bytes(page))
-        if loader.read_flash(stage, page) != before:
-            fail("the staged copy's guard let its own slot change")
+        # The resident slot, written from the copy standing beside it - the
+        # whole point of relocating. There is no running-slot guard to probe
+        # against: nothing here refuses an address, and a copy that erases the
+        # page it is executing from does not come back to report it.
+        # pbselfwrite.py gates that direction on a device it is allowed to
+        # destroy.
         marker = bytes((i * 3) & 0xFF for i in range(page))
         loader.write_page(base, marker)
         if loader.read_flash(base, page) != marker:
-            loader.write_page(base, marker)
-            if loader.read_flash(base, page) != marker:
-                fail("the staged copy could not write the resident slot, even on retry")
+            fail("the staged copy could not write the resident slot")
 
         # Restore the resident image through the staged copy, then 'J' back
         # into it and prove it lives.

test/pbselfwrite.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""The seal, gated on the one command that proves it: erase the page the
+loader is executing from.
+
+pureboot 9 dropped the running-slot write guard, so this command is now
+permitted - that is what lets a resident copy plant something in its own slot,
+which on a chip whose boot section *is* the loader slot is the only route a
+self-update has. Permitted means the loader must actually do it, and the only
+honest proof is the flash afterwards.
+
+What stands in the guard's place is the seal, and the two halves are tested
+against each other here: the identical destructive command, refused when its
+seal is wrong and honoured when it is right. A test that only showed the
+refusal would pass just as well against a loader that ignores SPM entirely.
+
+Usage: pbselfwrite.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                      <baud> <tool_py> <workdir>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def sealed_frame(pb, op, space, address, count):
+    """A command header and its seal, built here rather than borrowed from the
+    tool: this test is about what the loader accepts, and a probe that shares
+    the host's frame builder cannot tell a wrong frame from a wrong loader."""
+    head = bytes((op, pb.selector(space, address), address & 0xFF,
+                  (address >> 8) & 0xFF, count & 0xFF))
+    seal = pb.SEAL
+    for byte in head:
+        seal ^= byte
+    return head + bytes((seal,))
+
+
+def main():
+    device_bin, elf, mcu, hz, base_hex, page, baud, tool, workdir = sys.argv[1:]
+    base, page, baud = int(base_hex, 0), int(page), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+    import pureboot as pb
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "dump.bin")
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump)
+    try:
+        port = pb.Port(device.pty, baud)
+        loader = pb.Loader(port)
+        info = loader.connect(25)
+        if info.version < pb.SEALED_LOADER:
+            fail(f"this test is for pureboot {pb.SEALED_LOADER} and later, not {info.version}")
+
+        # Erase the first page of the running slot: the entry stub and the
+        # command loop are both in it, so a loader that performs this does not
+        # answer again. Nothing else in the protocol is as sharp a probe.
+        frame = sealed_frame(pb, pb.OP_WRITE, pb.SP_SPM, base, pb.SPM_ERASE)
+
+        # Red: the same command with one bit wrong in its seal. Refused before
+        # anything happens, and the loader is still there to say so.
+        broken = bytearray(frame)
+        broken[-1] ^= 0x01
+        port.write(bytes(broken))
+        if port.read_exact(1, 5.0) != pb.NAK:
+            fail("an unsealed erase of the running page was not refused")
+        if port.read_exact(1, 5.0) != pb.PROMPT:
+            fail("the loader did not re-prompt after refusing the erase")
+        alive = loader.read_flash(base, 8)
+        if alive == b"\xff" * 8:
+            fail("the refused erase happened anyway - the running page reads erased")
+
+        # Green: the identical command, correctly sealed. Nothing is required
+        # of the link from here on. The verdict is *issued* before the SPM, but
+        # the erase takes the code that would have finished saying it, and how
+        # much of it survives is the chip's business - an erase removes one page
+        # and nothing else, so a loader whose command loop lives past the page
+        # erased will prompt as usual where one with 128-byte pages goes with
+        # the stub. None of that is the claim. The claim is that the erase
+        # reached flash, and the dump is both the only witness for it and a
+        # better one: it tells "accepted and performed" from "merely answered".
+        port.write(frame)
+        try:
+            port.read_exact(2, 2.0)
+        except pb.Error:
+            pass
+        port.close()
+    finally:
+        device.stop()
+
+    # Ground truth: the simulator's flash, not the loader's opinion of it.
+    flash = open(dump, "rb").read()
+    if flash[base : base + page] != b"\xff" * page:
+        fail("the sealed erase did not reach flash - the running page is intact")
+    print("pbselfwrite: the running slot is refused unsealed and erased sealed")
+
+
+main()

test/pbsim.py

@@ -8,14 +8,17 @@ import subprocess
 
 
 class Device:
-    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None, link=None):
+    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None, link=None,
+                 window=False):
         cmd = [binary]
         if link:
             cmd += ["-l", link]
+        if window:
+            cmd.append("-w")  # report the first-transmit cycle, free-run idle
         cmd += [elf, mcu, hz, base_hex, str(page), str(baud), dump]
         if reset_hex is not None or resume is not None:
-            # Chips without a hardware boot section — the tinies and the
-            # m48s — reset to address 0 like silicon; the boot-sectioned
+            # Chips without a hardware boot section - the tinies and the
+            # m48s - reset to address 0 like silicon; the boot-sectioned
             # megas re-vector to the loader base (BOOTRST).
             patch = not mcu.startswith("atmega") or mcu.startswith("atmega48")
             cmd.append(reset_hex if reset_hex is not None else ("0" if patch else base_hex))
@@ -41,7 +44,7 @@ class Device:
         self.proc.send_signal(signal.SIGUSR1)
 
     def power_fail(self):
-        """SIGTERM: the runner dumps its flash and exits — the image a
+        """SIGTERM: the runner dumps its flash and exits - the image a
         restart resumes from."""
         self.stop()
         return self.dump

test/pbtest.py

@@ -11,6 +11,7 @@ loader built off the chip's natural serial default.
 """
 
 import os
+import re
 import sys
 
 
@@ -20,7 +21,7 @@ def fail(message):
 
 
 def rjmp_decode(word, at, flash_words):
-    """Where an rjmp word at word-address `at` lands — deliberately written
+    """Where an rjmp word at word-address `at` lands - deliberately written
     against the instruction-set definition (12-bit signed offset), not with
     the host tool's encoder, so an encoding bug cannot verify itself."""
     if word & 0xF000 != 0xC000:
@@ -55,6 +56,12 @@ def main():
     # the page byte is the wire's 0-means-256.
     mega = mcu.startswith("atmega")
     patch = not mega or mcu.startswith("atmega48")
+    # Where SRAM begins: the x8 and x4 megas push it past their extended I/O
+    # space, everything else starts right after the plain I/O registers. The
+    # loader keeps no statics and its stack sits at RAMEND, so the first SRAM
+    # byte is free for the data-space probe below.
+    classic = mcu in ("atmega8", "atmega8a", "atmega16", "atmega16a", "atmega32", "atmega32a")
+    ram_base = 0x0100 if mega and not classic else 0x0060
     word_flash = base + pb.SLOT > 0x10000
     wire_base = base // 2 if word_flash else base
     flags = (1 if patch else 0) | (2 if word_flash else 0)
@@ -64,21 +71,34 @@ def main():
         + bytes([flags])
     )
 
+    # A shared-line link (RX == TX in the -l spec) makes the host read every
+    # byte it sends back off the line; all sessions then discard the echo.
+    one_wire = bool(link) and re.fullmatch(r"sw:([A-H][0-7]),\1(@[01])?", link) is not None
+    extra = ("--one-wire",) if one_wire else ()
+
     device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, link=link)
     try:
         # Session 1: knock from reset, identify, program everything, stay.
-        out = pbsim.run_tool(tool, device.pty, baud, "--info", "--fuses", "--flash", app_bin,
+        out = pbsim.run_tool(tool, device.pty, baud, *extra, "--info", "--fuses", "--flash", app_bin,
                              "--eeprom", ee_path, "--stay")
         for needed in ("version", "signature", "fuses", "verify:", "stays"):
             if needed not in out:
                 fail(f"session 1 output lacks {needed!r}")
 
-        # Session 2: reconnect into the live session, verify, dump, hand over
-        # is deferred — the pty must be reopened for the APP banner first.
-        out = pbsim.run_tool(tool, device.pty, baud, "--verify-flash", app_bin, "--verify-eeprom", ee_path,
-                             "--read-flash", read_flash, "--read-eeprom", read_eeprom, "--stay")
+        # Session 2: reconnect into the live session, verify, dump, exercise
+        # the data space; hand over is deferred - the pty must be reopened for
+        # the APP banner first.
+        probe = "c0ffee"
+        out = pbsim.run_tool(tool, device.pty, baud, *extra, "--verify-flash", app_bin, "--verify-eeprom", ee_path,
+                             "--read-flash", read_flash, "--read-eeprom", read_eeprom,
+                             "--poke", f"{ram_base:#x}:{probe}", "--peek", f"{ram_base:#x}:3", "--stay")
         if out.count("verify:") != 2:
             fail("session 2 did not verify both memories")
+        # What went into SRAM must come back out of it: the data space is one
+        # more selector on the same transfer as flash and EEPROM, so a wrong
+        # selector decode would show up here and nowhere else.
+        if probe not in out.replace(" ", ""):
+            fail(f"data-space round trip at {ram_base:#x} did not read back {probe}\n{out}")
 
         eeprom_back = open(read_eeprom, "rb").read()
         if eeprom_back[: len(ee_image)] != ee_image:
@@ -97,11 +117,13 @@ def main():
         # land in the application, which banners on the same link.
         device.reset()
         port = pb.Port(device.pty, baud)
+        if one_wire:
+            port = pb.OneWirePort(port)
         try:
             loader = pb.Loader(port)
             live = loader.connect(15)
             # The loader built from this tree must report a version the tool
-            # beside it speaks — a bump the tool was never told about is a
+            # beside it speaks - a bump the tool was never told about is a
             # loader it would refuse to talk to. Not equality with the newest:
             # the tool now spans two loader generations, the fixed-baud one
             # here and the unified autobaud loader that follows it.
@@ -109,15 +131,42 @@ def main():
                 fail(f"loader reports pureboot {live.version}, the tool speaks "
                      f"{pb.OLDEST_LOADER}..{pb.NEWEST_LOADER}")
 
-            # A W addressed inside a page rather than at its base must still
-            # consume exactly one page and prompt. The loader's own slot is
-            # the target — it is drained and never programmed — and the
-            # payload is erased-state bytes, so the probe can disturb neither
-            # the image nor the page buffer it leaves behind.
-            wire = wire_base + 1
-            port.write(bytes((ord("W"), wire & 0xFF, wire >> 8)) + b"\xff" * page)
+            # A fill addressed inside a page rather than at its base must still
+            # consume exactly one page and prompt. The loader's own slot is the
+            # target and the payload is erased-state bytes, so the probe can
+            # disturb neither the image nor the page buffer it leaves behind: a
+            # fill only loads the buffer, and nothing commits it. Hand-built
+            # rather than through write_page(), which would follow the fill
+            # with its erase and write; the point here is that the fill alone
+            # consumes exactly one page whatever the address's low bits say.
+            # Hand-sealed too - a protocol probe that borrowed the tool's own
+            # frame builder could not tell a wrong frame from a wrong loader.
+            wire = base + 1
+            head = bytes((pb.OP_FILL, pb.selector(pb.SP_FLASH, wire), wire & 0xFF,
+                          (wire >> 8) & 0xFF, page & 0xFF))
+            seal = pb.SEAL
+            for byte in head:
+                seal ^= byte
+            port.write(head + bytes((seal,)))
             if port.read_exact(1, 5.0) != pb.PROMPT:
-                fail("unaligned W did not return to the prompt")
+                fail("the loader refused a correctly sealed fill")
+            port.write(b"\xff" * page)
+            if port.read_exact(1, 5.0) != pb.PROMPT:
+                fail("unaligned fill did not return to the prompt")
+
+            # And the seal itself, red: one wrong bit in the address of that
+            # same frame must be refused outright. The verdict has to arrive
+            # *before* the page would have been sent - that ordering is what
+            # keeps a refusal from turning into a desync - so the probe sends
+            # no payload at all and expects the loader straight back at the
+            # command level.
+            broken = bytearray(head + bytes((seal,)))
+            broken[2] ^= 0x01
+            port.write(bytes(broken))
+            if port.read_exact(1, 5.0) != pb.NAK:
+                fail("a header with a broken seal was not refused")
+            if port.read_exact(1, 5.0) != pb.PROMPT:
+                fail("the loader did not re-prompt after refusing a broken seal")
 
             loader.run_application()
             banner = port.read_exact(3, 5.0)
@@ -137,7 +186,7 @@ def main():
 
     # The surgery, decoded independently: the patched vector must land on the
     # loader, the trampoline on the application's own entry (patched-vector
-    # chips only — a boot-sectioned mega's word 0 stays the application's).
+    # chips only - a boot-sectioned mega's word 0 stays the application's).
     if patch:
         flash_words = (base + pb.SLOT) // 2
         app = open(app_bin, "rb").read()

test/pbupdate.py

@@ -4,8 +4,8 @@ itself with a re-timed build, and every power-fail phase is rehearsed by
 killing the device mid-write, restarting it from its flash dump, and letting
 a re-run complete the update.
 
-The boot-sectioned megas run the BOOTRST-unprogrammed profile — reset boots
-the application, whose 'L' is the application-owned loader entry — with
+The boot-sectioned megas run the BOOTRST-unprogrammed profile - reset boots
+the application, whose 'L' is the application-owned loader entry - with
 --assume-fuses standing in for the fuse read simavr cannot model.
 
 Usage: pbupdate.py <device_bin> <pureboot_elf> <update_elf> <mcu> <hz>
@@ -39,8 +39,8 @@ class PowerFail(Exception):
 
 def assumed_fuses(pb, image):
     """Synthetic 'F' bytes for --assume-fuses: the smallest boot section
-    covering both the resident and the staging slot (two slots — what a
-    self-update needs), BOOTRST unprogrammed — the per-chip BOOTSZ ladder
+    covering both the resident and the staging slot (two slots - what a
+    self-update needs), BOOTRST unprogrammed - the per-chip BOOTSZ ladder
     and fuse byte come from the tool's own table, keyed by the update
     image's embedded signature."""
     info = pb.image_info(image)
@@ -138,7 +138,7 @@ def main():
     device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=reset_hex)
     final = "v0"
     try:
-        # The application first — its planner output is the restore truth.
+        # The application first - its planner output is the restore truth.
         pbsim.run_tool(tool, device.pty, baud, "--flash", app_bin, "--stay")
         port, loader = connect(device)
         app_pages = pb.plan_flash(open(app_bin, "rb").read(), loader.info)
@@ -167,7 +167,7 @@ def main():
         # direction so the flash is never already at its target. The mega's
         # mid-resident-rewrite loss is exercised as a host crash instead:
         # with BOOTRST unprogrammed and the resident mid-erase, a power loss
-        # there has no reset path into the staging copy — the documented
+        # there has no reset path into the staging copy - the documented
         # cost of that profile (README).
         for kill_region, kill_hits, kill_device in (
             ("stage", 2, True),

test/pbwindow.py

@@ -0,0 +1,137 @@
+#!/usr/bin/env python3
+"""The activation window as a behavioral duration gate.
+
+The loader's window is a counted poll loop whose per-poll cost is hand-counted
+in the source (`link::poll_cycles`) - but the loop compiles in consumer
+context, so only the running image can prove the count. This test installs a
+real application beside the loader (the host tool's own `plan_flash` supplies
+the reset-vector surgery), starts the simulator with the line idle, and reads
+the cycle of the first transmit activity: nothing talks until the window
+closes and the application banners, so that cycle *is* the window, give or
+take a banner lead measured in microseconds. Asserted at +/-2 % - one
+mis-counted cycle per poll shifts a window by 10 % and more.
+
+Fixed-baud loaders declare their window in seconds (--seconds, the build's
+TIMEOUT). The autobaud loader's window is its calibration poll budget
+(--autobaud-polls); the seconds it amounts to are budget x 9 / f_cpu, the
+measured cost of the calibrate() wait loop this gate pins.
+"""
+import argparse
+import importlib.util
+import pathlib
+import select
+import sys
+import time
+
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent))
+from pbsim import Device
+
+# The calibrate() budget loop's cycles per poll in the built image - what the
+# README's window arithmetic rests on, verified here. A measured fact, not a
+# design constant: the wait's exit branches land where the compiler's block
+# layout puts them, and the bounded-calibration rework moved the loop from
+# ten cycles to nine.
+AUTOBAUD_POLL_CYCLES = 9
+
+
+def load_tool(path):
+    spec = importlib.util.spec_from_file_location("pureboot", path)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def compose_flash(pb, loader_bytes, app_bytes, mcu, base, page):
+    """The flash image a completed programming session leaves: application
+    (with the tinies' vector surgery), loader at base - built through the
+    host tool's own planner so the surgery is the shipped one, not a copy."""
+    flash_size = base + pb.SLOT
+    patch = not mcu.startswith("atmega") or mcu.startswith("atmega48")
+    word_flash = flash_size > 0x10000
+    wire_base = base // 2 if word_flash else base
+    flags = (1 if patch else 0) | (2 if word_flash else 0)
+    raw = bytes((ord("P"), ord("B"), 5, 0, 0, 0, page & 0xFF,
+                 wire_base & 0xFF, wire_base >> 8, 0, 0, flags))
+    info = pb.Info(raw)
+
+    flash = bytearray(b"\xff" * flash_size)
+    for address, content in pb.plan_flash(app_bytes, info).items():
+        flash[address:address + len(content)] = content
+    flash[base:base + len(loader_bytes)] = loader_bytes
+    return bytes(flash)
+
+
+def first_tx_cycle(device, deadline):
+    """The PB_WINDOW_TX report, or None. The runner prints it once."""
+    stream = device.proc.stdout
+    while True:
+        remaining = deadline - time.monotonic()
+        if remaining <= 0:
+            return None
+        ready, _, _ = select.select([stream], [], [], remaining)
+        if not ready:
+            return None
+        line = stream.readline()
+        if not line:
+            return None
+        if line.startswith("PB_WINDOW_TX"):
+            return int(line.split()[1])
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--device", required=True)
+    parser.add_argument("--loader", required=True)
+    parser.add_argument("--mcu", required=True)
+    parser.add_argument("--hz", type=int, required=True)
+    parser.add_argument("--base", required=True)
+    parser.add_argument("--page", type=int, required=True)
+    parser.add_argument("--baud", type=int, required=True)
+    parser.add_argument("--app", required=True)
+    parser.add_argument("--tool", required=True)
+    parser.add_argument("--workdir", required=True)
+    parser.add_argument("--link", default=None)
+    parser.add_argument("--seconds", type=float, default=None)
+    parser.add_argument("--autobaud-polls", type=int, default=None)
+    args = parser.parse_args()
+    if (args.seconds is None) == (args.autobaud_polls is None):
+        parser.error("exactly one of --seconds / --autobaud-polls")
+
+    pb = load_tool(args.tool)
+    base = int(args.base, 0)
+    expected = (args.seconds if args.seconds is not None
+                else args.autobaud_polls * AUTOBAUD_POLL_CYCLES / args.hz)
+
+    work = pathlib.Path(args.workdir)
+    work.mkdir(parents=True, exist_ok=True)
+    # Every loader target objcopies its slot content beside the ELF (.bin).
+    loader_bytes = pathlib.Path(args.loader + ".bin").read_bytes()
+    app_bytes = pathlib.Path(args.app).read_bytes()
+    flash_file = work / "window-flash.bin"
+    flash_file.write_bytes(compose_flash(pb, loader_bytes, app_bytes, args.mcu, base, args.page))
+
+    device = Device(args.device, args.loader, args.mcu, str(args.hz), args.base, args.page,
+                    args.baud, str(work / "window-dump.bin"), resume=str(flash_file),
+                    link=args.link, window=True)
+    try:
+        # Simulation speed is machine-dependent; a few hundred thousand
+        # cycles per wall second is the pessimistic floor.
+        budget = max(60.0, expected * args.hz / 300000)
+        cycle = first_tx_cycle(device, time.monotonic() + budget)
+    finally:
+        device.stop()
+
+    if cycle is None:
+        print(f"  [FAIL] no transmit activity within {budget:.0f} s wall "
+              f"(expected a {expected:.2f} s window)")
+        return 1
+    measured = cycle / args.hz
+    error = (measured - expected) / expected
+    ok = abs(error) <= 0.02
+    print(f"  [{'PASS' if ok else 'FAIL'}] window {measured:.3f} s vs declared "
+          f"{expected:.3f} s ({error:+.1%}, gate +/-2%)")
+    return 0 if ok else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

test/pureboot_device.c

@@ -1,461 +0,0 @@
-// simavr "device" for the pureboot protocol tests, every chip. Loads the
-// boot-linked ELF at the loader base, starts execution there (BOOTRST / the
-// patched vector are not what is under test), and exposes the loader's
-// serial link as a pty for the real host tool:
-//
-//   - Hardware USART builds: simavr's uart_pty on the selected instance.
-//   - Software UART builds: an 8N1 bridge between a pty and the GPIO pins,
-//     timed against the simulated cycle counter (drives the loader's RX,
-//     decodes its TX).
-//
-// The link follows the chip's natural default (USART0 on the megas, the
-// software UART on PB0/PB1 elsewhere) unless -l overrides it: `-l usart1`
-// for the second instance, `-l sw:B5,B1` for a software build's RX,TX pins.
-//
-// simavr's tiny cores decode the SPM opcode but attach no NVM module — SPM
-// is a silent no-op (the mega's boot section has one, avr_flash). The
-// missing module is supplied here: the SPM ioctl reads SPMCSR/Z/r1:r0 and
-// implements buffer fill, page erase, page write, and CTPB, completing
-// instantly. RFLB's LPM diversion (fuse readout) stays unmodeled, so the
-// 'F' command answers with flash bytes — the tests assert transport only.
-//
-// On exit (or SIGTERM) the flash and EEPROM are dumped to files for a
-// ground-truth cross-check against what the host read back.
-#include <fcntl.h>
-#include <pty.h>
-#include <signal.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <termios.h>
-#include <unistd.h>
-
-#include "avr_eeprom.h"
-#include "avr_flash.h"
-#include "avr_ioport.h"
-#include "avr_uart.h"
-#include "sim_avr.h"
-#include "sim_elf.h"
-#include "sim_io.h"
-#include "uart_pty.h"
-
-static avr_t *avr;
-static uart_pty_t uart_pty;
-static int link_software;
-static char uart_digit = '0';
-static char sw_rx_port = 'B', sw_tx_port = 'B';
-static int sw_rx_bit = 0, sw_tx_bit = 1;
-static const char *dump_path;
-static uint32_t reset_pc;
-static volatile sig_atomic_t reset_requested;
-
-static int parse_link(const char *spec)
-{
-	if (strcmp(spec, "usart0") == 0 || strcmp(spec, "usart1") == 0) {
-		link_software = 0;
-		uart_digit = spec[5];
-		return 0;
-	}
-	if (strncmp(spec, "sw", 2) == 0) {
-		link_software = 1;
-		if (spec[2] == '\0')
-			return 0;
-		if (sscanf(spec + 2, ":%c%d,%c%d", &sw_rx_port, &sw_rx_bit, &sw_tx_port, &sw_tx_bit) == 4)
-			return 0;
-	}
-	return -1;
-}
-
-// simavr 1.6's avr_flash PGERS handler erases spm_pagesize bytes starting at
-// Z & ~1 instead of the page containing Z (its PGWRT path masks correctly) —
-// hardware ignores the in-page bits (§26.8.1), so an erase issued with Z
-// anywhere inside the page wipes half the neighbouring page in simulation
-// only. Wrap the mega's registered flash ioctl and re-dispatch page erases
-// with Z forced to the page boundary; everything else passes through.
-//
-// A second gap on the boot-section-less m48s: their RWWSRE bit is the
-// temporary-buffer discard (Atmel-8271 §26.2/§26.3.1), but the stock model
-// gates its RWWSRE branch on AVR_SELFPROG_HAVE_RWW — absent on the m48
-// core — so the discard store falls through into the buffer-fill branch and
-// plants whatever Z/R1:R0 happen to hold. Perform the silicon's discard
-// here instead.
-static avr_flash_t *mega_flash;
-static int (*mega_flash_ioctl)(avr_io_t *io, uint32_t ctl, void *param);
-
-static int fixed_flash_ioctl(avr_io_t *io, uint32_t ctl, void *param)
-{
-	if (ctl == AVR_IOCTL_FLASH_SPM && avr_regbit_get(io->avr, mega_flash->pgers)) {
-		uint16_t z = (uint16_t)(io->avr->data[30] | (io->avr->data[31] << 8));
-		uint16_t masked = (uint16_t)(z & ~(mega_flash->spm_pagesize - 1));
-		io->avr->data[30] = (uint8_t)masked;
-		io->avr->data[31] = (uint8_t)(masked >> 8);
-		int result = mega_flash_ioctl(io, ctl, param);
-		io->avr->data[30] = (uint8_t)z;
-		io->avr->data[31] = (uint8_t)(z >> 8);
-		return result;
-	}
-	if (ctl == AVR_IOCTL_FLASH_SPM && !(mega_flash->flags & AVR_SELFPROG_HAVE_RWW) &&
-	    (io->avr->data[mega_flash->r_spm] & 0x11) == 0x11) { // RWWSRE|SELFPRGEN: the m48 buffer discard
-		for (int i = 0; i < mega_flash->spm_pagesize / 2; i++) {
-			mega_flash->tmppage[i] = 0xffff;
-			mega_flash->tmppage_used[i] = 0;
-		}
-		avr_regbit_clear(io->avr, mega_flash->selfprgen);
-		return 0;
-	}
-	return mega_flash_ioctl(io, ctl, param);
-}
-
-static void fix_mega_flash_erase(void)
-{
-	for (avr_io_t *io = avr->io_port; io; io = io->next) {
-		if (io->kind && strcmp(io->kind, "flash") == 0) {
-			mega_flash = (avr_flash_t *)io;
-			mega_flash_ioctl = io->ioctl;
-			io->ioctl = fixed_flash_ioctl;
-			return;
-		}
-	}
-	fprintf(stderr, "device: no flash module to fix — SPM page erases may misalign\n");
-}
-
-static void request_reset(int sig)
-{
-	(void)sig;
-	reset_requested = 1;
-}
-
-// ------------------------------------------------------------- tiny NVM ---
-
-typedef struct {
-	avr_io_t io;
-	uint8_t buffer[128];
-	uint8_t used[128]; // a buffer word loads once until erased — like silicon
-	unsigned page;
-} tiny_nvm_t;
-
-static tiny_nvm_t nvm;
-
-static int nvm_ioctl(avr_io_t *io, uint32_t ctl, void *param)
-{
-	(void)param;
-	if (ctl != AVR_IOCTL_FLASH_SPM)
-		return -1;
-	tiny_nvm_t *n = (tiny_nvm_t *)io;
-	avr_t *mcu = io->avr;
-	uint8_t command = mcu->data[0x57] & 0x1f; // SPMCSR, both tinies
-	uint16_t z = (uint16_t)(mcu->data[30] | (mcu->data[31] << 8));
-	uint32_t page_base = (uint32_t)(z & ~(n->page - 1)) % (mcu->flashend + 1);
-	if (command == 0x01) { // SPMEN alone: buffer fill from r1:r0
-		unsigned offset = z & (n->page - 1) & ~1u;
-		if (!n->used[offset]) { // first write wins until the buffer clears
-			n->buffer[offset] = mcu->data[0];
-			n->buffer[offset + 1] = mcu->data[1];
-			n->used[offset] = 1;
-		}
-	} else if (command == 0x03) { // PGERS
-		memset(mcu->flash + page_base, 0xff, n->page);
-	} else if (command == 0x05) { // PGWRT: programming only clears bits
-		for (unsigned i = 0; i < n->page; i++)
-			mcu->flash[page_base + i] &= n->buffer[i];
-		memset(n->buffer, 0xff, n->page);
-		memset(n->used, 0, n->page);
-	} else if (command == 0x11) { // CTPB
-		memset(n->buffer, 0xff, n->page);
-		memset(n->used, 0, n->page);
-	}
-	mcu->data[0x57] &= (uint8_t)~0x1f; // the operation completes instantly
-	return 0;
-}
-
-// ----------------------------------------------------------- GPIO bridge ---
-
-static int pty_master = -1;
-static avr_irq_t *rx_pin; // the loader's RX (PB0), driven from the pty
-static avr_cycle_count_t bit_cycles;
-
-static int tx_level = 1, tx_active, tx_bit;
-static uint8_t tx_shift;
-
-static avr_cycle_count_t tx_sample(avr_t *mcu, avr_cycle_count_t when, void *param)
-{
-	(void)mcu;
-	(void)param;
-	tx_shift = (uint8_t)((tx_shift >> 1) | (tx_level ? 0x80 : 0));
-	if (++tx_bit < 8)
-		return when + bit_cycles;
-	if (write(pty_master, &tx_shift, 1) != 1)
-		fprintf(stderr, "device: pty write lost a byte\n");
-	tx_active = 0;
-	return 0;
-}
-
-static void tx_hook(avr_irq_t *irq, uint32_t value, void *param)
-{
-	(void)irq;
-	(void)param;
-	int level = value & 1;
-	if (!tx_active && tx_level == 1 && level == 0) { // start edge
-		tx_active = 1;
-		tx_bit = 0;
-		avr_cycle_timer_register(avr, bit_cycles + bit_cycles / 2, tx_sample, NULL);
-	}
-	tx_level = level;
-}
-
-static uint8_t rx_queue[8192];
-static unsigned rx_head, rx_tail; // ring: head = next to send
-static int rx_active, rx_bit;
-static uint8_t rx_byte;
-
-static void rx_start_next(void);
-
-static avr_cycle_count_t rx_step(avr_t *mcu, avr_cycle_count_t when, void *param)
-{
-	(void)mcu;
-	(void)param;
-	if (rx_bit < 8) {
-		avr_raise_irq(rx_pin, (rx_byte >> rx_bit) & 1);
-		rx_bit++;
-		return when + bit_cycles;
-	}
-	if (rx_bit == 8) { // stop bit, plus one idle bit of margin
-		avr_raise_irq(rx_pin, 1);
-		rx_bit++;
-		return when + 2 * bit_cycles;
-	}
-	rx_active = 0;
-	rx_start_next();
-	return 0;
-}
-
-static void rx_start_next(void)
-{
-	if (rx_active || rx_head == rx_tail)
-		return;
-	rx_byte = rx_queue[rx_head];
-	rx_head = (rx_head + 1) % sizeof(rx_queue);
-	rx_active = 1;
-	rx_bit = 0;
-	avr_raise_irq(rx_pin, 0); // start bit
-	avr_cycle_timer_register(avr, bit_cycles, rx_step, NULL);
-}
-
-// A reset abandons whatever the bridge was mid-transfer: bytes still queued
-// for a chip that no longer has the context to receive them meaningfully,
-// and a decode in progress on a TX line the reset may have already changed.
-// The pending cycle timers must go with the state: avr_reset drops the TX
-// output latch, whose falling edge starts a spurious decode before this
-// runs, and a stale tx_sample would then interleave with the loader's first
-// real answer through the shared shift state, corrupting it.
-static void bridge_reset(void)
-{
-	avr_cycle_timer_cancel(avr, tx_sample, NULL);
-	avr_cycle_timer_cancel(avr, rx_step, NULL);
-	rx_head = rx_tail = 0;
-	rx_active = 0;
-	tx_active = 0;
-	tx_level = 1;
-	avr_raise_irq(rx_pin, 1); // idle line
-}
-
-static void poll_pty(void)
-{
-	uint8_t chunk[256];
-	ssize_t got = read(pty_master, chunk, sizeof(chunk));
-	for (ssize_t i = 0; i < got; i++) {
-		unsigned next = (rx_tail + 1) % sizeof(rx_queue);
-		if (next == rx_head)
-			break; // full: the host will retry on timeout
-		rx_queue[rx_tail] = chunk[i];
-		rx_tail = next;
-	}
-	if (got > 0)
-		rx_start_next();
-}
-
-// ------------------------------------------------------------------ main ---
-
-static void finish(int sig)
-{
-	(void)sig;
-	if (dump_path) {
-		FILE *f = fopen(dump_path, "wb");
-		if (f) {
-			fwrite(avr->flash, 1, avr->flashend + 1, f);
-			fclose(f);
-		}
-		avr_eeprom_desc_t ee = {.ee = NULL, .offset = 0, .size = 0};
-		if (avr_ioctl(avr, AVR_IOCTL_EEPROM_GET, &ee) == 0 && ee.ee && ee.size) {
-			char path[512];
-			snprintf(path, sizeof(path), "%s.eeprom", dump_path);
-			f = fopen(path, "wb");
-			if (f) {
-				fwrite(ee.ee, 1, ee.size, f);
-				fclose(f);
-			}
-		}
-	}
-	if (!link_software)
-		uart_pty_stop(&uart_pty);
-	_exit(0);
-}
-
-int main(int argc, char *argv[])
-{
-	int link_given = 0;
-	for (int opt; (opt = getopt(argc, argv, "l:")) != -1;) {
-		if (opt != 'l' || parse_link(optarg) != 0) {
-			fprintf(stderr, "device: bad link spec (usart0, usart1, sw, or sw:B0,B1 as RX,TX)\n");
-			return 2;
-		}
-		link_given = 1;
-	}
-	int args = argc - optind;
-	if (args < 7 || args > 9) {
-		fprintf(stderr,
-		        "usage: %s [-l link] <pureboot.elf> <mcu> <hz> <base_hex> <page> <baud> <flash_dump>"
-		        " [reset_hex] [resume_flash]\n"
-		        "  -l link:      usart0 | usart1 | sw[:B0,B1] (RX,TX); default: the chip's own\n"
-		        "  reset_hex:    reset vector (default: base with a boot section, else 0)\n"
-		        "  resume_flash: raw full-flash image loaded instead of the ELF — a prior\n"
-		        "                run's dump, for power-fail resume tests\n",
-		        argv[0]);
-		return 2;
-	}
-	argv += optind - 1; // argv[1] is the ELF again, whatever was parsed
-	const char *mcu_name = argv[2];
-	uint32_t base = (uint32_t)strtoul(argv[4], NULL, 0);
-	unsigned page = (unsigned)atoi(argv[5]);
-	unsigned baud = (unsigned)atoi(argv[6]);
-	dump_path = argv[7];
-	int is_mega = strncmp(mcu_name, "atmega", 6) == 0;
-	if (!link_given)
-		link_software = !is_mega; // the chips' natural links: USART0, or PB0/PB1
-
-	avr = avr_make_mcu_by_name(mcu_name);
-	if (!avr) {
-		fprintf(stderr, "device: no %s core\n", mcu_name);
-		return 1;
-	}
-	avr_init(avr);
-	avr->frequency = (uint32_t)strtoul(argv[3], NULL, 0);
-	memset(avr->flash, 0xff, avr->flashend + 1); // real flash powers up erased
-
-	if (args > 8) {
-		// Resume: the full flash image of an interrupted prior run.
-		FILE *f = fopen(argv[9], "rb");
-		if (!f || fread(avr->flash, 1, avr->flashend + 1, f) == 0) {
-			fprintf(stderr, "device: cannot read %s\n", argv[9]);
-			return 1;
-		}
-		fclose(f);
-	} else {
-		elf_firmware_t fw = {0};
-		if (elf_read_firmware(argv[1], &fw) != 0) {
-			fprintf(stderr, "device: cannot read %s\n", argv[1]);
-			return 1;
-		}
-		memcpy(avr->flash + base, fw.flash, fw.flashsize);
-	}
-	// The boot-sectioned megas enter the loader in hardware (BOOTRST, not
-	// modeled — the argument picks the modeled fuse's target); the tinies
-	// and the boot-section-less m48s reset to word 0 like silicon — erased
-	// flash walks up into the loader, and after the host's surgery the
-	// patched vector routes there.
-	int boot_section = is_mega && strncmp(mcu_name, "atmega48", 8) != 0;
-	reset_pc = args > 7 ? (uint32_t)strtoul(argv[8], NULL, 0) : (boot_section ? base : 0);
-	avr->pc = reset_pc;
-	avr->codeend = avr->flashend;
-
-	// Erased EEPROM, as hardware powers up (simavr zeroes it).
-	uint8_t blank[1024];
-	memset(blank, 0xff, sizeof(blank));
-	avr_eeprom_desc_t seed = {.ee = blank, .offset = 0, .size = 0};
-	if (avr_ioctl(avr, AVR_IOCTL_EEPROM_GET, &seed) == 0 && seed.size <= sizeof(blank)) {
-		seed.ee = blank;
-		avr_ioctl(avr, AVR_IOCTL_EEPROM_SET, &seed);
-	}
-
-	// The megas carry simavr's avr_flash module (and its two gaps the wrap
-	// above fixes); the tinies get the NVM module simavr lacks. Which serial
-	// bridge runs is the link's business, not the chip class's.
-	if (is_mega) {
-		fix_mega_flash_erase();
-	} else {
-		nvm.page = page;
-		memset(nvm.buffer, 0xff, sizeof(nvm.buffer));
-		nvm.io.kind = "tiny_nvm";
-		nvm.io.ioctl = nvm_ioctl;
-		avr_register_io(avr, &nvm.io);
-	}
-
-	if (!link_software) {
-		// POLL_SLEEP paces an idle-polling loader in host real time (a
-		// no-hardware CPU-saving hack); clear it so cycles run free.
-		uint32_t flags = 0;
-		avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS(uart_digit), &flags);
-		flags &= ~AVR_UART_FLAG_POLL_SLEEP;
-		avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS(uart_digit), &flags);
-		uart_pty_init(avr, &uart_pty);
-		uart_pty_connect(&uart_pty, uart_digit);
-		printf("PB_PTY %s\n", uart_pty.pty.slavename);
-	} else {
-		bit_cycles = (avr->frequency + baud / 2) / baud; // matches uart.hpp's own rounding exactly
-		rx_pin = avr_io_getirq(avr, AVR_IOCTL_IOPORT_GETIRQ(sw_rx_port), (unsigned)sw_rx_bit);
-		avr_irq_register_notify(avr_io_getirq(avr, AVR_IOCTL_IOPORT_GETIRQ(sw_tx_port), (unsigned)sw_tx_bit), tx_hook,
-		                        NULL);
-		avr_raise_irq(rx_pin, 1); // idle line
-
-		int slave;
-		struct termios raw;
-		cfmakeraw(&raw);
-		if (openpty(&pty_master, &slave, NULL, &raw, NULL) != 0) {
-			fprintf(stderr, "device: openpty failed\n");
-			return 1;
-		}
-		fcntl(pty_master, F_SETFL, O_NONBLOCK);
-		printf("PB_PTY %s\n", ttyname(slave));
-	}
-	fflush(stdout);
-
-	signal(SIGTERM, finish);
-	signal(SIGINT, finish);
-	signal(SIGUSR1, request_reset); // an external reset line, for the tests
-
-	long since_poll = 0;
-	for (;;) {
-		int state = avr_run(avr);
-		if (state == cpu_Done || state == cpu_Crashed)
-			break;
-		if (reset_requested) {
-			reset_requested = 0;
-			avr_reset(avr);
-			avr->pc = reset_pc;
-			if (!link_software) { // reset restores the pacing hack; re-clear it
-				uint32_t flags = 0;
-				avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS(uart_digit), &flags);
-				flags &= ~AVR_UART_FLAG_POLL_SLEEP;
-				avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS(uart_digit), &flags);
-			} else {
-				bridge_reset();
-			}
-		}
-		if (link_software && ++since_poll >= 2000) {
-			since_poll = 0;
-			poll_pty();
-			// An unthrottled idle simulation runs the activation window out
-			// from under the host's real-time knock cadence: a 1 MHz build's
-			// 8 s window is 8 M cycles — tens of wall milliseconds — so a
-			// first knock lost to an in-flight reset misses the window
-			// entirely. Pace the simulation only while the bridge is fully
-			// quiet (nothing decoding, nothing queued); transfers keep full
-			// speed, and a quiet window stretches toward real time.
-			if (!rx_active && !tx_active && rx_head == rx_tail)
-				usleep(200);
-		}
-	}
-	finish(0);
-	return 0;
-}

test/pureboot_device.cpp

@@ -0,0 +1,703 @@
+// simavr "device" for the pureboot protocol tests, every chip. Loads the
+// boot-linked ELF at the loader base, starts execution there (BOOTRST / the
+// patched vector are not what is under test), and exposes the loader's
+// serial link as a pty for the real host tool:
+//
+//   - Hardware USART builds: simavr's uart_pty on the selected instance.
+//   - Software UART builds: an 8N1 bridge between a pty and the GPIO pins,
+//     timed against the simulated cycle counter (drives the loader's RX,
+//     decodes its TX).
+//
+// The link follows the chip's natural default (USART0 on the megas, the
+// software UART on PB0/PB1 elsewhere) unless -l overrides it: `-l usart1`
+// for the second instance, `-l sw:B5,B1` for a software build's RX,TX pins,
+// and `-l sw:D0,D1@0` where those pins are a USART's own - see the pin
+// ownership the bridge models below.
+//
+// simavr's tiny cores decode the SPM opcode but attach no NVM module - SPM
+// is a silent no-op (the mega's boot section has one, avr_flash). The
+// missing module is supplied here: the SPM ioctl reads SPMCSR/Z/r1:r0 and
+// implements buffer fill, page erase, page write, and CTPB, completing
+// instantly. RFLB's LPM diversion (fuse readout) stays unmodeled, so the
+// 'F' command answers with flash bytes - the tests assert transport only.
+//
+// On exit (or SIGTERM) the flash and EEPROM are dumped to files for a
+// ground-truth cross-check against what the host read back.
+#include <array>
+#include <csignal>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <print>
+#include <string_view>
+
+#include <fcntl.h>
+#include <pty.h>
+#include <termios.h>
+#include <unistd.h>
+
+// The parts headers (uart_pty.h) carry no C++ linkage guards of their own,
+// unlike simavr's core headers - the block covers both harmlessly.
+extern "C" {
+#include "avr_eeprom.h"
+#include "avr_flash.h"
+#include "avr_ioport.h"
+#include "avr_uart.h"
+#include "sim_avr.h"
+#include "sim_elf.h"
+#include "sim_io.h"
+#include "uart_pty.h"
+}
+
+namespace {
+
+avr_t *avr;
+uart_pty_t uart_pty;
+bool link_software;
+avr_uart_t *hw_uart; // the pty-driven USART, for the datasheet-reset fix below
+char uart_digit = '0';
+char sw_rx_port = 'B', sw_tx_port = 'B';
+int sw_rx_bit = 0, sw_tx_bit = 1;
+char sw_tx_owner = 0; // the USART whose TXD the software link sits on
+const char *dump_path;
+std::uint32_t reset_pc;
+volatile std::sig_atomic_t reset_requested;
+
+// -w: report the cycle of the first transmit activity, once. What the
+// activation-window gate reads - with an idle line and an application
+// installed, the first thing that ever talks is the application's banner,
+// so this cycle *is* the loader's window plus a banner lead measured in
+// microseconds. Idle pacing is skipped in this mode: there is no real-time
+// host in the loop, and a paced multi-second window would take hours.
+bool window_report;
+bool window_tx_seen;
+
+void window_first_tx()
+{
+	if (!window_report || window_tx_seen) {
+		return;
+	}
+	window_tx_seen = true;
+	std::println("PB_WINDOW_TX {}", avr->cycle);
+	std::fflush(stdout);
+}
+
+void window_uart_hook(avr_irq_t *, std::uint32_t, void *)
+{
+	window_first_tx();
+}
+
+// One-wire (RX == TX in the link spec): both directions on one GPIO line
+// idling on the firmware's pull-up. The bridge then follows the pin's
+// direction the way the real wiring does: it drives only while the
+// firmware's DDR bit reads input, decodes transitions as the firmware's
+// transmit only while the firmware owns the line, ignores its own raises
+// coming back through the shared irq - and echoes every byte it drives back
+// to the pty, which is what the host-side FTDI tie does and what the host
+// tool's --one-wire mode reads back and discards.
+bool link_one_wire;
+bool mcu_owns_line;
+bool self_drive;
+
+int parse_link(std::string_view spec)
+{
+	if (spec == "usart0" || spec == "usart1") {
+		link_software = false;
+		uart_digit = spec[5];
+		return 0;
+	}
+	if (spec.starts_with("sw")) {
+		link_software = true;
+		if (spec.size() == 2) {
+			return 0;
+		}
+		char owner = 0;
+		int fields =
+		    std::sscanf(spec.data() + 2, ":%c%d,%c%d@%c", &sw_rx_port, &sw_rx_bit, &sw_tx_port, &sw_tx_bit, &owner);
+		if (fields == 4 || fields == 5) {
+			sw_tx_owner = owner;
+			link_one_wire = sw_rx_port == sw_tx_port && sw_rx_bit == sw_tx_bit;
+			return 0;
+		}
+	}
+	return -1;
+}
+
+// simavr 1.6's avr_flash PGERS handler erases spm_pagesize bytes starting at
+// Z & ~1 instead of the page containing Z (its PGWRT path masks correctly)  -
+// hardware ignores the in-page bits (section 26.8.1), so an erase issued with Z
+// anywhere inside the page wipes half the neighbouring page in simulation
+// only. Wrap the mega's registered flash ioctl and re-dispatch page erases
+// with Z forced to the page boundary; everything else passes through.
+//
+// A second gap on the boot-section-less m48s: their RWWSRE bit is the
+// temporary-buffer discard (Atmel-8271 section 26.2/section 26.3.1), but the stock model
+// gates its RWWSRE branch on AVR_SELFPROG_HAVE_RWW - absent on the m48
+// core - so the discard store falls through into the buffer-fill branch and
+// plants whatever Z/R1:R0 happen to hold. Perform the silicon's discard
+// here instead.
+avr_flash_t *mega_flash;
+int (*mega_flash_ioctl)(avr_io_t *io, std::uint32_t ctl, void *param);
+
+int fixed_flash_ioctl(avr_io_t *io, std::uint32_t ctl, void *param)
+{
+	if (ctl == AVR_IOCTL_FLASH_SPM && avr_regbit_get(io->avr, mega_flash->pgers)) {
+		auto z = static_cast<std::uint16_t>(io->avr->data[30] | (io->avr->data[31] << 8));
+		auto masked = static_cast<std::uint16_t>(z & ~(mega_flash->spm_pagesize - 1));
+		io->avr->data[30] = static_cast<std::uint8_t>(masked);
+		io->avr->data[31] = static_cast<std::uint8_t>(masked >> 8);
+		int result = mega_flash_ioctl(io, ctl, param);
+		io->avr->data[30] = static_cast<std::uint8_t>(z);
+		io->avr->data[31] = static_cast<std::uint8_t>(z >> 8);
+		return result;
+	}
+	if (ctl == AVR_IOCTL_FLASH_SPM && !(mega_flash->flags & AVR_SELFPROG_HAVE_RWW) &&
+	    (io->avr->data[mega_flash->r_spm] & 0x11) == 0x11) { // RWWSRE|SELFPRGEN: the m48 buffer discard
+		for (int i = 0; i < mega_flash->spm_pagesize / 2; i++) {
+			mega_flash->tmppage[i] = 0xffff;
+			mega_flash->tmppage_used[i] = 0;
+		}
+		avr_regbit_clear(io->avr, mega_flash->selfprgen);
+		return 0;
+	}
+	return mega_flash_ioctl(io, ctl, param);
+}
+
+void fix_mega_flash_erase()
+{
+	for (avr_io_t *io = avr->io_port; io; io = io->next) {
+		if (io->kind && std::string_view{io->kind} == "flash") {
+			mega_flash = reinterpret_cast<avr_flash_t *>(io);
+			mega_flash_ioctl = io->ioctl;
+			io->ioctl = fixed_flash_ioctl;
+			return;
+		}
+	}
+	std::println(stderr, "device: no flash module to fix - SPM page erases may misalign");
+}
+
+void request_reset(int)
+{
+	reset_requested = 1;
+}
+
+// ------------------------------------------------------------- tiny NVM ---
+
+struct tiny_nvm_t {
+	avr_io_t io;
+	std::array<std::uint8_t, 128> buffer;
+	std::array<std::uint8_t, 128> used; // a buffer word loads once until erased - like silicon
+	unsigned page;
+};
+
+tiny_nvm_t nvm;
+
+int nvm_ioctl(avr_io_t *io, std::uint32_t ctl, void *)
+{
+	if (ctl != AVR_IOCTL_FLASH_SPM) {
+		return -1;
+	}
+	auto *n = reinterpret_cast<tiny_nvm_t *>(io);
+	avr_t *mcu = io->avr;
+	std::uint8_t command = mcu->data[0x57] & 0x1f; // SPMCSR, both tinies
+	auto z = static_cast<std::uint16_t>(mcu->data[30] | (mcu->data[31] << 8));
+	std::uint32_t page_base = static_cast<std::uint32_t>(z & ~(n->page - 1)) % (mcu->flashend + 1);
+	if (command == 0x01) { // SPMEN alone: buffer fill from r1:r0
+		unsigned offset = z & (n->page - 1) & ~1u;
+		if (!n->used[offset]) { // first write wins until the buffer clears
+			n->buffer[offset] = mcu->data[0];
+			n->buffer[offset + 1] = mcu->data[1];
+			n->used[offset] = 1;
+		}
+	} else if (command == 0x03) { // PGERS
+		std::memset(mcu->flash + page_base, 0xff, n->page);
+	} else if (command == 0x05) { // PGWRT: programming only clears bits
+		for (unsigned i = 0; i < n->page; i++) {
+			mcu->flash[page_base + i] &= n->buffer[i];
+		}
+		std::memset(n->buffer.data(), 0xff, n->page);
+		std::memset(n->used.data(), 0, n->page);
+	} else if (command == 0x11) { // CTPB
+		std::memset(n->buffer.data(), 0xff, n->page);
+		std::memset(n->used.data(), 0, n->page);
+	}
+	mcu->data[0x57] &= static_cast<std::uint8_t>(~0x1f); // the operation completes instantly
+	return 0;
+}
+
+// ----------------------------------------------------------- GPIO bridge ---
+
+int pty_master = -1;
+avr_irq_t *rx_pin; // the loader's RX (PB0), driven from the pty
+avr_cycle_count_t bit_cycles;
+
+int tx_level = 1, tx_active, tx_bit;
+std::uint8_t tx_shift;
+
+avr_cycle_count_t tx_sample(avr_t *, avr_cycle_count_t when, void *)
+{
+	if (tx_bit < 0) {
+		// Half a bit into the start bit: a real receiver re-samples here and
+		// abandons a false start. The device's own init produces one - DDR
+		// drives the pin low for the instructions until the idle level is
+		// written - and without this check that glitch decodes as a stray
+		// byte (and would read as first transmit activity under -w).
+		if (tx_level) {
+			tx_active = 0;
+			return 0;
+		}
+		window_first_tx();
+		tx_bit = 0;
+		return when + bit_cycles;
+	}
+	if (tx_bit < 8) {
+		tx_shift = static_cast<std::uint8_t>((tx_shift >> 1) | (tx_level ? 0x80 : 0));
+		if (++tx_bit < 8) {
+			return when + bit_cycles;
+		}
+		// The byte is delivered at the stop bit's sampling point (9.5 bit
+		// times), where a hardware receiver raises its RXC - not sooner: a
+		// host answering before the stop bit would put its start bit on the
+		// wire while the device is still driving, which the device,
+		// transmitting, is not watching for.
+		return when + bit_cycles;
+	}
+	if (write(pty_master, &tx_shift, 1) != 1) {
+		std::println(stderr, "device: pty write lost a byte");
+	}
+	tx_active = 0;
+	return 0;
+}
+
+// A USART owns its TxD pin whenever its transmitter is enabled, and the port
+// register cannot drive it (section 20.2 / Atmel-8271 section 19.2) - which is why a
+// bit-banged link deployed on those pins is mute until it clears UCSRnB.
+// simavr wires a USART entirely through IRQs and never touches the port pin
+// model, so the ownership does not exist there and the mute cannot happen:
+// supply it, or the very state this models is untestable. The link spec's
+// trailing @n names the USART; without one the pins are nobody's.
+avr_uart_t *tx_owner;
+
+bool tx_pin_taken()
+{
+	if (!tx_owner) {
+		return false;
+	}
+	if (avr_regbit_get(avr, tx_owner->txen)) {
+		return true;
+	}
+	// One-wire on the USART's RXD: RXEN forces the shared pin's direction to
+	// input (section 20.7.3), so the firmware's drive goes nowhere until the
+	// release - the receive-side twin of the TXD hold.
+	return link_one_wire && avr_regbit_get(avr, tx_owner->rxen);
+}
+
+// simavr leaves TXEN set in UCSRnB out of reset, where silicon clears the
+// whole register (section 20.11.3) - which would hand the pin to a USART no code has
+// enabled, making a freshly reset chip mute for reasons hardware does not
+// have. Reset it the way the datasheet does, so the ownership starts from
+// nobody's and only an application that really enables the USART takes it.
+void reset_tx_owner()
+{
+	if (tx_owner) {
+		avr_regbit_clear(avr, tx_owner->txen);
+	}
+}
+
+void find_tx_owner()
+{
+	for (avr_io_t *io = avr->io_port; io; io = io->next) {
+		if (io->kind && std::string_view{io->kind} == "uart" &&
+		    reinterpret_cast<avr_uart_t *>(io)->name == sw_tx_owner) {
+			tx_owner = reinterpret_cast<avr_uart_t *>(io);
+			reset_tx_owner();
+			return;
+		}
+	}
+	std::println(stderr, "device: no USART{} to own the software link's TX pin", sw_tx_owner);
+}
+
+void tx_hook(avr_irq_t *, std::uint32_t value, void *)
+{
+	if (link_one_wire && (self_drive || !mcu_owns_line)) {
+		// The bridge's own drive coming back through the shared irq, or a
+		// transition while the line is the bridge's - either way not the
+		// firmware talking: the decoder sees an idle line.
+		tx_level = 1;
+		return;
+	}
+	if (tx_pin_taken()) { // the USART holds the line; the port write goes nowhere
+		tx_level = 1;
+		return;
+	}
+	int level = value & 1;
+	if (!tx_active && tx_level == 1 && level == 0) { // start edge, confirmed mid-bit
+		tx_active = 1;
+		tx_bit = -1;
+		avr_cycle_timer_register(avr, bit_cycles / 2, tx_sample, nullptr);
+	}
+	tx_level = level;
+}
+
+std::array<std::uint8_t, 8192> rx_queue;
+unsigned rx_head, rx_tail; // ring: head = next to send
+int rx_active, rx_bit;
+std::uint8_t rx_byte;
+
+void rx_start_next();
+
+// Every level the bridge itself puts on the line goes through here, so the
+// shared-pin decoder can tell its own drive from the firmware's.
+void bridge_drive(int level)
+{
+	self_drive = true;
+	avr_raise_irq(rx_pin, static_cast<std::uint32_t>(level));
+	self_drive = false;
+}
+
+avr_cycle_count_t rx_step(avr_t *, avr_cycle_count_t when, void *)
+{
+	if (rx_bit < 8) {
+		bridge_drive((rx_byte >> rx_bit) & 1);
+		rx_bit++;
+		return when + bit_cycles;
+	}
+	if (rx_bit == 8) { // stop bit, plus one idle bit of margin
+		bridge_drive(1);
+		// The host-side tie: an FTDI adapter on a one-wire line reads every
+		// byte it transmits - supply that echo, which the host tool's
+		// --one-wire mode consumes as its wiring check.
+		if (link_one_wire && write(pty_master, &rx_byte, 1) != 1) {
+			std::println(stderr, "device: pty echo lost a byte");
+		}
+		rx_bit++;
+		return when + 2 * bit_cycles;
+	}
+	rx_active = 0;
+	rx_start_next();
+	return 0;
+}
+
+void rx_start_next()
+{
+	if (rx_active || rx_head == rx_tail) {
+		return;
+	}
+	// The firmware is answering on the shared line: hold the byte - a real
+	// host's transmission waits out the reply on the wire too. The next
+	// poll_pty tick retries once the line is handed back.
+	if (link_one_wire && mcu_owns_line) {
+		return;
+	}
+	rx_byte = rx_queue[rx_head];
+	rx_head = (rx_head + 1) % rx_queue.size();
+	rx_active = 1;
+	rx_bit = 0;
+	bridge_drive(0); // start bit
+	avr_cycle_timer_register(avr, bit_cycles, rx_step, nullptr);
+}
+
+// The shared pin's direction is the line's ownership: DDR-out is the
+// firmware driving a frame, DDR-in hands the line back to the bridge.
+void on_ddr(avr_irq_t *, std::uint32_t value, void *)
+{
+	const bool owns = (value >> sw_rx_bit) & 1;
+	if (mcu_owns_line && !owns) {
+		bridge_drive(1); // hand-back: a turn-based host idles here, and the cache stays truthful
+	}
+	mcu_owns_line = owns;
+	// A byte held back while the firmware answered starts from the next
+	// poll_pty tick, never from inside the DDR write itself - the port
+	// model's own pull-up re-derivation runs right after this notify and
+	// would erase a start edge raised here.
+}
+
+// A reset abandons whatever the bridge was mid-transfer: bytes still queued
+// for a chip that no longer has the context to receive them meaningfully,
+// and a decode in progress on a TX line the reset may have already changed.
+// The pending cycle timers must go with the state: avr_reset drops the TX
+// output latch, whose falling edge starts a spurious decode before this
+// runs, and a stale tx_sample would then interleave with the loader's first
+// real answer through the shared shift state, corrupting it.
+void bridge_reset()
+{
+	avr_cycle_timer_cancel(avr, tx_sample, nullptr);
+	avr_cycle_timer_cancel(avr, rx_step, nullptr);
+	rx_head = rx_tail = 0;
+	rx_active = 0;
+	tx_active = 0;
+	tx_level = 1;
+	mcu_owns_line = false; // avr_reset zeroed DDR: every pin reads input again
+	// Re-drive the idle line through a forced transition: ioport pin irqs are
+	// IRQ_FLAG_FILTERED, and avr_reset zeroes the port latch while the irq
+	// keeps its pre-reset cached value - so a plain raise(1) against a cached
+	// 1 is dropped and the device reads the line stuck low. A loader entering
+	// calibration on that line measures reset-to-first-edge as one giant
+	// pulse and mis-locks or boots the application on the first real knock.
+	// No cycles run between the two raises, so the device only ever sees the
+	// final idle-high.
+	bridge_drive(0);
+	bridge_drive(1);
+}
+
+void poll_pty()
+{
+	std::array<std::uint8_t, 256> chunk;
+	ssize_t got = read(pty_master, chunk.data(), chunk.size());
+	for (ssize_t i = 0; i < got; i++) {
+		unsigned next = (rx_tail + 1) % rx_queue.size();
+		if (next == rx_head) {
+			break; // full: the host will retry on timeout
+		}
+		rx_queue[rx_tail] = chunk[i];
+		rx_tail = next;
+	}
+	// Unconditional: a byte held back while the firmware owned a shared
+	// line restarts from here once the hand-back has happened.
+	rx_start_next();
+}
+
+// ------------------------------------------------------------------ main ---
+
+[[noreturn]] void finish(int)
+{
+	if (dump_path) {
+		std::FILE *f = std::fopen(dump_path, "wb");
+		if (f) {
+			std::fwrite(avr->flash, 1, avr->flashend + 1, f);
+			std::fclose(f);
+		}
+		avr_eeprom_desc_t ee = {
+		    .ee = nullptr,
+		    .offset = 0,
+		    .size = 0,
+		};
+		if (avr_ioctl(avr, AVR_IOCTL_EEPROM_GET, &ee) == 0 && ee.ee && ee.size) {
+			std::array<char, 512> path;
+			std::snprintf(path.data(), path.size(), "%s.eeprom", dump_path);
+			f = std::fopen(path.data(), "wb");
+			if (f) {
+				std::fwrite(ee.ee, 1, ee.size, f);
+				std::fclose(f);
+			}
+		}
+	}
+	if (!link_software) {
+		uart_pty_stop(&uart_pty);
+	}
+	_exit(0);
+}
+
+} // namespace
+
+int main(int argc, char *argv[])
+{
+	bool link_given = false;
+	for (int opt; (opt = getopt(argc, argv, "l:w")) != -1;) {
+		if (opt == 'w') {
+			window_report = true;
+			continue;
+		}
+		if (opt != 'l' || parse_link(optarg) != 0) {
+			std::println(stderr, "device: bad link spec (usart0, usart1, sw, or sw:B0,B1 as RX,TX)");
+			return 2;
+		}
+		link_given = true;
+	}
+	int args = argc - optind;
+	if (args < 7 || args > 9) {
+		std::print(stderr,
+		           "usage: {} [-l link] [-w] <pureboot.elf> <mcu> <hz> <base_hex> <page> <baud> <flash_dump>"
+		           " [reset_hex] [resume_flash]\n"
+		           "  -l link:      usart0 | usart1 | sw[:B0,B1[@0]] (RX,TX, then the USART owning\n"
+		           "                them); default: the chip's own\n"
+		           "  -w:           print PB_WINDOW_TX <cycle> at the first transmit activity and\n"
+		           "                free-run idle time (window measurement mode)\n"
+		           "  reset_hex:    reset vector (default: base with a boot section, else 0)\n"
+		           "  resume_flash: raw full-flash image loaded instead of the ELF - a prior\n"
+		           "                run's dump, for power-fail resume tests\n",
+		           argv[0]);
+		return 2;
+	}
+	argv += optind - 1; // argv[1] is the ELF again, whatever was parsed
+	const std::string_view mcu_name = argv[2];
+	auto base = static_cast<std::uint32_t>(std::strtoul(argv[4], nullptr, 0));
+	auto page = static_cast<unsigned>(std::atoi(argv[5]));
+	auto baud = static_cast<unsigned>(std::atoi(argv[6]));
+	dump_path = argv[7];
+	const bool is_mega = mcu_name.starts_with("atmega");
+	if (!link_given) {
+		link_software = !is_mega; // the chips' natural links: USART0, or PB0/PB1
+	}
+
+	avr = avr_make_mcu_by_name(mcu_name.data());
+	if (!avr) {
+		std::println(stderr, "device: no {} core", mcu_name);
+		return 1;
+	}
+	avr_init(avr);
+	avr->frequency = static_cast<std::uint32_t>(std::strtoul(argv[3], nullptr, 0));
+	std::memset(avr->flash, 0xff, avr->flashend + 1); // real flash powers up erased
+
+	if (args > 8) {
+		// Resume: the full flash image of an interrupted prior run.
+		std::FILE *f = std::fopen(argv[9], "rb");
+		if (!f || std::fread(avr->flash, 1, avr->flashend + 1, f) == 0) {
+			std::println(stderr, "device: cannot read {}", argv[9]);
+			return 1;
+		}
+		std::fclose(f);
+	} else {
+		elf_firmware_t fw{};
+		if (elf_read_firmware(argv[1], &fw) != 0) {
+			std::println(stderr, "device: cannot read {}", argv[1]);
+			return 1;
+		}
+		// An image past flash end would smash the simulator's heap and turn
+		// into phantom peripheral behavior (lessons: believe the size gate
+		// first) - refuse it loudly instead.
+		if (base + fw.flashsize > avr->flashend + 1) {
+			std::println(stderr, "device: {} B at {:#x} runs past flash end {:#x} - image does not fit its slot",
+			             fw.flashsize, base, avr->flashend);
+			return 1;
+		}
+		std::memcpy(avr->flash + base, fw.flash, fw.flashsize);
+	}
+	// The boot-sectioned megas enter the loader in hardware (BOOTRST, not
+	// modeled - the argument picks the modeled fuse's target); the tinies
+	// and the boot-section-less m48s reset to word 0 like silicon - erased
+	// flash walks up into the loader, and after the host's surgery the
+	// patched vector routes there.
+	const bool boot_section = is_mega && !mcu_name.starts_with("atmega48");
+	reset_pc = args > 7 ? static_cast<std::uint32_t>(std::strtoul(argv[8], nullptr, 0)) : (boot_section ? base : 0);
+	avr->pc = reset_pc;
+	avr->codeend = avr->flashend;
+
+	// Erased EEPROM, as hardware powers up (simavr zeroes it).
+	std::array<std::uint8_t, 1024> blank;
+	std::memset(blank.data(), 0xff, blank.size());
+	avr_eeprom_desc_t seed = {
+	    .ee = blank.data(),
+	    .offset = 0,
+	    .size = 0,
+	};
+	if (avr_ioctl(avr, AVR_IOCTL_EEPROM_GET, &seed) == 0 && seed.size <= blank.size()) {
+		seed.ee = blank.data();
+		avr_ioctl(avr, AVR_IOCTL_EEPROM_SET, &seed);
+	}
+
+	// The megas carry simavr's avr_flash module (and its two gaps the wrap
+	// above fixes); the tinies get the NVM module simavr lacks. Which serial
+	// bridge runs is the link's business, not the chip class's.
+	if (is_mega) {
+		fix_mega_flash_erase();
+	} else {
+		nvm.page = page;
+		std::memset(nvm.buffer.data(), 0xff, nvm.buffer.size());
+		nvm.io.kind = "tiny_nvm";
+		nvm.io.ioctl = nvm_ioctl;
+		avr_register_io(avr, &nvm.io);
+	}
+
+	if (!link_software) {
+		// POLL_SLEEP paces an idle-polling loader in host real time (a
+		// no-hardware CPU-saving hack); clear it so cycles run free.
+		std::uint32_t flags = 0;
+		avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS(uart_digit), &flags);
+		flags &= ~AVR_UART_FLAG_POLL_SLEEP;
+		avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS(uart_digit), &flags);
+		// simavr leaves TXEN set out of reset where silicon clears the whole
+		// UCSR#B (section 20.11.3). Harmless to a loader that enables TXEN itself  -
+		// but a half-duplex build's receiver-only init then *drops* TXEN,
+		// and this uart model clears UDRE on that edge and never re-raises
+		// it on a later enable: the first transmitter after the hand-over
+		// waits UDRE forever, a wedge silicon does not have. Start from the
+		// datasheet's zero, as the software bridge's tx-owner model does.
+		for (avr_io_t *io = avr->io_port; io; io = io->next) {
+			if (io->kind && std::string_view{io->kind} == "uart" &&
+			    reinterpret_cast<avr_uart_t *>(io)->name == uart_digit) {
+				hw_uart = reinterpret_cast<avr_uart_t *>(io);
+			}
+		}
+		if (hw_uart) {
+			avr_regbit_clear(avr, hw_uart->txen);
+		}
+		uart_pty_init(avr, &uart_pty);
+		uart_pty_connect(&uart_pty, uart_digit);
+		if (window_report) {
+			avr_irq_register_notify(avr_io_getirq(avr, AVR_IOCTL_UART_GETIRQ(uart_digit), UART_IRQ_OUTPUT),
+			                        window_uart_hook, nullptr);
+		}
+		std::println("PB_PTY {}", uart_pty.pty.slavename);
+	} else {
+		bit_cycles = (avr->frequency + baud / 2) / baud; // matches uart.hpp's own rounding exactly
+		if (sw_tx_owner) {
+			find_tx_owner();
+		}
+		rx_pin = avr_io_getirq(avr, AVR_IOCTL_IOPORT_GETIRQ(sw_rx_port), static_cast<unsigned>(sw_rx_bit));
+		avr_irq_register_notify(
+		    avr_io_getirq(avr, AVR_IOCTL_IOPORT_GETIRQ(sw_tx_port), static_cast<unsigned>(sw_tx_bit)), tx_hook,
+		    nullptr);
+		if (link_one_wire) {
+			avr_irq_register_notify(avr_io_getirq(avr, AVR_IOCTL_IOPORT_GETIRQ(sw_rx_port), IOPORT_IRQ_DIRECTION_ALL),
+			                        on_ddr, nullptr);
+		}
+		bridge_drive(1); // idle line
+
+		int slave;
+		struct termios raw;
+		cfmakeraw(&raw);
+		if (openpty(&pty_master, &slave, nullptr, &raw, nullptr) != 0) {
+			std::println(stderr, "device: openpty failed");
+			return 1;
+		}
+		fcntl(pty_master, F_SETFL, O_NONBLOCK);
+		std::println("PB_PTY {}", ttyname(slave));
+	}
+	std::fflush(stdout);
+
+	std::signal(SIGTERM, finish);
+	std::signal(SIGINT, finish);
+	std::signal(SIGUSR1, request_reset); // an external reset line, for the tests
+
+	long since_poll = 0;
+	for (;;) {
+		int state = avr_run(avr);
+		if (state == cpu_Done || state == cpu_Crashed) {
+			break;
+		}
+		if (reset_requested) {
+			reset_requested = 0;
+			avr_reset(avr);
+			avr->pc = reset_pc;
+			if (!link_software) { // reset restores the pacing hack; re-clear it
+				std::uint32_t flags = 0;
+				avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS(uart_digit), &flags);
+				flags &= ~AVR_UART_FLAG_POLL_SLEEP;
+				avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS(uart_digit), &flags);
+				if (hw_uart) { // and simavr's bogus reset TXEN (section 20.11.3: zero)
+					avr_regbit_clear(avr, hw_uart->txen);
+				}
+			} else {
+				bridge_reset();
+				reset_tx_owner();
+			}
+		}
+		if (link_software && ++since_poll >= 2000) {
+			since_poll = 0;
+			poll_pty();
+			// An unthrottled idle simulation runs the activation window out
+			// from under the host's real-time knock cadence: a 1 MHz build's
+			// 8 s window is 8 M cycles - tens of wall milliseconds - so a
+			// first knock lost to an in-flight reset misses the window
+			// entirely. Pace the simulation only while the bridge is fully
+			// quiet (nothing decoding, nothing queued); transfers keep full
+			// speed, and a quiet window stretches toward real time.
+			if (!window_report && !rx_active && !tx_active && rx_head == rx_tail) {
+				usleep(200);
+			}
+		}
+	}
+	finish(0);
+}

test/test_handshake.py

@@ -0,0 +1,212 @@
+#!/usr/bin/env python3
+"""Host-tool activation handshake: bounded against a line that misbehaves.
+
+`_handshake` drains the line after it sees a prompt, to absorb a real loader's
+trailing bytes before it asks for the identity. That drain must be bounded: a
+target that never falls quiet - a board stuck in a reset loop presents exactly
+this, ~60 reboots/s of UART-reset garbage in which a stray 0x2b reads as a
+prompt - otherwise spins the tool forever. Regression for that hang, plus a
+control that a well-behaved loader still connects.
+
+The handshake must also survive its own leftovers: after `--stay` the loader's
+final prompt can still be in the USB pipeline when the next invocation opens
+the port, and on a board wired to reset on open, that opening starts a fresh
+activation window the stale prompt then betrays - the tool commits to an
+identity read against a device that never heard its knock, and what it finally
+collects is the application's banner. StaleDTRPort is that moment as a port.
+
+Stdlib only, no device: host-tool logic, so it runs on every chip's preset
+beside pureboot.planner.
+"""
+import importlib.util
+import pathlib
+import threading
+import time
+
+PB = pathlib.Path(__file__).resolve().parents[1] / "pureboot" / "pureboot.py"
+_spec = importlib.util.spec_from_file_location("pureboot", PB)
+pb = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(pb)
+
+P = F = 0
+
+
+def check(name, ok):
+    global P, F
+    P, F = P + (1 if ok else 0), F + (0 if ok else 1)
+    print(f"  [{'PASS' if ok else 'FAIL'}] {name}")
+
+
+class FloodPort:
+    """A line that never falls quiet: read_available always returns bytes, and
+    they contain a prompt. No identity ever completes."""
+
+    def flush_input(self):
+        pass
+
+    def write(self, data):
+        pass
+
+    def read_available(self, wait):
+        time.sleep(0.01)  # a real read waits; keep the busy loop off a core
+        return b"+\x00\xff"
+
+    def read_exact(self, count, timeout):
+        raise pb.Error("no identity")
+
+
+class LoaderPort:
+    """A well-behaved pureboot 5: a prompt to the knock, then quiet, then the
+    slim identity (version 5 + m328p signature) and a closing prompt."""
+
+    def __init__(self):
+        self.pending = b""
+        self.exacts = 0
+
+    def flush_input(self):
+        self.pending = b""
+
+    def write(self, data):
+        if b"p" in data:
+            self.pending = b"+"  # the prompt answers the knock, nothing else
+
+    def read_available(self, wait):
+        data, self.pending = self.pending, b""
+        return data
+
+    def read_exact(self, count, timeout):
+        self.exacts += 1
+        return b"\x05\x1e\x95\x0f" if self.exacts == 1 else b"+"  # identity, then prompt
+
+
+class StaleDTRPort:
+    """`--stay`, then a fresh invocation on a board that resets when its port
+    opens. Three facts of that moment, all timed from the open: the previous
+    session's final prompt is still in transit and lands only after the
+    opening flush has already run; the reset holds the device off the line
+    at first, eating anything written before it completes; and the fresh
+    window is finite - once it expires the application boots and prints a
+    banner whose bytes are what a pending identity read collects. A
+    handshake that trusts the stale prompt spends the whole window waiting
+    on a device that never heard its knock; one that drains the line first
+    knocks into the real window and connects."""
+
+    STALE_AT = 0.02   # the leftover prompt becomes visible (post-flush)
+    READY_AT = 0.05   # reset complete, activation window opens
+    WINDOW = 1.0      # window length; expiry boots the application
+
+    def __init__(self):
+        self.t0 = time.monotonic()
+        # (visible-from, bytes): the line as a timed queue.
+        self.queue = [(self.t0 + self.STALE_AT, b"+")]
+        self.armed = False  # a 'p' heard inside the window arms 'b'
+        self.booted = False
+
+    def _boot_check(self):
+        if not self.booted and time.monotonic() > self.t0 + self.READY_AT + self.WINDOW:
+            self.booted = True
+            self.queue.append((self.t0 + self.READY_AT + self.WINDOW,
+                               b"W r libavr tempmon\r\n"))
+
+    def _visible(self):
+        self._boot_check()
+        now = time.monotonic()
+        return b"".join(d for t, d in self.queue if t <= now)
+
+    def _consume(self, n):
+        now = time.monotonic()
+        left = []
+        for t, d in self.queue:
+            if t <= now and n:
+                take = min(n, len(d))
+                d = d[take:]
+                n -= take
+            if d:
+                left.append((t, d))
+        self.queue = left
+
+    def flush_input(self):
+        self._consume(len(self._visible()))
+
+    def write(self, data):
+        self._boot_check()
+        now = time.monotonic()
+        if now < self.t0 + self.READY_AT or self.booted:
+            return  # still in reset, or the application owns the line
+        if b"p" in data:
+            self.armed = True
+            self.queue.append((now + 0.01, b"+"))
+        if b"b" in data and self.armed:
+            # The slim identity (version 5 + m328p signature) and a prompt.
+            self.queue.append((now + 0.01, b"\x05\x1e\x95\x0f+"))
+
+    def read_available(self, wait):
+        deadline = time.monotonic() + wait
+        while True:
+            data = self._visible()
+            if data:
+                self._consume(len(data))
+                return data
+            if time.monotonic() >= deadline:
+                return b""
+            time.sleep(0.005)
+
+    def read_exact(self, count, timeout):
+        deadline = time.monotonic() + timeout
+        data = b""
+        while len(data) < count:
+            visible = self._visible()
+            if visible:
+                take = visible[:count - len(data)]
+                self._consume(len(take))
+                data += take
+            elif time.monotonic() >= deadline:
+                raise pb.Error(f"timeout: got {len(data)} of {count} bytes")
+            else:
+                time.sleep(0.005)
+        return data
+
+
+def terminates(port, wait, budget):
+    """Run connect_autobaud in a thread; True if it returns/raises within
+    `budget` seconds rather than hanging."""
+    done = threading.Event()
+
+    def run():
+        try:
+            pb.Loader(port).connect_autobaud(wait)
+        except Exception:
+            pass
+        finally:
+            done.set()
+
+    threading.Thread(target=run, daemon=True).start()
+    return done.wait(budget)
+
+
+def main():
+    # the hang: a flooding target must not spin the drain forever. With wait=0.5
+    # the whole handshake has to give up well inside a few seconds.
+    check("flooding target: handshake terminates, drain is bounded",
+          terminates(FloodPort(), wait=0.5, budget=4.0))
+
+    # the control: a real loader still connects and reads identity.
+    info = pb.Loader(LoaderPort()).connect_autobaud(2.0)
+    check("well-behaved loader still connects (version 5)", info.version == 5)
+
+    # the stale prompt: a --stay leftover plus reset-on-open must not burn the
+    # fresh window - the pre-knock drain absorbs it and the first real knock
+    # lands inside the window.
+    try:
+        stale_ok = pb.Loader(StaleDTRPort()).connect(2.5).version == 5
+    except pb.Error as failed:
+        print(f"        ({failed})")
+        stale_ok = False
+    check("stale --stay prompt + reset-on-open: connects in the fresh window", stale_ok)
+
+    print(f"\n  {P} passed, {F} failed")
+    return 1 if F else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

test/test_planner.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Host-tool unit tests — the planning and policy logic, no simulator:
+"""Host-tool unit tests - the planning and policy logic, no simulator:
 programming orders and their recovery properties, the reset-vector surgery,
 the staging composition, the boot-fuse decode, and the update preflight over
 fuse combinations simavr cannot model.
@@ -30,8 +30,14 @@ def info_of(pb, base, page, patch, flash, signature=(0x1E, 0x93, 0x0B), word_fla
     scale = 2 if word_flash else 1
     wire_base = base // scale
     flags = (1 if patch else 0) | (2 if word_flash else 0)
+    # The EEPROM size comes from the signature, as it must: pureboot 5 derives
+    # the whole geometry from the signature rather than sending it, so a
+    # synthetic block that disagreed with its own signature would describe a
+    # chip that cannot exist.
+    eeprom = pb.CHIP_GEOMETRY[signature][2]
     raw = bytes((0x50, 0x42, pb.NEWEST_LOADER if version is None else version,
-                 *signature, page & 0xFF, wire_base & 0xFF, wire_base >> 8, 0, 2, flags))
+                 *signature, page & 0xFF, wire_base & 0xFF, wire_base >> 8,
+                 eeprom & 0xFF, eeprom >> 8, flags))
     info = pb.Info(raw)
     if info.flash_size != flash:
         fail(f"info_of({base:#x}) decodes to {info.flash_size:#x} of flash, not {flash:#x}")
@@ -70,7 +76,7 @@ def main():
         "newer tool",
     )
 
-    # mega_boot: BOOTSZ words and the BOOTRST sense per chip — the fuse byte
+    # mega_boot: BOOTSZ words and the BOOTRST sense per chip - the fuse byte
     # index (EXTENDED on the x8 line except the m328s' HIGH, HIGH elsewhere)
     # and the per-family ladders (Atmel-2486/2466/2503/2545/8271/DS40002065/
     # 8272/8011/2593/42719). Synthetic 'F' replies: only the boot byte
@@ -108,15 +114,15 @@ def main():
                 fail(f"mega_boot {signature[1]:02x}{signature[2]:02b} unprogrammed: {prog} {at:#07x}")
 
     # Word-addressed info decode: the 1284P's base and page ride the wire
-    # scaled — a 17-bit base halved into the block's two bytes, a 256-byte page
-    # spelled 0 — and its slot is the same 512 bytes as everywhere else, so its
+    # scaled - a 17-bit base halved into the block's two bytes, a 256-byte page
+    # spelled 0 - and its slot is the same 512 bytes as everywhere else, so its
     # staging slot lands inside the 1 KiB minimum boot section.
     big = info_of(pb, 0x1FE00, 0, False, 0x20000, signature=(0x1E, 0x97, 0x05), word_flash=True)
     if big.page != 256 or big.base != 0x1FE00 or big.stage != 0x1FC00:
         fail(f"word-addressed info decode: page {big.page}, base {big.base:#x}, stage {big.stage:#x}")
 
     # Surgery: word 0 lands on the loader, the trampoline on the original
-    # entry — checked with an independent decoder.
+    # entry - checked with an independent decoder.
     app = bytes((0xC0 | 0x00, 0xC0)) + bytes((0x12,)) * 300  # rjmp .+0x00C0... entry word 0xC0C0
     entry = rjmp_decode(app[0] | (app[1] << 8), 0, tiny.flash_size // 2)
     pages = pb.plan_flash(app, tiny)
@@ -152,7 +158,7 @@ def main():
 
     # Staging content: the identical image plus the through-word on a
     # patched-vector chip; hard size clamps either way.
-    image = bytes(range(256)) * 2  # 512 B — too big for a tiny slot
+    image = bytes(range(256)) * 2  # 512 B - too big for a tiny slot
     expect_error("tiny staging size", lambda: pb.staging_content(image, tiny), "510")
     staged = pb.staging_content(image[:508], tiny)
     through = staged[510] | (staged[511] << 8)
@@ -162,11 +168,16 @@ def main():
         fail("mega staging content should be the bare image")
     expect_error("mega staging size", lambda: pb.staging_content(image + b"!", mega), "512")
 
-    # The embedded info block: found in a synthetic binary, absent in noise.
-    binary = bytes((0xAA,)) * 10 + tiny.raw + bytes((0xBB,)) * 10
+    # The image stamp: found in a synthetic binary, absent in noise. pureboot
+    # 5 stamps the magic, its version and the signature, and the geometry is
+    # looked up from there - so what comes back must equal what a live device
+    # of the same chip reports.
+    stamp = bytes((0x50, 0x42, pb.NEWEST_LOADER)) + bytes(tiny.signature)
+    binary = bytes((0xAA,)) * 10 + stamp + bytes((0xBB,)) * 10
     found = pb.image_info(binary)
     if found is None or found.raw != tiny.raw:
-        fail("image_info misses the embedded block")
+        fail(f"image_info misreads the v{pb.NEWEST_LOADER} stamp: "
+             f"{found.raw.hex() if found else None} != {tiny.raw.hex()}")
     if pb.image_info(bytes((0xAA,)) * 40) is not None:
         fail("image_info invents a block")
     # An older loader's image stays readable, so a deployed build can be
@@ -217,7 +228,7 @@ def main():
 
     # The 1284s' smallest boot section (512 words) is exactly the resident
     # slot plus its staging slot, so self-update is possible at the minimum
-    # BOOTSZ — no fuse step up, the 644's geometry. That holds only while a
+    # BOOTSZ - no fuse step up, the 644's geometry. That holds only while a
     # slot is 512 B: at 1 KiB the staging slot would fall outside the section
     # and the preflight would refuse.
     notes = pb.update_preflight(bytes((0xAA,)) * 8 + big.raw, big, fuses(0xFE))
@@ -280,7 +291,7 @@ def main():
     if device.writes != pb.RETRIES + 1:
         fail(f"unrepairable page took {device.writes} writes, expected {pb.RETRIES + 1}")
 
-    # The knock handshake against a device that is not listening yet — the
+    # The knock handshake against a device that is not listening yet - the
     # state a port open leaves behind: it resets the chip into a fresh
     # activation window while the previous session's prompt is still in
     # flight, so the first knock is lost and a prompt arrives anyway.

test/test_scan.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""--scan's walk and report logic, no simulator: the probe order, the rate
+arithmetic, and the advice's direction. The rate physics itself is not
+sim-testable - a pty carries bytes at any termios rate - so what the wire
+would arbitrate is pinned here as logic instead.
+
+Usage: test_scan.py <tool_py>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    sys.path.insert(0, os.path.dirname(os.path.abspath(sys.argv[1])))
+    import pureboot as pb
+
+    walk = pb.scan_ratios()
+    if walk != [0, -2, 2, -4, 4, -6, 6, -8, 8, -10, 10]:
+        fail(f"probe walk is not built-rate-first, nearest-out: {walk}")
+
+    if pb.scan_rate(9600, 4) != 9984 or pb.scan_rate(9600, -4) != 9216:
+        fail("probe rate arithmetic")
+    if pb.scan_rate(115200, 0) != 115200:
+        fail("the built rate must probe unchanged")
+
+    # A loader answering fast means a fast oscillator: the trim goes down.
+    report = "\n".join(pb.scan_report(9600, 4, 6))
+    for needle in ("9984", "+4 %", "--baud 9984", "4 steps lower", "pureboot 6"):
+        if needle not in report:
+            fail(f"+4 % report lacks {needle!r}:\n{report}")
+    report = "\n".join(pb.scan_report(9600, -6, 6))
+    if "6 steps higher" not in report:
+        fail(f"-6 % report advises the wrong direction:\n{report}")
+
+    report = "\n".join(pb.scan_report(9600, 0, 6))
+    if "none" not in report or "steps" in report:
+        fail(f"an on-rate answer must advise no trim:\n{report}")
+
+    report = "\n".join(pb.scan_report(9600, 4, 6, clock=9600000))
+    if "9984000" not in report:
+        fail(f"the absolute clock must scale with the found ratio:\n{report}")
+
+    # The walk's rates mostly have no termios B-constant, so the POSIX port
+    # must set them through termios2 - probed on a pty, which accepts the
+    # ioctl without caring about the speed. Without this every off-nominal
+    # probe would abort the walk on the platform --scan matters most on.
+    if os.name == "posix":
+        import pty
+
+        master, slave = pty.openpty()
+        try:
+            port = pb.Port(os.ttyname(slave), pb.scan_rate(9600, 4))
+            port.set_baud(pb.scan_rate(9600, -4))
+            port.close()
+        except pb.Error as error:
+            fail(f"PosixPort refused an off-nominal probe rate: {error}")
+        finally:
+            os.close(master)
+            os.close(slave)
+
+    print("OK")
+
+
+if __name__ == "__main__":
+    main()

test/test_update_link.py

@@ -0,0 +1,167 @@
+#!/usr/bin/env python3
+"""Self-update across a link change: the host must follow the staging copy.
+
+`--update-loader` installs the new image in the staging slot and then *enters
+it* to have it rewrite the resident. That copy is the new image, so it speaks the
+new image's baud and backend - but the host was talking to the *resident*. Where
+the two differ, the host kept knocking at the old rate in the old mode, the
+staging copy never answered, and the update stranded: staging installed, resident
+untouched, and on a 1 KiB tiny the application region (which *is* the staging
+slot there) already gone.
+
+The wire cannot be probed for this - 512 bytes of position-independent code carry
+no header saying what rate they were built for - so the operator declares it, and
+a mismatch with nothing declared has to say so instead of reporting a bare
+timeout.
+
+Stdlib only, no device: host-tool logic, so it runs on every chip's preset beside
+pureboot.planner.
+"""
+import importlib.util
+import pathlib
+
+PB = pathlib.Path(__file__).resolve().parents[1] / "pureboot" / "pureboot.py"
+_spec = importlib.util.spec_from_file_location("pureboot", PB)
+pb = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(pb)
+
+IDENTITY = b"\x05\x1e\x95\x0f"  # pureboot 5 + m328p signature
+P = F = 0
+
+
+def check(name, ok, detail=""):
+    global P, F
+    P, F = P + (1 if ok else 0), F + (0 if ok else 1)
+    print(f"  [{'PASS' if ok else 'FAIL'}] {name}" + (f" - {detail}" if detail else ""))
+
+
+class TwoLinkPort:
+    """A board whose resident and staging copy answer on different links.
+
+    Only the rate currently set decides who can be heard, which is the physical
+    truth: a loader's bit timing is a cycle count, so a copy built for another
+    rate is unreadable until the host retunes. The knock bytes carry the mode, so
+    a backend mismatch is caught the same way.
+    """
+
+    def __init__(self, resident=(57600, False), staged=(38400, False)):
+        self.resident, self.staged = resident, staged
+        self.baud = resident[0]
+        self.entered = False        # a 'J' has handed control to the staging copy
+        self.switches = []          # every retune the host asked for
+        self.pending = bytearray()  # what the device has queued to send
+
+    # --- the part under test needs this to exist at all
+    def set_baud(self, baud):
+        self.baud = baud
+        self.switches.append(baud)
+
+    def flush_input(self):
+        self.pending.clear()
+
+    def _audible(self, knock=None):
+        baud, autobaud = self.staged if self.entered else self.resident
+        if self.baud != baud:
+            return False
+        if knock is None:
+            return True
+        return knock == (bytes((pb.CALIBRATE, ord("p"))) if autobaud else b"pb")
+
+    def write(self, data):
+        data = bytes(data)
+        if data[:1] == b"J" and len(data) == 3:
+            # The resident acks the jump, then control moves to the copy.
+            if self._audible():
+                self.pending += pb.PROMPT
+            self.entered = True
+        elif data in (b"pb", bytes((pb.CALIBRATE, ord("p")))):
+            if self._audible(data):
+                self.pending += pb.PROMPT
+        elif data == b"b":
+            if self._audible():
+                self.pending += IDENTITY + pb.PROMPT
+
+    def read_available(self, wait):
+        out, self.pending = bytes(self.pending), bytearray()
+        return out
+
+    def read_exact(self, count, timeout):
+        if len(self.pending) < count:
+            raise pb.Error(f"timeout: got {len(self.pending)} of {count} bytes")
+        out, self.pending = bytes(self.pending[:count]), self.pending[count:]
+        return out
+
+
+def connected(port):
+    """A Loader already in session with the resident."""
+    loader = pb.Loader(port)
+    loader.connect(2.0)
+    return loader
+
+
+def main():
+    # The control first: where the staged image keeps the resident's link, the
+    # flow works and needs no retune. This is the case that always passed, and
+    # it is what made the bug look like "self-update is broken" rather than
+    # "self-update cannot change the link".
+    port = TwoLinkPort(resident=(57600, False), staged=(57600, False))
+    loader = connected(port)
+    try:
+        loader.enter_copy(0x7C00, 2.0)
+        check("same link: staging copy entered", True)
+    except pb.Error as error:
+        check("same link: staging copy entered", False, str(error))
+
+    # A baud change, declared. The host must retune before knocking.
+    port = TwoLinkPort(resident=(57600, False), staged=(38400, False))
+    loader = connected(port)
+    try:
+        loader.enter_copy(0x7C00, 2.0, link=(38400, False))
+        check("baud change declared: entered after retuning", 38400 in port.switches,
+              f"switches={port.switches}")
+    except (pb.Error, TypeError) as error:
+        check("baud change declared: entered after retuning", False, repr(error))
+
+    # A backend change, declared: the knock itself has to become the calibration
+    # pulse, or an autobaud staging copy never hears a thing.
+    port = TwoLinkPort(resident=(57600, False), staged=(57600, True))
+    loader = connected(port)
+    try:
+        loader.enter_copy(0x7C00, 2.0, link=(57600, True))
+        check("backend change declared: entered as autobaud", True)
+    except (pb.Error, TypeError) as error:
+        check("backend change declared: entered as autobaud", False, repr(error))
+
+    # Nothing declared against a changed link: it still cannot work, but the
+    # error has to name the cause. A bare "no answer" sent the operator looking
+    # at the wiring while the application region sat erased.
+    port = TwoLinkPort(resident=(57600, False), staged=(38400, False))
+    loader = connected(port)
+    try:
+        loader.enter_copy(0x7C00, 0.3)
+        check("undeclared mismatch: reported", False, "unexpectedly succeeded")
+    except pb.Error as error:
+        text = str(error).lower()
+        check("undeclared mismatch: error names the link, not just a timeout",
+              "link" in text or "baud" in text or "backend" in text, str(error))
+    except TypeError as error:
+        check("undeclared mismatch: error names the link, not just a timeout",
+              False, repr(error))
+
+    # The resident's own link must be restored for the caller: a declared
+    # staging link is for the copy, and the tool talks to the new resident after.
+    port = TwoLinkPort(resident=(57600, False), staged=(38400, False))
+    loader = connected(port)
+    try:
+        loader.enter_copy(0x7C00, 2.0, link=(38400, False))
+        check("session records the link it is now speaking", loader.baud == 38400,
+              f"loader.baud={getattr(loader, 'baud', None)}")
+    except (pb.Error, TypeError, AttributeError) as error:
+        check("session records the link it is now speaking", False, repr(error))
+
+    print(f"\n  {P} passed, {F} failed")
+    return 1 if F else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

test/tsbtest.py

@@ -144,7 +144,7 @@ class Host:
         self._expect(CONFIRM, "C end")
         return echo
 
-    # Activation when the config page carries a password: 3×'@' then the
+    # Activation when the config page carries a password: 3x'@' then the
     # password bytes, then the info block + mainloop '!'.
     def activate_password(self, password):
         self.s.reset_input_buffer()
@@ -167,6 +167,20 @@ class Host:
         self._expect(CONFIRM, "emergency mainloop ready")
 
 
+    # A wrong password byte hangs the loader, still draining the line. Two
+    # things must not happen: it must not activate, and it must not fall
+    # through to the emergency erase - a byte the gate has already refused
+    # reaching the erase would let a guess wipe the part.
+    def refuse_password(self, byte):
+        self.s.reset_input_buffer()
+        self.s.write(bytes([KNOCK, KNOCK, KNOCK, byte]))
+        return self.s.read(1)
+
+    def say(self, byte):
+        self.s.write(bytes([byte]))
+        return self.s.read(1)
+
+
 def check(cond, msg):
     if not cond:
         raise AssertionError(msg)
@@ -181,7 +195,7 @@ PW_BYTES = bytes([0x50, 0x57])
 
 def scenario_roundtrip(host):
     """Activation + info block + flash/EEPROM/config read-write round-trips, on
-    a device with a blank (erased) config page — the usual no-password case."""
+    a device with a blank (erased) config page - the usual no-password case."""
     info = host.activate()
     check(info[0:3] == b"TSB", f"magic 'TSB' (got {info[0:3]!r})")
     check(info[6:9] == bytes([0x1E, 0x95, 0x0F]), f"signature 1E 95 0F (got {info[6:9].hex()})")
@@ -219,6 +233,15 @@ def scenario_emergency(host):
     check(host.read_eeprom(1) == b"\xff" * PAGE, "EEPROM wiped")
 
 
+def scenario_wrong_password(host):
+    """A wrong password byte neither activates the loader nor opens the
+    emergency erase behind it - the oracle carries a dedicated fix for the
+    second, and nothing here exercised either half."""
+    check(host.refuse_password(PW_BYTES[0] ^ 1) == b"", "a wrong password byte draws no reply")
+    check(host.say(0x00) == b"", "a 0 byte after it does not request the erase")
+    check(host.say(CONFIRM) == b"", "and neither does a confirm")
+
+
 def main():
     binary, elf, boot_base = sys.argv[1], sys.argv[2], sys.argv[3]
     failures = []
@@ -229,6 +252,7 @@ def main():
         ("round-trip", None, scenario_roundtrip),
         ("password activation", PW_CONFIG, scenario_password),
         ("emergency erase", PW_CONFIG, scenario_emergency),
+        ("wrong password", PW_CONFIG, scenario_wrong_password),
     ]
     for name, config, fn in groups:
         print(f"--- {name} ---")

tools/check.sh

@@ -1,39 +1,68 @@
 #!/bin/bash
-# The port's gate: every chip's generated workflow — build, size matrix, and
+# The port's gate: every chip's generated workflow - build, size matrix, and
 # the simulator-driven protocol suites. --full adds the reflect-spot builds
 # (libavr's rule: reflect compiles are bounded to its spot set, never the
 # full matrix) and swaps the compact size matrix for the exhaustive
-# clock × baud × backend cross product. LIBAVR_ROOT must point at the libavr
-# checkout.
+# clock x baud x backend cross product. libavr resolves from the `libavr/`
+# submodule; LIBAVR_ROOT overrides it for a working tree.
 set -e
 cd "$(dirname "$0")/.."
 
 full=0
 [[ "$1" == "--full" ]] && { full=1; shift; export PUREBOOT_FULL_MATRIX=1; }
 
-CHIPS=(attiny13 attiny13a attiny25 attiny45 attiny85
-       atmega8 atmega8a atmega16 atmega16a atmega32 atmega32a
-       atmega48 atmega48a atmega48p atmega48pa
-       atmega88 atmega88a atmega88p atmega88pa
-       atmega168 atmega168a atmega168p atmega168pa
-       atmega328 atmega328p
-       atmega164a atmega164p atmega164pa
-       atmega324a atmega324p atmega324pa
-       atmega644 atmega644a atmega644p atmega644pa
-       atmega1284 atmega1284p)
-REFLECT_SPOT=(attiny13a attiny85 atmega8 atmega16a atmega32a atmega48pa
-              atmega88 atmega168pa atmega328p atmega164a atmega644p atmega1284)
+# The chip lists come from the presets rather than being spelled a second time
+# here: a chip added to make_presets.py and missed in a copy of its list would
+# be a gate that silently never builds it, which is the one failure mode a gate
+# cannot report. tools/make_presets.py is the single source, CMakePresets.json
+# is its output, and this reads that.
+readarray -t WORKFLOWS < <(python3 -c '
+import json, sys
+presets = json.load(open("CMakePresets.json"))["workflowPresets"]
+print("\n".join(p["name"] for p in presets))')
+if ((${#WORKFLOWS[@]} == 0)); then
+	echo "no workflow presets in CMakePresets.json - run tools/make_presets.py" >&2
+	exit 1
+fi
+CHIPS=()
+REFLECT_SPOT=()
+for workflow in "${WORKFLOWS[@]}"; do
+	case $workflow in
+	*-generated) CHIPS+=("${workflow%-generated}") ;;
+	*-reflect) REFLECT_SPOT+=("${workflow%-reflect}") ;;
+	esac
+done
+
+# Every preset runs even after one goes red, and the gate fails at the end
+# naming all of them: stopping at the first failure turns a red - a stale size
+# canary above all - into an alibi for every chip behind it, and a loader can
+# ship on a chip this gate has not compiled since.
+red=()
+run_preset() {
+	echo "==== $1 ===="
+	cmake --workflow --preset "$1" "${@:2}" || red+=("$1")
+}
 
 for chip in "${CHIPS[@]}"; do
-	echo "==== $chip ===="
-	cmake --workflow --preset "$chip-generated" "$@"
+	run_preset "$chip-generated" "$@"
 done
 
 if ((full)); then
 	for chip in "${REFLECT_SPOT[@]}"; do
-		echo "==== $chip reflect ===="
-		cmake --workflow --preset "$chip-reflect" "$@"
+		run_preset "$chip-reflect" "$@"
 	done
 fi
 
+if ((${#red[@]})); then
+	printf '==== red presets ====\n' >&2
+	printf '  %s\n' "${red[@]}" >&2
+	exit 1
+fi
+
+# Every tree is freshly built now - the one moment the README's size table
+# can be held to what the images measure (a per-preset ctest sees only its
+# own chip; the table needs all of them, and ungated it drifts: a
+# common-code shave moves every row at once with nothing over budget).
+python3 tools/sizes.py check-readme
+
 echo "check: every chip green"

tools/make_presets.py

@@ -1,17 +1,20 @@
 #!/usr/bin/env python3
-"""Regenerate CMakePresets.json — one uniform pipeline per chip.
+"""Regenerate CMakePresets.json - one uniform pipeline per chip.
 
 Every chip gets generated-mode configure/build/test presets and a workflow
-running all three. Reflect-mode presets (configure + build, no tests — the
+running all three. Reflect-mode presets (configure + build, no tests - the
 port's TUs compile identically; the sims prove nothing new there) exist for
 libavr's reflect spot set only, mirroring its rule: the full reflect matrix
 is never built, one chip per hardware class and pack vintage is.
 
-Run from the repo root: tools/make_presets.py
+Run from the repo root: tools/make_presets.py - or with --check, which
+verifies the committed file matches this generator and edits nothing (the
+ctest entry `presets.generated` runs that, so drift reds the gate).
 """
 
 import json
 import os
+import sys
 
 CHIPS = [
     "attiny13", "attiny13a", "attiny25", "attiny45", "attiny85",
@@ -41,7 +44,7 @@ def main():
         "hidden": True,
         "generator": "Ninja",
         "binaryDir": "${sourceDir}/build/${presetName}",
-        "toolchainFile": "$env{LIBAVR_ROOT}/cmake/avr-toolchain.cmake",
+        "toolchainFile": "${sourceDir}/libavr/cmake/avr-toolchain.cmake",
         "cacheVariables": {
             "CMAKE_BUILD_TYPE": "Release",
             "CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
@@ -72,6 +75,9 @@ def main():
     for chip in REFLECT_SPOT:
         add(chip, "reflect")
 
+    # CMake rejects unknown fields in the presets root, $comment included, so
+    # the file cannot carry a generated-file marker; the --check ctest is the
+    # whole of rule 10's guard here.
     presets = {
         "version": 8,
         "configurePresets": configure,
@@ -79,12 +85,19 @@ def main():
         "testPresets": test,
         "workflowPresets": workflows,
     }
+    rendered = json.dumps(presets, indent=1) + "\n"
     path = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "CMakePresets.json")
+    if "--check" in sys.argv[1:]:
+        current = open(path).read() if os.path.exists(path) else ""
+        if current != rendered:
+            print("CMakePresets.json does not match its generator - run tools/make_presets.py")
+            return 1
+        return 0
     with open(path, "w") as f:
-        json.dump(presets, f, indent=1)
-        f.write("\n")
+        f.write(rendered)
     print(f"{len(CHIPS)} chips, {len(REFLECT_SPOT)} reflect: {os.path.normpath(path)}")
+    return 0
 
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main())

tools/pbhw.py

@@ -0,0 +1,377 @@
+#!/usr/bin/env python3
+"""Hardware acceptance suite for a pureboot deployment.
+
+`tools/check.sh` proves the protocol under simavr on every chip. This proves one
+*board*: that the loader actually installed on it answers, that the memories
+round-trip over the real link, that the application it flashes runs afterwards,
+and that the refusals which keep a 512-byte slot alive still fire. Run it once
+when a board is brought up, and again whenever the deployment moves - a new
+clock, a new backend, new pins.
+
+Every check derives its bounds from the info block the loader itself reports, so
+nothing here is per-chip: the same run covers a 1 KiB tiny whose application
+region is 510 usable bytes and a 128 KiB mega whose flash needs a bank in the
+selector.
+
+**This overwrites the board's application flash and EEPROM.** Capture them first
+with `pbrig.py backup`, which verifies what it captured.
+
+    tools/pbhw.py --programmer atmelice_isp --part t13 --port COM6 \
+                  --autobaud --loader build/ab.bin --app build/pbapp.hex \
+                  --marker APP
+"""
+
+from __future__ import annotations
+
+import argparse
+import pathlib
+import sys
+import tempfile
+
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent))
+import pbrig  # noqa: E402
+
+
+class Suite:
+    def __init__(self, rig: pbrig.Rig, work: pathlib.Path):
+        self.rig = rig
+        self.work = work
+        self.results: list[tuple[str, bool, str]] = []
+
+    def check(self, name: str, ok: bool, detail: str = "") -> bool:
+        self.results.append((name, ok, detail))
+        print(f"  {'PASS' if ok else 'FAIL'}  {name}" + (f"   {detail}" if detail else ""))
+        return ok
+
+    @staticmethod
+    def _brief(text: str, limit: int = 78) -> str:
+        return " | ".join(l.strip() for l in text.splitlines() if l.strip())[:limit]
+
+    # ----------------------------------------------------------------- checks
+
+    def identity(self) -> object | None:
+        """The info block, which every later check takes its bounds from."""
+        module = pbrig.load_pureboot(self.rig.d.pureboot)
+        self.rig.reset()
+        port = self.rig.open_port()  # wrapped for the echo where the line is shared
+        try:
+            loader = module.Loader(port)
+            if self.rig.d.autobaud:
+                loader.connect_autobaud(self.rig.d.wait)
+            else:
+                loader.connect(self.rig.d.wait)
+            info = loader.info
+            self.check("identity read", True, info.describe())
+            return info
+        except Exception as error:  # noqa: BLE001 - a dead link is a result
+            self.check("identity read", False, str(error)[:70])
+            return None
+        finally:
+            try:
+                port.close()
+            except Exception:
+                pass
+
+    def scan(self) -> None:
+        """The --scan walk against real termios and a real oscillator: every
+        probe rate must open a port (the off-nominal rates exist only through
+        termios2), and one probe must answer - the nominal on a healthy board,
+        a neighbor on a drifted one. The rig injects the one reset per probe
+        the operator supplies in the field; this is the rate physics the
+        simulator cannot arbitrate (a pty carries bytes at any rate), pinned
+        on silicon."""
+        module = pbrig.load_pureboot(self.rig.d.pureboot)
+        found = None
+        try:
+            for pct in module.scan_ratios():
+                rate = module.scan_rate(self.rig.d.baud, pct)
+                self.rig.reset()
+                try:
+                    # Same wrap as identity(): on a shared line an undiscarded
+                    # echo answers every rate a scan probes, so the walk would
+                    # report the first one it tried.
+                    port = self.rig.open_port(rate)
+                except module.Error as error:
+                    self.check("scan opens every probe rate", False, f"{rate} Bd: {error}")
+                    return
+                try:
+                    module.Loader(port).connect(min(self.rig.d.wait, 6.0))
+                    found = pct
+                    break
+                except module.Error:
+                    continue
+                finally:
+                    port.close()
+        except Exception as error:  # noqa: BLE001 - a rig hiccup is a result
+            self.check("scan walks the probe ladder", False, str(error)[:70])
+            return
+        self.check("scan finds the board's rate", found is not None,
+                   "no probe answered" if found is None else f"{found:+d} % of {self.rig.d.baud} Bd")
+
+    def eeprom(self, info) -> None:
+        size = info.eeprom_size
+        if not size:
+            print("  skip  EEPROM (this part has none)")
+            return
+        # A pattern no erase or partial write could produce by accident.
+        pattern = bytes((i * 7 + 3) & 0xFF for i in range(size))
+        image = self.work / "ee.bin"
+        image.write_bytes(pattern)
+
+        rc, out = self.rig.pureboot("--eeprom", str(image), "--verify-eeprom", str(image))
+        self.check(f"EEPROM write + verify ({size} B)", rc == 0, self._brief(out))
+
+        back = self.work / "ee-back.bin"
+        rc, out = self.rig.pureboot("--read-eeprom", str(back))
+        got = back.read_bytes() if back.exists() else b""
+        self.check("EEPROM reads back what was written", got == pattern, f"{len(got)} B")
+
+        self.rig.pureboot("--erase-eeprom")
+        erased = self.work / "ee-erased.bin"
+        self.rig.pureboot("--read-eeprom", str(erased))
+        got = erased.read_bytes() if erased.exists() else b""
+        self.check("EEPROM erase leaves 0xff", got == b"\xff" * size, f"{len(got)} B")
+
+    def application(self, info, app: pathlib.Path, marker: str,
+                    marker_wait: float = 2.5) -> None:
+        rc, out = self.rig.pureboot("--flash", str(app), "--verify-flash", str(app))
+        self.check(f"application flash + verify ({app.name})", rc == 0, self._brief(out))
+
+        if marker:
+            # The tool hands over as it ends its session, so the application is
+            # already running - but only on a board whose DTR is unwired, where
+            # opening a port simply listens. Where DTR *is* wired to reset (an
+            # Arduino, most USB-serial dev boards), this open resets the part
+            # and the activation window comes first, so a marker emitted once at
+            # startup happens on the far side of a wait this cannot know the
+            # length of: the window is a compile-time constant and nothing on
+            # the wire reports it. Hence --marker-wait, and a fixture that
+            # repeats its banner (PUREBOOT_HEARTBEAT) rather than saying it once.
+            data = self.rig.capture(seconds=marker_wait)
+            seen = marker.encode() in data
+            sample = "".join(chr(b) if 32 <= b < 127 else "." for b in data[:40])
+            self.check(f"application runs (emits {marker!r})", seen,
+                       f"|{sample}|" if seen or data else
+                       f"nothing in {marker_wait:g} s - if this board resets when its port "
+                       f"opens, that wait has to outlast the activation window")
+
+        back = self.work / "app-back.bin"
+        rc, out = self.rig.pureboot("--read-flash", str(back))
+        got = back.read_bytes() if back.exists() else b""
+        self.check("application flash reads back", rc == 0 and len(got) == info.base,
+                   f"{len(got)} B of {info.base}")
+
+    def _witness(self, info, slot_length: int):
+        """Read back the erased region and the loader slot: (erased, slot, how).
+
+        Prefers ISP, because an independent reader is the only one that can
+        testify about a loader just asked to erase around itself. Where no
+        programmer is attached the link answers instead - which is weaker for
+        exactly the reason it is worth having, a destroyed loader being unable
+        to report anything at all. The two are never printed under one word:
+        an absent probe is a fact about the bench, a wrong byte is a verdict on
+        the loader, and a check that conflates them stops being read.
+        """
+        limit = info.base - 2 if info.patch_vector else info.base
+        whole = self.work / "whole.bin"
+        if self.rig.read_memory("flash", whole, "r"):
+            image = whole.read_bytes()
+            image += b"\xff" * (info.flash_size - len(image))
+            return image[0:limit], image[info.base:info.base + slot_length], "ISP"
+
+        module = pbrig.load_pureboot(self.rig.d.pureboot)
+        port = self.rig.open_port()
+        try:
+            loader = module.Loader(port)
+            if self.rig.d.autobaud:
+                loader.connect_autobaud(self.rig.d.wait)
+            else:
+                loader.connect(self.rig.d.wait)
+            return (loader.read_flash(0, limit),
+                    loader.read_flash(info.base, slot_length),
+                    "the link, no probe attached - the loader's own account")
+        except Exception as error:  # noqa: BLE001 - a dead link is a result
+            print(f"  skip  slot checks: no programmer, and the link did not "
+                  f"answer either ({str(error)[:60]})")
+            return None, None, ""
+        finally:
+            try:
+                port.close()
+            except Exception:  # noqa: BLE001
+                pass
+
+    def erase_and_slot(self, info, loader_image: pathlib.Path | None) -> None:
+        rc, out = self.rig.pureboot("--erase-flash")
+        self.check("application region erases", rc == 0, self._brief(out))
+
+        want = loader_image.read_bytes() if loader_image and loader_image.exists() else b""
+        erased, slot, how = self._witness(info, len(want))
+        if erased is None:
+            return
+
+        # Erased application flash, up to the trampoline word the host composes
+        # on a patched-vector part.
+        limit = info.base - 2 if info.patch_vector else info.base
+        self.check("erased application region is 0xff",
+                   set(erased) <= {0xFF}, f"0x0000..{limit:#06x} via {how}")
+
+        if want:
+            self.check("loader slot survives the erase", slot == want,
+                       f"{len(want)} B at {info.base:#06x} via {how}")
+        else:
+            print("  skip  loader slot comparison (pass --loader <image.bin>)")
+
+    def seal(self, info, rounds: int = 1) -> None:
+        """The seal, adversarially, over the real link.
+
+        pureboot 9 has no running-slot guard: what stops a mangled command from
+        erasing the loader is the seal and nothing else. So this aims the worst
+        command the protocol has - an SPM erase at the loader's own first page -
+        and damages one header byte at a time. Every one must come back NAK with
+        the slot untouched and the session still in step.
+
+        On a board whose link drops or mangles bytes of its own accord this is
+        also the stress test: `--seal-rounds` repeats it, and a link fault
+        during a round is indistinguishable to the loader from the damage being
+        injected, which is the point.
+        """
+        module = pbrig.load_pureboot(self.rig.d.pureboot)
+        self.rig.reset()
+        port = self.rig.open_port()
+        try:
+            loader = module.Loader(port)
+            if self.rig.d.autobaud:
+                loader.connect_autobaud(self.rig.d.wait)
+            else:
+                loader.connect(self.rig.d.wait)
+            if loader.info.version < module.SEALED_LOADER:
+                print(f"  skip  seal checks (loader is pureboot {loader.info.version})")
+                return
+
+            head_of = lambda dmg: self._sealed(module, module.OP_WRITE, module.SP_SPM,
+                                               info.base, module.SPM_ERASE, dmg)
+            refused = 0
+            attempts = 0
+            for _ in range(rounds):
+                for index in range(6):
+                    attempts += 1
+                    port.write(head_of((index, 0x01)))
+                    verdict = port.read_exact(1, 5.0)
+                    if verdict != module.NAK:
+                        self.check(f"damaged byte {index} refused", False,
+                                   f"verdict {verdict.hex()}")
+                        return
+                    if port.read_exact(1, 5.0) != module.PROMPT:
+                        self.check(f"re-prompt after byte {index}", False, "no prompt")
+                        return
+                    refused += 1
+            self.check("damaged headers refused", refused == attempts,
+                       f"{refused}/{attempts}, every header byte")
+            self.check("session still in step", loader.identity().raw == info.raw)
+
+            # And the slot itself, read back over the link: the loader is the
+            # thing that would have been erased, so its own account of its
+            # first bytes is a real witness - an erased page reads all 0xff.
+            head = loader.read_flash(info.base, 16)
+            self.check("loader slot intact", set(head) != {0xFF}, head[:8].hex())
+        except Exception as error:  # noqa: BLE001 - a dead link is a result
+            self.check("seal checks", False, str(error)[:70])
+        finally:
+            try:
+                port.close()
+            except Exception:  # noqa: BLE001
+                pass
+
+    @staticmethod
+    def _sealed(module, op, space, address, count, damage=None):
+        """A sealed header, damaged after sealing - the shape a link fault has."""
+        head = bytearray((op, module.selector(space, address), address & 0xFF,
+                          (address >> 8) & 0xFF, count & 0xFF))
+        seal = module.SEAL
+        for byte in head:
+            seal ^= byte
+        out = bytearray(head + bytes((seal,)))
+        if damage:
+            out[damage[0]] ^= damage[1]
+        return bytes(out)
+
+    def refusals(self, info) -> None:
+        # One word too many: a patched-vector part spends the slot's last word
+        # on the trampoline, so its application stops two bytes short.
+        limit = info.base - 2 if info.patch_vector else info.base
+        oversized = self.work / "oversized.bin"
+        oversized.write_bytes(bytes(limit + 2))
+        rc, out = self.rig.pureboot("--flash", str(oversized))
+        self.check(f"image over {limit} B refused", rc != 0, self._brief(out))
+
+    # ------------------------------------------------------------------- run
+
+    def run(self, app: pathlib.Path | None, loader_image: pathlib.Path | None,
+            marker: str, marker_wait: float = 2.5, seal_rounds: int = 1) -> int:
+        print("identity")
+        info = self.identity()
+        if info is None:
+            print("\nthe loader never answered; nothing below can be trusted")
+            return 1
+
+        if not self.rig.d.autobaud:
+            print("\nscan")
+            self.scan()
+
+        print("\nEEPROM")
+        self.eeprom(info)
+
+        if app:
+            print("\napplication")
+            self.application(info, app, marker, marker_wait)
+        else:
+            print("\nskip  application checks (pass --app <image.hex>)")
+
+        print("\nerase and the slot boundary")
+        self.erase_and_slot(info, loader_image)
+
+        print("\nthe seal")
+        self.seal(info, seal_rounds)
+
+        print("\nrefusals")
+        self.refusals(info)
+
+        passed = sum(1 for _, ok, _ in self.results if ok)
+        print(f"\n{passed}/{len(self.results)} passed")
+        return 0 if passed == len(self.results) else 1
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(
+        description="hardware acceptance suite for one pureboot deployment",
+        epilog="overwrites the board's application flash and EEPROM - back them up first")
+    pbrig.Deployment.add_arguments(parser)
+    parser.add_argument("--app", type=pathlib.Path,
+                        help="application image to flash (test/pbapp.cpp built for this deployment)")
+    parser.add_argument("--loader", type=pathlib.Path,
+                        help="the resident loader's .bin, to prove the slot survives an erase")
+    parser.add_argument("--seal-rounds", type=int, default=1,
+                        help="repeat the adversarial seal sweep N times (a lossy board's stress test)")
+    parser.add_argument("--marker", default="",
+                        help="text the application emits when it runs, e.g. APP")
+    parser.add_argument("--marker-wait", type=float, default=2.5,
+                        help="seconds to listen for it. On a board whose DTR is wired to "
+                             "reset, opening the port resets the part, so this must outlast "
+                             "the activation window (default 2.5)")
+    args = parser.parse_args(argv)
+
+    rig = pbrig.Rig(pbrig.Deployment.from_args(args))
+    print(f"rig: {args.part} on {args.programmer}, link {args.port} at {args.baud} Bd"
+          f"{' (autobaud)' if args.autobaud else ''}")
+    print("this overwrites the application flash and EEPROM\n")
+    with tempfile.TemporaryDirectory(prefix="pbhw-") as temporary:
+        return Suite(rig, pathlib.Path(temporary)).run(args.app, args.loader, args.marker,
+                                                      args.marker_wait, args.seal_rounds)
+
+
+if __name__ == "__main__":
+    try:
+        sys.exit(main())
+    except pbrig.Error as error:
+        print(f"error: {error}", file=sys.stderr)
+        sys.exit(2)

tools/pbrig.py

@@ -0,0 +1,445 @@
+#!/usr/bin/env python3
+"""Hardware rig driver for pureboot: an ISP programmer beside a serial link.
+
+The simulated suites (`test/pb*.py`) prove the protocol; this drives the same
+loader on real silicon, where the things a cycle-exact simulator cannot model
+live - an RC oscillator off its nominal, a reset edge that has to come from
+somewhere, a serial bridge with its own idea of what a baud is.
+
+Nothing here knows a port name, a part or a programmer. Every deployment fact
+arrives from the command line or the environment, so the same script serves any
+board: see `Deployment`. As a module it is the reset/flash/talk primitives that
+`pbhw.py` builds its acceptance suite from; as a command it is the handful of
+one-shot operations worth having on a rig - most importantly `backup`, which is
+the only thing standing between a fuse experiment and an unrecoverable part.
+
+Two rig facts are encoded here because they are not guessable and cost a
+session each to learn:
+
+* **An ISP access resets the part**, and it runs again the moment the programmer
+  releases it. That is the only reset edge available when the serial adapter's
+  DTR is not wired to reset - so a loader session begins with an ISP touch and
+  knocks immediately after, which is what `Rig.pureboot()` does.
+* **avrdude splits `-U memory:op:file:format` on colons**, so a Windows path's
+  drive letter breaks the spec. Every file argument is therefore passed as a
+  bare filename with avrdude run in that file's own directory.
+"""
+
+from __future__ import annotations
+
+import argparse
+import dataclasses
+import importlib.util
+import os
+import pathlib
+import subprocess
+import sys
+import time
+
+HERE = pathlib.Path(__file__).resolve().parent
+DEFAULT_PUREBOOT = HERE.parent / "pureboot" / "pureboot.py"
+
+# Memories worth capturing before an experiment, and the format each is read in.
+# Fuses and lock are per-part: a part without an extended fuse simply fails that
+# one read, which `backup` reports and steps over rather than aborting on.
+BACKUP_MEMORIES = (
+    ("flash", "i", "hex"),
+    ("flash", "r", "bin"),
+    ("eeprom", "i", "hex"),
+    ("eeprom", "r", "bin"),
+    ("lfuse", "h", "hex"),
+    ("hfuse", "h", "hex"),
+    ("efuse", "h", "hex"),
+    ("lock", "h", "hex"),
+    ("calibration", "h", "hex"),
+)
+
+
+class Error(Exception):
+    pass
+
+
+def bitclock_for(hz: int) -> str:
+    """A safe ISP bitclock for a part *currently running* at `hz`.
+
+    SCK must stay under a quarter of the target clock, so the bitclock follows
+    the clock in force - not the one about to be fused in. Halving that ceiling
+    again costs nothing on a link that moves a few hundred bytes and buys margin
+    against an oscillator that is already known to be off its nominal.
+    """
+    ceiling = hz // 8
+    rungs = (1000, 4000, 8000, 32000, 125000, 400000)
+    if ceiling < rungs[0]:
+        raise Error(f"a part at {hz} Hz is too slow to reach over ISP safely")
+    best = max(rung for rung in rungs if rung <= ceiling)
+    return f"{best // 1000}kHz"
+
+
+@dataclasses.dataclass
+class Deployment:
+    """Everything about one board. No default names a real device."""
+
+    port: str = ""              # serial device the loader speaks on
+    baud: int = 57600           # host rate; for autobaud, the rate to drive
+    autobaud: bool = False      # send the calibration pulse instead of p+b
+    one_wire: bool = False      # shared line: the host discards its own echo
+    programmer: str = ""        # avrdude -c
+    part: str = ""              # avrdude -p
+    avrdude: str = "avrdude"
+    bitclock: str = "125kHz"    # see bitclock_for()
+    pureboot: pathlib.Path = DEFAULT_PUREBOOT
+    wait: int = 12              # seconds the host keeps knocking
+
+    @classmethod
+    def from_env(cls) -> "Deployment":
+        """Environment defaults, so a rig's facts live in one place per machine."""
+        return cls(
+            port=os.environ.get("PUREBOOT_PORT", ""),
+            baud=int(os.environ.get("PUREBOOT_BAUD", "57600")),
+            autobaud=os.environ.get("PUREBOOT_AUTOBAUD", "") not in ("", "0"),
+            one_wire=os.environ.get("PUREBOOT_ONE_WIRE", "") not in ("", "0"),
+            programmer=os.environ.get("PUREBOOT_PROGRAMMER", ""),
+            part=os.environ.get("PUREBOOT_PART", ""),
+            avrdude=os.environ.get("AVRDUDE", "avrdude"),
+            bitclock=os.environ.get("PUREBOOT_BITCLOCK", "125kHz"),
+            pureboot=pathlib.Path(os.environ.get("PUREBOOT_TOOL", str(DEFAULT_PUREBOOT))),
+        )
+
+    @staticmethod
+    def add_arguments(parser: argparse.ArgumentParser) -> None:
+        """Deployment flags, shared by this tool and pbhw.py."""
+        env = Deployment.from_env()
+        parser.add_argument("--port", default=env.port, help="serial device the loader speaks on")
+        parser.add_argument("--baud", type=int, default=env.baud,
+                            help="host rate (for autobaud, the rate to drive)")
+        parser.add_argument("--autobaud", action="store_true", default=env.autobaud,
+                            help="send the calibration pulse instead of the p+b knock")
+        parser.add_argument("--one-wire", action="store_true", default=env.one_wire,
+                            help="shared line: pass the tool its echo discard")
+        parser.add_argument("--programmer", default=env.programmer, help="avrdude -c, e.g. atmelice_isp")
+        parser.add_argument("--part", default=env.part, help="avrdude -p, e.g. t13 or m328p")
+        parser.add_argument("--avrdude", default=env.avrdude, help="path to avrdude")
+        parser.add_argument("--bitclock", default=env.bitclock, help="ISP bitclock, e.g. 125kHz or 8kHz")
+        parser.add_argument("--pureboot", type=pathlib.Path, default=env.pureboot,
+                            help="path to pureboot.py")
+        parser.add_argument("--wait", type=int, default=env.wait, help="seconds to keep knocking")
+
+    @classmethod
+    def from_args(cls, args: argparse.Namespace) -> "Deployment":
+        return cls(port=args.port, baud=args.baud, autobaud=args.autobaud,
+                   one_wire=args.one_wire,
+                   programmer=args.programmer, part=args.part, avrdude=args.avrdude,
+                   bitclock=args.bitclock, pureboot=args.pureboot, wait=args.wait)
+
+
+def load_pureboot(path: pathlib.Path = DEFAULT_PUREBOOT):
+    """The host tool as a module - its Port and Loader, not a subprocess.
+
+    Used where a subprocess cannot express what is needed: a poke followed by a
+    peek in the *same* session, or a raw read at an arbitrary baud.
+    """
+    spec = importlib.util.spec_from_file_location("pureboot", path)
+    if spec is None or spec.loader is None:
+        raise Error(f"cannot load the host tool from {path}")
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+class Rig:
+    """One board: its programmer on one side, its serial link on the other."""
+
+    def __init__(self, deployment: Deployment):
+        self.d = deployment
+        if not deployment.programmer or not deployment.part:
+            raise Error("a rig needs --programmer and --part")
+
+    # ------------------------------------------------------------- programmer
+
+    def avrdude(self, *args: str, cwd: pathlib.Path | None = None,
+                bitclock: str | None = None, timeout: int = 300) -> subprocess.CompletedProcess:
+        command = [self.d.avrdude, "-c", self.d.programmer, "-p", self.d.part,
+                   "-B", bitclock or self.d.bitclock, *args]
+        return subprocess.run(command, capture_output=True, text=True,
+                              cwd=None if cwd is None else str(cwd), timeout=timeout)
+
+    @staticmethod
+    def _ok(result: subprocess.CompletedProcess) -> bool:
+        return result.returncode == 0
+
+    def reset(self, bitclock: str | None = None) -> None:
+        """An ISP access, which resets the part; it runs when avrdude exits."""
+        self.avrdude("-U", "signature:r:-:h", bitclock=bitclock)
+
+    def signature(self, bitclock: str | None = None) -> str:
+        result = self.avrdude("-U", "signature:r:-:h", bitclock=bitclock)
+        for line in reversed(result.stdout.splitlines()):
+            if line.strip().startswith("0x"):
+                return line.strip()
+        raise Error(f"no signature read: {(result.stderr or result.stdout).strip()[:200]}")
+
+    def read_memory(self, memory: str, destination: pathlib.Path, fmt: str = "r",
+                    bitclock: str | None = None) -> bool:
+        """Read `memory` into `destination`, whose directory avrdude runs in."""
+        destination = pathlib.Path(destination).resolve()
+        destination.parent.mkdir(parents=True, exist_ok=True)
+        result = self.avrdude("-U", f"{memory}:r:{destination.name}:{fmt}",
+                              cwd=destination.parent, bitclock=bitclock)
+        # A memory the part does not have (a tiny's extended fuse) leaves avrdude
+        # happy and the file empty. An empty capture is a miss, not a backup.
+        return self._ok(result) and destination.exists() and destination.stat().st_size > 0
+
+    def write_memory(self, memory: str, source: pathlib.Path, fmt: str = "i",
+                     erase: bool = False, bitclock: str | None = None) -> bool:
+        source = pathlib.Path(source).resolve()
+        args = ["-U", f"{memory}:w:{source.name}:{fmt}"]
+        if erase:
+            args.insert(0, "-e")
+        result = self.avrdude(*args, cwd=source.parent, bitclock=bitclock)
+        return "verified" in (result.stdout + result.stderr)
+
+    def flash_hex(self, image: pathlib.Path, erase: bool = True,
+                  bitclock: str | None = None) -> bool:
+        return self.write_memory("flash", image, "i", erase=erase, bitclock=bitclock)
+
+    def read_fuses(self, bitclock: str | None = None) -> dict[str, str]:
+        out: dict[str, str] = {}
+        for fuse in ("lfuse", "hfuse", "efuse", "lock"):
+            result = self.avrdude("-U", f"{fuse}:r:-:h", bitclock=bitclock)
+            values = [l.strip() for l in result.stdout.splitlines() if l.strip().startswith("0x")]
+            if values:
+                out[fuse] = values[-1]
+        return out
+
+    def write_fuses(self, bitclock: str | None = None, **fuses: str) -> bool:
+        """Write named fuses. A fuse change moves the clock the *next* access is
+        timed against, so pass a bitclock safe for both sides of the change."""
+        args: list[str] = []
+        for name, value in fuses.items():
+            args += ["-U", f"{name}:w:{value}:m"]
+        if not args:
+            return True
+        result = self.avrdude(*args, bitclock=bitclock)
+        text = result.stdout + result.stderr
+        return "verified" in text or "written" in text
+
+    # ------------------------------------------------------------ backup
+
+    def backup(self, directory: pathlib.Path, prefix: str = "") -> dict[str, bool]:
+        """Capture every memory worth keeping, then prove it by a second read.
+
+        A backup nobody verified is a guess. Each memory is read twice and the
+        two reads compared; a mismatch is reported rather than quietly stored.
+        """
+        directory = pathlib.Path(directory).resolve()
+        directory.mkdir(parents=True, exist_ok=True)
+        stem = prefix or self.d.part
+        status: dict[str, bool] = {}
+        for memory, fmt, extension in BACKUP_MEMORIES:
+            name = f"{stem}-{memory}.{extension}"
+            if not self.read_memory(memory, directory / name, fmt):
+                status[f"{memory}.{extension}"] = False
+                continue
+            if extension == "bin":  # only the raw form is worth comparing byte-wise
+                again = directory / f".{name}.again"
+                self.read_memory(memory, again, fmt)
+                same = again.exists() and again.read_bytes() == (directory / name).read_bytes()
+                again.unlink(missing_ok=True)
+                status[f"{memory}.{extension}"] = same
+            else:
+                status[f"{memory}.{extension}"] = True
+        return status
+
+    # ------------------------------------------------------------ serial link
+
+    def pureboot(self, *args: str, reset_first: bool = True, baud: int | None = None,
+                 autobaud: bool | None = None, timeout: int = 300,
+                 bitclock: str | None = None) -> tuple[int, str]:
+        """Reset, then knock immediately - see the module docstring.
+
+        Returns the host tool's exit status and its combined output, so a caller
+        can assert on what it printed as well as on whether it succeeded.
+        """
+        if reset_first:
+            self.reset(bitclock=bitclock)
+        command = [sys.executable, str(self.d.pureboot), "--port", self.d.port,
+                   "--baud", str(self.d.baud if baud is None else baud),
+                   "--wait", str(self.d.wait)]
+        if self.d.autobaud if autobaud is None else autobaud:
+            command.append("--autobaud")
+        if self.d.one_wire:
+            command.append("--one-wire")
+        command += [str(a) for a in args]
+        try:
+            result = subprocess.run(command, capture_output=True, text=True, timeout=timeout)
+        except subprocess.TimeoutExpired as expired:
+            return 99, f"TIMEOUT after {timeout}s\n{expired.stdout or ''}{expired.stderr or ''}"
+        return result.returncode, (result.stdout or "") + (result.stderr or "")
+
+    def open_port(self, baud: int | None = None):
+        """A port opened the way this deployment says to speak to the board.
+
+        Everything the rig runs as a *subprocess* gets its flags from
+        `pureboot()` above; anything that drives the protocol in-process has
+        to reach the same facts, and until this existed only the subprocess
+        path could. A shared line is the one where that gap is fatal rather
+        than untidy: the host reads back every byte it writes, so an
+        undiscarded echo answers the knock before the device does. Open
+        through here and a one-wire deployment cannot be silently driven as
+        a two-wire one.
+        """
+        module = load_pureboot(self.d.pureboot)
+        port = module.Port(self.d.port, self.d.baud if baud is None else baud)
+        return module.OneWirePort(port) if self.d.one_wire else port
+
+    def capture(self, seconds: float = 2.0, baud: int | None = None) -> bytes:
+        """Listen to whatever the board is saying, at an arbitrary rate.
+
+        Opening the port does not reset a board whose DTR is unwired, so this can
+        sample a running application repeatedly without disturbing it - which is
+        what makes the rate sweep below possible.
+        """
+        module = load_pureboot(self.d.pureboot)
+        port = module.Port(self.d.port, self.d.baud if baud is None else baud)
+        try:
+            data = b""
+            deadline = time.monotonic() + seconds
+            while time.monotonic() < deadline:
+                chunk = port.read_available(0.2)
+                if chunk:
+                    data += chunk
+            return data
+        finally:
+            try:
+                port.close()
+            except Exception:
+                pass
+
+
+def measure_rate(rig: Rig, marker: bytes, built_baud: int, nominal_hz: int | None = None,
+                 span_percent: float = 12.0, step_percent: float = 0.5,
+                 seconds: float = 0.75) -> dict:
+    """Find a transmitting board's true bit rate, using only the serial port.
+
+    The board must be emitting something recognisable at a *fixed* cycles-per-bit
+    - `test/pbapp.cpp` built with PUREBOOT_HEARTBEAT does. Since its bit timing is
+    a cycle count, its wire rate scales with its actual clock, so the host rates
+    at which `marker` still decodes bracket that rate; the centre of the band is
+    the answer, and with the clock the image was built for it gives the real one.
+
+    This is the measurement that turns "the loader is silent, so the wiring must
+    be wrong" into a number, and it needs no instrument beyond the adapter
+    already attached.
+    """
+    steps = int(span_percent / step_percent)
+    clean: list[int] = []
+    samples: list[tuple[int, int, bool]] = []
+    for index in range(-steps, steps + 1):
+        baud = int(round(built_baud * (1 + index * step_percent / 100.0)))
+        if baud <= 0:
+            continue
+        data = rig.capture(seconds=seconds, baud=baud)
+        hit = marker in data
+        samples.append((baud, len(data), hit))
+        if hit:
+            clean.append(baud)
+    result: dict = {"samples": samples, "clean": clean, "built_baud": built_baud}
+    if clean:
+        low, high = min(clean), max(clean)
+        centre = (low + high) / 2.0
+        result |= {"low": low, "high": high, "centre": centre,
+                   "half_width_percent": (high - low) / 2.0 / centre * 100.0,
+                   "error_percent": (centre / built_baud - 1.0) * 100.0}
+        if nominal_hz:
+            result["measured_hz"] = nominal_hz * centre / built_baud
+    return result
+
+
+# ------------------------------------------------------------------- command
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(
+        description="pureboot hardware rig: ISP reset/flash beside the serial link")
+    Deployment.add_arguments(parser)
+    sub = parser.add_subparsers(dest="command", required=True)
+
+    sub.add_parser("signature", help="read the part signature over ISP")
+    sub.add_parser("reset", help="reset the part (an ISP access) and let it run")
+    sub.add_parser("fuses", help="read the fuse and lock bytes")
+
+    p = sub.add_parser("flash", help="program a hex image over ISP")
+    p.add_argument("image", type=pathlib.Path)
+    p.add_argument("--no-erase", action="store_true", help="do not chip-erase first")
+
+    p = sub.add_parser("backup", help="capture and verify every memory")
+    p.add_argument("directory", type=pathlib.Path)
+    p.add_argument("--prefix", default="", help="filename stem (default: the part name)")
+
+    p = sub.add_parser("rate", help="measure the board's true bit rate and clock")
+    p.add_argument("--marker", default="APP", help="text the board emits (default: APP)")
+    p.add_argument("--built-baud", type=int, required=True,
+                   help="the baud the running image was built for")
+    p.add_argument("--nominal-hz", type=int, default=0,
+                   help="the clock the image was built for, to report the real one")
+    p.add_argument("--span", type=float, default=12.0, help="sweep +-this many percent")
+    p.add_argument("--step", type=float, default=0.5, help="sweep step in percent")
+    p.add_argument("--verbose", action="store_true", help="print every step")
+
+    p = sub.add_parser("bitclock", help="a safe ISP bitclock for a clock in force")
+    p.add_argument("hz", type=int)
+
+    args = parser.parse_args(argv)
+
+    if args.command == "bitclock":
+        print(bitclock_for(args.hz))
+        return 0
+
+    rig = Rig(Deployment.from_args(args))
+
+    if args.command == "signature":
+        print(rig.signature())
+    elif args.command == "reset":
+        rig.reset()
+        print("reset")
+    elif args.command == "fuses":
+        for name, value in rig.read_fuses().items():
+            print(f"{name:<6} {value}")
+    elif args.command == "flash":
+        ok = rig.flash_hex(args.image, erase=not args.no_erase)
+        print(f"{args.image.name}: {'verified' if ok else 'FAILED'}")
+        return 0 if ok else 1
+    elif args.command == "backup":
+        status = rig.backup(args.directory, args.prefix)
+        for name, ok in status.items():
+            print(f"  {'ok  ' if ok else 'FAIL'} {name}")
+        missing = [n for n, ok in status.items() if not ok]
+        # Fuses a part does not have are expected misses, not failures.
+        fatal = [n for n in missing if not n.startswith(("efuse", "calibration"))]
+        print(f"\n{len(status) - len(missing)}/{len(status)} captured into {args.directory}")
+        return 1 if fatal else 0
+    elif args.command == "rate":
+        result = measure_rate(rig, args.marker.encode(), args.built_baud,
+                              args.nominal_hz or None, args.span, args.step)
+        if args.verbose:
+            for baud, size, hit in result["samples"]:
+                print(f"  {baud:7d} Bd  {size:5d} B  {'MARKER' if hit else ''}")
+        if not result["clean"]:
+            print(f"no capture contained {args.marker!r} at any rate - is the board "
+                  f"transmitting, and on the pin this port is wired to?")
+            return 1
+        print(f"clean band  {result['low']}..{result['high']} Bd")
+        print(f"centre      {result['centre']:.0f} Bd "
+              f"(+-{result['half_width_percent']:.1f} %)")
+        print(f"vs built    {result['built_baud']} Bd ({result['error_percent']:+.1f} %)")
+        if "measured_hz" in result:
+            print(f"true clock  {result['measured_hz'] / 1e6:.3f} MHz")
+    return 0
+
+
+if __name__ == "__main__":
+    try:
+        sys.exit(main())
+    except Error as error:
+        print(f"error: {error}", file=sys.stderr)
+        sys.exit(2)

tools/sizes.py

@@ -0,0 +1,194 @@
+#!/usr/bin/env python3
+"""What the loader images actually measure, and whether the README still agrees.
+
+The size matrix asserts every image fits its slot; it says nothing about the
+numbers the README prints, and those drift. Every row of that table was eight
+bytes stale once `startup::caller_page()` landed - common code, so every build
+moved at once and no test noticed, because none of them was over budget.
+
+Two questions, both answered from built trees:
+
+  sizes.py max            the largest image per chip, and anything over budget
+  sizes.py check-readme   the README's per-chip table against what is built
+
+Nothing here knows a chip's geometry. The (image, budget) pairs come from each
+build's own `CTestTestfile.cmake` - the same values the gate checks - so the
+slot rules stay where they belong, in `pureboot/CMakeLists.txt`, and a chip
+added or a budget changed needs no edit here. Only trees a configure preset
+still owns are read: a stale directory keeps its last build, and a loader built
+before a slot changed will happily report a size that was true once
+(`tools/prune-build-trees.sh` in libavr removes them).
+
+Sizes come from `avr-size`, and a target is only as current as its last build -
+run the gate first if you want the table checked against today's source.
+"""
+
+from __future__ import annotations
+
+import argparse
+import pathlib
+import re
+import shutil
+import subprocess
+import sys
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+# add_test(<name>.size ... -DELF=<path> ... -DLIMIT=<n> ...) - the gate's own
+# pairing of an image with the budget it must fit.
+# ctest writes the name as a bracket argument ([=[name.size]=]) and quotes the
+# rest, so the name starts after the bracket and the path ends at the quote.
+SIZE_TEST = re.compile(r'add_test\(\s*\[=\[(?P<name>[^\]]+?)\.size\]=\][^\n]*?'
+                       r'-DELF=(?P<elf>[^"\s]+)[^\n]*?-DLIMIT=(?P<limit>\d+)')
+
+
+def avr_size() -> str:
+    for env in (ROOT / "../../toolchain").resolve().glob("avr-gcc-*/bin/avr-size"):
+        if env.is_file():
+            return str(env)
+    found = shutil.which("avr-size")
+    if not found:
+        sys.exit("no avr-size found (build the toolchain, or put it on PATH)")
+    return found
+
+
+def preset_dirs() -> list[pathlib.Path]:
+    """Build trees a configure preset still owns, newest-listed first."""
+    listing = subprocess.run(["cmake", "--list-presets"], cwd=ROOT, capture_output=True, text=True)
+    names = re.findall(r'^\s*"(.+)"$', listing.stdout, re.MULTILINE)
+    if not names:
+        sys.exit("cmake --list-presets returned nothing - run from a configured checkout")
+    return [d for d in (ROOT / "build" / n for n in names) if (d / "CTestTestfile.cmake").is_file()]
+
+
+def measure(paths: list[str], tool: str) -> dict[str, int]:
+    """.text per ELF, in one avr-size call per batch."""
+    sizes: dict[str, int] = {}
+    for start in range(0, len(paths), 400):
+        batch = [p for p in paths[start:start + 400] if pathlib.Path(p).is_file()]
+        if not batch:
+            continue
+        out = subprocess.run([tool, *batch], capture_output=True, text=True).stdout
+        for line in out.splitlines()[1:]:
+            fields = line.split()
+            if len(fields) >= 6 and fields[0].isdigit():
+                sizes[fields[5]] = int(fields[0])
+    return sizes
+
+
+def collect() -> dict[str, list[tuple[str, int, int]]]:
+    """chip -> [(target, text, limit)], from every owned build tree."""
+    tool = avr_size()
+    found: dict[str, list[tuple[str, str, int]]] = {}
+    for tree in preset_dirs():
+        chip = tree.name.split("-")[0]
+        for match in SIZE_TEST.finditer((tree / "CTestTestfile.cmake").read_text()):
+            found.setdefault(chip, []).append((match["name"], match["elf"], int(match["limit"])))
+    sizes = measure([elf for rows in found.values() for _, elf, _ in rows], tool)
+    # A chip's generated and reflect trees must answer with the same bytes
+    # (the identity invariant), so the same target measuring two sizes means
+    # a stale tree - or an identity breach. Either is a finding; picking one
+    # silently is how a gate reports another build's numbers as today's.
+    for chip, rows in found.items():
+        seen: dict[str, tuple[int, str]] = {}
+        for name, elf, _ in rows:
+            if elf not in sizes:
+                continue
+            if name in seen and seen[name][0] != sizes[elf]:
+                sys.exit(f"{chip} {name}: {seen[name][0]} B in {seen[name][1]} but "
+                         f"{sizes[elf]} B in {elf} - a stale tree (rebuild or remove it) "
+                         f"or a cross-mode identity breach")
+            seen.setdefault(name, (sizes[elf], elf))
+    measured = {
+        chip: sorted(((name, sizes[elf], limit) for name, elf, limit in rows if elf in sizes),
+                     key=lambda row: -row[1])
+        for chip, rows in sorted(found.items())
+    }
+    # A configured-but-unbuilt preset registers its tests with no images behind
+    # them; it is not a chip with nothing to say, it is a chip not built yet.
+    return {chip: rows for chip, rows in measured.items() if rows}
+
+
+def cmd_max(args) -> int:
+    measured = collect()
+    if not measured:
+        sys.exit("nothing built - configure and build a preset first")
+    over = []
+    print(f"{'chip':<13} {'largest image':<34} {'.text':>6} {'budget':>7}  headroom")
+    for chip, rows in measured.items():
+        name, text, limit = rows[0]
+        flag = "OVER" if text > limit else f"{limit - text:>5} B"
+        print(f"{chip:<13} {name:<34} {text:>6} {limit:>7}  {flag}")
+        over += [(chip, n, t, l) for n, t, l in rows if t > l]
+    total = sum(len(rows) for rows in measured.values())
+    print(f"\n{total} images across {len(measured)} chips")
+    if over:
+        print("\nOVER BUDGET:")
+        for chip, name, text, limit in over:
+            print(f"  {chip} {name}: {text} > {limit}")
+        return 1
+    tightest = min(((chip, n, t, l) for chip, rows in measured.items() for n, t, l in rows),
+                   key=lambda row: row[3] - row[2])
+    chip, name, text, limit = tightest
+    print(f"tightest fit: {chip} {name} - {text} of {limit}, {limit - text} B spare")
+    return 0
+
+
+def cmd_check_readme(args) -> int:
+    """The README's per-chip table, against the stock build and the worst
+    autobaud configuration (OSCCAL baked, plus the USART-pin release where
+    the chip has a USART; the one-wire fold of the same build is its twin
+    and competes for the same cell) - the config the Autobaud column
+    documents."""
+    readme = (ROOT / "pureboot" / "README.md").read_text()
+    measured = collect()
+    rows = re.findall(r"^\|\s*(AT\w+[^|]*?)\s*\|[^|]*\|[^|]*\|[^|]*\|\s*(\d+) B\s*\|\s*(\d+) B\s*\|$",
+                      readme, re.MULTILINE)
+    if not rows:
+        sys.exit("no size table found in pureboot/README.md")
+    bad = skipped = 0
+    for chips, stock_doc, auto_doc in rows:
+        # "ATmega48, 48A, 48P, 48PA" plus any footnote mark - the first name
+        # is the family's base, and the sub strips the rest.
+        chip = re.sub(r"[^a-z0-9]", "", chips.split(",")[0].strip().lower())
+        built = {name: text for name, text, _ in measured.get(chip, [])}
+        # The on-USART pair defines the column where the chip has a USART;
+        # the default-pin pair is the whole space elsewhere. Whichever twin
+        # measures larger is the number the cell must state.
+        candidates = [name for name in ("pureboot_autobaud_osccal_on_usart0",
+                                        "pureboot_1w_autobaud_osccal_on_usart0") if name in built]
+        if not candidates:
+            candidates = [name for name in ("pureboot_autobaud_osccal",
+                                            "pureboot_1w_autobaud_osccal") if name in built]
+        worst = max(candidates, key=lambda name: built[name], default="pureboot_autobaud_osccal")
+        for target, documented in (("pureboot", stock_doc), (worst, auto_doc)):
+            if target not in built:
+                skipped += 1
+                continue
+            if built[target] != int(documented):
+                print(f"  {chip:<12} {target:<18} README says {documented} B, built is {built[target]} B")
+                bad += 1
+    if bad:
+        print(f"\n{bad} row(s) stale - update pureboot/README.md")
+        return 1
+    # A row whose target was not built is only skipped, so a chip-name change
+    # or a build tree that holds nothing would otherwise skip every row and
+    # report a match over an empty comparison.
+    if skipped == 2 * len(rows):
+        sys.exit(f"none of the {len(rows)} README rows matched a built image - "
+                 f"refusing to report a match over nothing")
+    print(f"README size table matches every built image ({len(rows)} rows"
+          + (f", {skipped} not built" if skipped else "") + ")")
+    return 0
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    subs = parser.add_subparsers(dest="cmd", required=True)
+    subs.add_parser("max", help="largest image per chip, and anything over budget")
+    subs.add_parser("check-readme", help="the README's size table against what is built")
+    args = parser.parse_args()
+    return {"max": cmd_max, "check-readme": cmd_check_readme}[args.cmd](args)
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tsb/tsb_asm.cpp

@@ -1,14 +1,15 @@
-// TinySafeBoot on libavr — tier 3: full feature parity in the 512-byte boot
+// TinySafeBoot on libavr - tier 3: full feature parity in the 512-byte boot
 // section, in C++ except where the C ABI itself is the cost.
 //
-// The complete TinySafeBoot feature set — watchdog-reset bail, one-wire
+// The complete TinySafeBoot feature set - watchdog-reset bail, one-wire
 // half-duplex UART, a config-page activation timeout, the password gate,
-// emergency erase, and config/flash/EEPROM read-write — at 510 bytes in the
-// 512-byte BOOTSZ=11 section the hand-written oracle occupies (500 B). This tier used to be one
-// monolithic inline-asm routine; it is now the tricks tier's C++ (same
-// register protocol, same structure — see tsb_tricks.cpp, including the
-// global-register miscompile rules) with exactly two routines kept in
-// assembly, the two whose remaining cost *is* the calling convention:
+// emergency erase, and config/flash/EEPROM read-write - inside the 512-byte
+// BOOTSZ=11 section the hand-written oracle occupies (oracle/README.md holds
+// what each tier measures, in one table rather than four). The
+// body is the tricks tier's C++ (same register protocol, same structure - see
+// tsb_tricks.cpp, including the global-register miscompile rules) with exactly
+// two routines kept in assembly, the two whose remaining cost *is* the calling
+// convention:
 //
 //   rx     the bounded receive: C++ must re-floor the timeout window on every
 //          call (the global-register-store miscompile) and split it across
@@ -16,12 +17,12 @@
 //          countdown.
 //   store  the page-store loop: C++ cannot hold the receive byte pair and the
 //          walked Z pointer across the rx calls without call-saved staging
-//          (push/pop + a Y→Z copy per word); the asm calls rx knowing exactly
+//          (push/pop + a Y->Z copy per word); the asm calls rx knowing exactly
 //          which registers it touches and walks Z live across the whole page.
 //
-// Everything else — bring-up, activation, password gate, emergency erase,
+// Everything else - bring-up, activation, password gate, emergency erase,
 // dispatch, every SPM/EEPROM/flash primitive, every geometry/baud/info
-// constant — is C++ on libavr, and the two asm routines splice into the same
+// constant - is C++ on libavr, and the two asm routines splice into the same
 // global-register protocol the C++ uses (g_addr in Y, g_cnt in r16, g_window
 // in r7, g_receiving in r6), so calls cross the boundary with no marshalling.
 //
@@ -41,10 +42,16 @@ namespace hw = avr::hw;
 namespace tsb {
 namespace {
 
-// The loader is purely polled — it never enables interrupts — so every SPM and
+// The loader is purely polled - it never enables interrupts - so every SPM and
 // EEPROM lock folds to nothing under this posture.
 constexpr auto off = avr::irq::guard_policy::unused;
 
+// Strict request/response: every SPM operation is waited out before the next
+// byte moves, so no flash operation is ever in flight at an EEPROM access -
+// the write procedure's step 2 has nothing to guard, the omission the
+// datasheet grants (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
 constexpr std::uint8_t confirm = '!';
 constexpr std::uint8_t request = '?';
 constexpr std::uint8_t knock = '@';
@@ -57,28 +64,34 @@ constexpr std::uint16_t boot_bytes = 512;
 constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
 constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
 
-// Lockout-proof floor for the activation window (the oracle's F_CPU/1MHz).
-constexpr std::uint8_t act_min = 16;
+// Lockout-proof floor for the activation window: the oracle's F_CPU/1MHz, so
+// it follows the clock rather than restating it (rule 41).
+constexpr auto act_min = static_cast<std::uint8_t>((16_MHz).hz / 1'000'000);
 // Post-activation window: the host gets seconds, not milliseconds, mid-session.
 constexpr std::uint8_t comm_window = 200;
 
 constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 20;
 
 // Fixed 115200 8N1; the library solves UBRR + U2X from clock and baud.
-constexpr auto baud = avr::uart::detail::solve_baud(16_MHz, 115200_Bd);
+constexpr auto baud = avr::uart::solve_baud(16_MHz, 115200_Bd, 8, avr::uart::parity::none);
+
+// One bit time on the wire: the turn-around a shared-line peer needs to stop
+// driving before this one starts. Derived from the solved rate, so it follows
+// the link rather than a count measured against one.
+constexpr auto guard_cycles = static_cast<std::uint32_t>((16_MHz).hz / baud.actual);
 
 // The 16-byte device-info block, streamed out on activation.
 // clang-format off
-[[gnu::progmem]] constexpr std::uint8_t info[16] = {
+[[gnu::progmem]] constexpr auto info = std::to_array<std::uint8_t>({
     'T', 'S', 'B',
     build_date & 0xFF, build_date >> 8,
     0xF3,             // status: native-UART fixed-baud lineage
-    0x1E, 0x95, 0x0F, // ATmega328P signature
+    avr::hw::db.signature[0], avr::hw::db.signature[1], avr::hw::db.signature[2],
     page / 2,         // page size in words
     (app_end / 2) & 0xFF, (app_end / 2) >> 8,
     eeprom_end & 0xFF, eeprom_end >> 8,
     0xAA, 0xAA,
-};
+});
 // clang-format on
 
 register std::uint16_t g_addr asm("r28");
@@ -94,7 +107,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 // Bounded byte receive (asm 1 of 2): release the one-wire line on a direction
 // change, poll RXC0 under the oracle's nested X-register countdown seeded from
 // g_window (floored against lockout), byte or 0-on-silence in r24. Z survives
-// — the property the store's word loop rides on.
+// - the property the store's word loop rides on.
 [[gnu::noinline, gnu::noclone]] std::uint8_t rx()
 {
 	std::uint8_t byte;
@@ -115,7 +128,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	             "	brne	3b			\n\t"
 	             "	sbiw	r26, 1			\n\t"
 	             "	brcc	2b			\n\t"
-	             "	clr	%[b]			\n\t" // silence → 0, which no compare accepts
+	             "	clr	%[b]			\n\t" // silence -> 0, which no compare accepts
 	             "	rjmp	5f			\n\t"
 	             "4:	lds	%[b], %[udr0]		\n\t"
 	             "5:					\n\t"
@@ -129,14 +142,13 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 // One-wire transmit: take the line (TXEN0 alone) on a direction change with a
 // turn-around guard, put the byte out, hold the line until the whole frame is
 // out (TXC0, not UDRE0), W1C TXC0 by storing the sampled status back (keeps
-// U2X0). Plain C++ — it compiles *smaller* than the oracle's routine.
+// U2X0). Plain C++ - it compiles *smaller* than the oracle's routine.
 [[gnu::noinline, gnu::noclone]] void tx(std::uint8_t byte)
 {
 	if (g_receiving) {
 		g_receiving = 0;
 		hw::ucsr0b::write(hw::ucsr0b::txen0(1));
-		for (std::uint8_t guard = 46; guard; --guard)
-			;
+		avr::delay::cycles<guard_cycles>();
 	}
 	hw::udr0::write(byte);
 	std::uint8_t status;
@@ -153,7 +165,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	return rx();
 }
 
-// One flash byte ← [g_addr++] (the advance right before ret — the
+// One flash byte <- [g_addr++] (the advance right before ret - the
 // global-register rule, see tsb_tricks.cpp).
 [[gnu::noinline, gnu::noclone]] std::uint8_t sflash()
 {
@@ -162,18 +174,18 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	return byte;
 }
 
-// One EEPROM byte ← [g_addr++].
+// One EEPROM byte <- [g_addr++].
 [[gnu::noinline, gnu::noclone]] std::uint8_t eerd()
 {
-	std::uint8_t byte = ee::read(g_addr);
+	std::uint8_t byte = ee::read<no_spm>(g_addr);
 	++g_addr;
 	return byte;
 }
 
-// One EEPROM byte → [g_addr++].
+// One EEPROM byte -> [g_addr++].
 [[gnu::noinline, gnu::noclone]] void eewr(std::uint8_t byte)
 {
-	ee::write<off>(g_addr, byte);
+	ee::write<off, no_spm>(g_addr, byte);
 	++g_addr;
 }
 
@@ -185,7 +197,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	} while (--g_cnt);
 }
 
-// Wait out a running SPM op, then re-open the RWW section — after every page
+// Wait out a running SPM op, then re-open the RWW section - after every page
 // op and before handing over, as the oracle does.
 [[gnu::noinline, gnu::noclone]] void settle()
 {
@@ -201,12 +213,12 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 	tsb_app();
 }
 
-// Step g_addr one page down and erase that page (the decrement lives here —
+// Step g_addr one page down and erase that page (the decrement lives here  -
 // the global-register rule).
 [[gnu::noinline, gnu::noclone]] void erase_below()
 {
 	g_addr -= page;
-	spm::erase_page<off>(g_addr);
+	spm::command<off>(spm::op::erase, g_addr);
 	settle();
 }
 
@@ -221,7 +233,7 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 }
 
 // Stream one host page into the erased flash page at g_addr (asm 2 of 2): the
-// word pair stages in r0:r1 straight from rx (whose register set is known —
+// word pair stages in r0:r1 straight from rx (whose register set is known  -
 // the cross-call liveness C++ cannot express), Z walks the page and PGWRT
 // programs it. g_addr is left at the next page base.
 [[gnu::noinline, gnu::noclone]] void store_flash()
@@ -256,14 +268,15 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 {
 	// A watchdog reset hands straight back to the application, as the
 	// reference loader does, rather than re-entering the bootloader.
-	if (hw::mcusr::wdrf.test())
+	if (hw::mcusr::wdrf.test()) {
 		appjump();
+	}
 
 	// Lean bring-up from reset state: UCSR0C already reads 8N1, UBRR0H reads
-	// 0, and rx()/tx() raise RXEN0/TXEN0 on first use — only the divisor low
+	// 0, and rx()/tx() raise RXEN0/TXEN0 on first use - only the divisor low
 	// byte and U2X0 need a store. The library still does the datasheet work.
 	static_assert(baud.u2x && baud.ubrr < 256, "lean bring-up writes UBRR0L only, with U2X0");
-	hw::reg<"UBRR0">::write(static_cast<std::uint8_t>(baud.ubrr));
+	hw::ubrr0::write(static_cast<std::uint8_t>(baud.ubrr));
 	hw::ucsr0a::write(hw::ucsr0a::u2x0(1));
 	// General-purpose registers are undefined at power-on (no crt zeroes them);
 	// the direction latch must start "not receiving" so the first rx() enables
@@ -271,13 +284,15 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 	// same reason.
 	g_receiving = 0;
 
-	// Activation: 3×'@', each inside the config page's timeout window (rx
-	// floors it so a corrupt page cannot lock the loader out); anything else —
-	// including silence — hands over.
+	// Activation: 3x'@', each inside the config page's timeout window (rx
+	// floors it so a corrupt page cannot lock the loader out); anything else  -
+	// including silence - hands over.
 	g_window = avr::flash_load(flash_ptr(app_end + 2));
-	for (std::uint8_t k = 3; k; --k)
-		if (rx() != knock)
+	for (std::uint8_t k = 3; k; --k) {
+		if (rx() != knock) {
 			appjump();
+		}
+	}
 	g_window = comm_window;
 
 	// Password gate (config page from app_end+3, 0xff-terminated; a blank
@@ -291,17 +306,19 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 		std::uint8_t expected = avr::flash_load(flash_ptr(g_addr)) & mask;
 		++g_addr;
 		if (expected == 0xff) {
-			g_addr = reinterpret_cast<std::uint16_t>(&info[0]);
+			g_addr = reinterpret_cast<std::uint16_t>(info.data());
 			g_cnt = sizeof(info);
 			sendf();
 			break;
 		}
 		std::uint8_t got = rx();
 		if (got == 0) {
-			if (mask == 0)
+			if (mask == 0) {
 				continue;
-			if (rcnf() != confirm || rcnf() != confirm)
+			}
+			if (rcnf() != confirm || rcnf() != confirm) {
 				appjump();
+			}
 			erase_application(); // leaves g_addr = 0 for the EEPROM walk
 			do {
 				eewr(0xff);
@@ -310,8 +327,9 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 			erase_below();
 			break;
 		}
-		if (got != expected)
+		if (got != expected) {
 			mask = 0;
+		}
 	}
 
 	for (;;) {
@@ -320,23 +338,27 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 		switch (rx()) {
 		case 'f': // read application flash, one page per host '!'
 			for (;;) {
-				if (rx() != confirm)
+				if (rx() != confirm) {
 					break;
+				}
 				g_cnt = page;
 				sendf();
-				if (g_addr >= app_end)
+				if (g_addr >= app_end) {
 					break;
+				}
 			}
 			break;
 		case 'F':                // erase the application, then take pages behind '?'
 			erase_application(); // leaves g_addr = 0, the write start
-			while (rcnf() == confirm)
+			while (rcnf() == confirm) {
 				store_flash();
+			}
 			break;
 		case 'e': // read EEPROM, one page per host '!', until the host stops
 			for (;;) {
-				if (rx() != confirm)
+				if (rx() != confirm) {
 					break;
+				}
 				g_cnt = page;
 				do {
 					tx(eerd());
@@ -358,8 +380,9 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 			sendf();
 			break;
 		case 'C': // replace the config page, then echo it back to verify
-			if (rcnf() != confirm)
+			if (rcnf() != confirm) {
 				break;
+			}
 			g_addr = app_end + page;
 			erase_below(); // leaves g_addr = app_end, the store target
 			store_flash();
@@ -373,14 +396,6 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 } // namespace
 } // namespace tsb
 
-// Reset lands here: BOOTRST vectors to the boot section base and .vectors is
-// laid first, so this is the first instruction executed. No crt ran, so set
-// the stack pointer before anything is called.
-extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
-{
-	SP = RAMEND;
-	// The one line of crt this loader needs: compiled code assumes
-	// __zero_reg__ (r1) is 0, and power-on registers are undefined.
-	asm volatile("clr __zero_reg__");
-	tsb::run();
-}
+// Reset lands at the boot section base (BOOTRST): the entry stub in .vectors
+// is laid first and does the one line of crt a crt-less image needs.
+template struct avr::startup::entry<tsb::run, avr::startup::stack::hardware>;

tsb/tsb_policy.cpp

@@ -0,0 +1,329 @@
+// TinySafeBoot on libavr - the policy floor: pureboot's rules, measured.
+//
+// The full TinySafeBoot feature set - watchdog bail, one-wire half-duplex,
+// config-page activation timeout, password gate, emergency erase, and
+// config/flash/EEPROM read-write - under philosophy #5 exactly as pureboot
+// obeys it: no assembly, no register variables; code, attributes, and flags
+// only. Every lesson pureboot's development produced is applied - the
+// library's half-duplex serial and startup entry, lean bring-up from reset
+// state, one merged send loop over both memories, oracle-shaped loop bounds,
+// locals threaded through noinline primitives, pureboot's codegen flags  -
+// and the result sits below the idiomatic tier and above the 512 B boot
+// section the tricks/asm tiers reach with the banned mechanisms
+// (oracle/README.md holds all four). This tier exists to keep that gap an
+// artifact
+// rather than a claim: the gap to 512 is the rent of policy-clean C++  -
+// helpers that hold a cursor across rx()/tx() pay push/pop and argument
+// threading where a global-register protocol pays nothing, and both
+// control-flow merges tried (a parametrized paged session, a merged store
+// loop) measured larger than the split cases they replaced. TSB's wire fixes
+// the per-command loop shapes on the device, so pureboot 5's one-transfer-
+// loop collapse has no purchase here.
+//
+// The wire protocol is strict request/response, which is what makes the
+// shared line safe: the device drives it only between a received command and
+// its reply, and releases it (the library's half-duplex choreography)
+// whenever it waits.
+
+#include <libavr/libavr.hpp>
+
+using namespace avr::literals;
+namespace spm = avr::spm;
+namespace ee = avr::eeprom;
+
+using dev = avr::device<{.clock = 16_MHz}>;
+// One-wire: RX and TX share the line, exactly as the native-UART TSB expects.
+// 115200 at 16 MHz lands +2.1 % off, past the receiver-tolerance table the
+// solver holds rates to - the oracle's own deployment has run there for a
+// decade, so the override states that it is meant.
+using serial_t = dev::uart0<{
+    .baud = 115200_Bd,
+    .allow_baud_error = true,
+    .half_duplex = true,
+}>;
+inline constexpr serial_t serial{};
+
+namespace tsb {
+namespace {
+
+// The loader is purely polled - it never enables interrupts - so every SPM and
+// EEPROM lock folds to nothing under this posture.
+constexpr auto off = avr::irq::guard_policy::unused;
+
+// Strict request/response: every SPM operation is waited out before the next
+// byte moves, so no flash operation is ever in flight at an EEPROM access -
+// the write procedure's step 2 has nothing to guard, the omission the
+// datasheet grants (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
+// The handshake bytes, identical across every TSB host.
+constexpr std::uint8_t confirm = '!';
+constexpr std::uint8_t request = '?';
+constexpr std::uint8_t knock = '@';
+
+// Boot geometry for the 1 KB boot section (BOOTSZ=10); the page size and the
+// flash/EEPROM extents are the chip database's to know. app_end is the config
+// page (TSB's LASTPAGE), one page below the boot section.
+constexpr std::uint16_t page = spm::page_bytes;
+constexpr std::uint16_t boot_bytes = 1024;
+constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
+constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
+
+// Lockout-proof floor for the activation window: the oracle's F_CPU/1MHz, so
+// it follows the clock rather than restating it (rule 41).
+constexpr auto act_min = static_cast<std::uint8_t>(dev::clock.hz / 1'000'000);
+// Post-activation window: the host gets seconds, not milliseconds, mid-session.
+constexpr std::uint8_t comm_window = 200;
+
+// Firmware version stamp: YY*512 + MM*32 + DD, the encoding the host decodes.
+constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 27;
+
+// The 16-byte device-info block, streamed out on activation.
+// clang-format off
+[[gnu::progmem]] constexpr auto info = std::to_array<std::uint8_t>({
+    'T', 'S', 'B',
+    build_date & 0xFF, build_date >> 8,
+    0xF3,             // status: native-UART fixed-baud lineage
+    avr::hw::db.signature[0], avr::hw::db.signature[1], avr::hw::db.signature[2],
+    page / 2,         // page size in words
+    (app_end / 2) & 0xFF, (app_end / 2) >> 8, // app-flash boundary, words
+    eeprom_end & 0xFF, eeprom_end >> 8,
+    0xAA, 0xAA,       // ATmega processor-type marker (bytes 14 == 15)
+});
+// clang-format on
+
+// The receive window, pre-floored where it is set. In .noinit: there is no
+// crt to clear a .bss image, and run() stores it before the first receive.
+[[gnu::section(".noinit")]] std::uint8_t window;
+
+const std::uint8_t *flash_ptr(std::uint16_t addr)
+{
+	return reinterpret_cast<const std::uint8_t *>(addr);
+}
+
+// Bounded byte receive: poll under nested countdowns, 0 on silence. The 0
+// then falls through every compare - not a knock, not a confirm, not a
+// command - so a silent host unwinds the loader to the application from
+// anywhere, and a mid-session cable pull cannot wedge it. The line release on
+// a direction change is the serial backend's.
+[[gnu::noinline]] std::uint8_t rx()
+{
+	std::uint16_t outer = static_cast<std::uint16_t>(window) << 8;
+	do {
+		std::uint8_t fine = 0;
+		do {
+			if (auto byte = serial.read()) {
+				return *byte;
+			}
+		} while (--fine);
+	} while (--outer);
+	return 0;
+}
+
+// One-wire transmit: the backend takes the line with a turn-around guard and
+// holds it until the whole frame is out.
+[[gnu::noinline]] void tx(std::uint8_t byte)
+{
+	serial.write(byte);
+}
+
+// '?', then hand back the host's reply for the callers' one-byte compare.
+[[gnu::noinline]] std::uint8_t rcnf()
+{
+	tx(request);
+	return rx();
+}
+
+// The one send loop: the info block, the config page, application flash and
+// EEPROM pages all stream through here.
+[[gnu::noinline]] void send_block(bool eep, std::uint16_t at, std::uint8_t count)
+{
+	do {
+		tx(eep ? ee::read<no_spm>(at) : avr::flash_load(flash_ptr(at)));
+		++at;
+	} while (--count);
+}
+
+// One EEPROM byte in - shared by the emergency wipe and the 'E' stream.
+[[gnu::noinline]] void eeput(std::uint16_t at, std::uint8_t value)
+{
+	ee::write<off, no_spm>(at, value);
+}
+
+// Wait out a running SPM op, then re-open the RWW section - after every page
+// op and before handing over, as the oracle does.
+[[gnu::noinline]] void settle()
+{
+	spm::wait();
+	spm::rww_enable<off>();
+}
+
+// One host page straight into the erased flash page at `at` - through the SPM
+// word buffer (low byte then high), no SRAM staging - then committed. `at`
+// names a page base, so the cursor's low byte reaching the boundary ends the
+// walk.
+[[gnu::noinline]] void store_flash_page(std::uint16_t at)
+{
+	const auto open = spm::page::begin<spm::from::boot_section, off>(at);
+	do {
+		std::uint8_t low = rx();
+		std::uint8_t high = rx();
+		spm::fill<off>(open, at, std::bit_cast<std::uint16_t>(std::array{low, high}));
+		at += 2;
+	} while (static_cast<std::uint8_t>(at) & (page - 1));
+	spm::command<off>(spm::op::write, at - page);
+	settle();
+}
+
+extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --defsym=tsb_app=0
+
+[[noreturn]] void appjump()
+{
+	settle();
+	tsb_app();
+}
+
+// Step one page down and erase it - the erase shared by the whole-app walk,
+// the config rewrite and the emergency wipe; hands the stepped address back.
+[[gnu::noinline]] std::uint16_t erase_below(std::uint16_t at)
+{
+	at -= page;
+	spm::command<off>(spm::op::erase, at);
+	settle();
+	return at;
+}
+
+// Erase the whole application, top-down like the oracle: the loop bound is a
+// compare with zero, and the returned 0 is the address every caller wants
+// next.
+[[gnu::noinline]] std::uint16_t erase_application()
+{
+	std::uint16_t at = app_end;
+	do {
+		at = erase_below(at);
+	} while (at != 0);
+	return at;
+}
+
+[[noreturn]] void run()
+{
+	// A watchdog reset hands straight back to the application, as the
+	// reference loader does, rather than re-entering the bootloader.
+	if (avr::hw::mcusr::wdrf.test()) {
+		appjump();
+	}
+
+	// Lean bring-up from reset state: UCSR0C already reads 8N1, UBRR0H reads
+	// 0, and the half-duplex write()/read() raise TXEN0/RXEN0 on first use  -
+	// only the divisor low byte and U2X0 need a store. The solver still does
+	// the datasheet work; the asserts pin the reset-state assumptions.
+	{
+		constexpr auto sol = avr::uart::solve_baud(dev::clock, 115200_Bd, 8, avr::uart::parity::none);
+		static_assert(sol.u2x && sol.ubrr < 256, "lean bring-up writes UBRR0L only, with U2X0");
+		avr::hw::ubrr0::write(static_cast<std::uint8_t>(sol.ubrr));
+		avr::hw::ucsr0a::write(avr::hw::ucsr0a::u2x0(1));
+	}
+
+	// Activation: 3x'@', each inside the config page's timeout window
+	// (floored so a corrupt page cannot lock the loader out); anything else  -
+	// including silence - hands over.
+	window = avr::flash_load(flash_ptr(app_end + 2)) | act_min;
+	for (std::uint8_t k = 3; k; --k) {
+		if (rx() != knock) {
+			appjump();
+		}
+	}
+	window = comm_window;
+
+	// Password gate (config page from app_end+3, 0xff-terminated; a blank
+	// page is no password). A wrong byte blanks the comparison and drains the
+	// line forever, so a wrong password can never fall through; a 0 requests
+	// emergency erase behind two confirms. On pass the info block goes out;
+	// the emergency path skips it and drops into the command loop.
+	std::uint16_t at = app_end + 3;
+	std::uint8_t mask = 0xff;
+	for (;;) {
+		std::uint8_t expected = avr::flash_load(flash_ptr(at)) & mask;
+		++at;
+		if (expected == 0xff) {
+			send_block(false, reinterpret_cast<std::uint16_t>(info.data()), info.size());
+			break;
+		}
+		std::uint8_t got = rx();
+		if (got == 0) {
+			if (mask == 0) {
+				continue;
+			}
+			if (rcnf() != confirm || rcnf() != confirm) {
+				appjump();
+			}
+			std::uint16_t a = erase_application();
+			do {
+				eeput(a, 0xff);
+			} while (++a <= eeprom_end);
+			erase_below(app_end + page);
+			break;
+		}
+		if (got != expected) {
+			mask = 0;
+		}
+	}
+
+	for (;;) {
+		tx(confirm); // Mainloop ready
+		const std::uint8_t command = rx();
+		switch (command) {
+		case 'f': // read application flash, one page per host '!'
+			for (std::uint16_t a = 0; a < app_end; a += page) {
+				if (rx() != confirm) {
+					break;
+				}
+				send_block(false, a, page);
+			}
+			break;
+		case 'e': // read EEPROM, one page per host '!', until the host stops
+			for (std::uint16_t a = 0;; a += page) {
+				if (rx() != confirm) {
+					break;
+				}
+				send_block(true, a, page);
+			}
+			break;
+		case 'F': { // erase the application, then take pages behind '?'
+			std::uint16_t a = erase_application();
+			for (; rcnf() == confirm; a += page) {
+				store_flash_page(a);
+			}
+			break;
+		}
+		case 'E': // take EEPROM pages behind '?', each write host-paced
+			for (std::uint16_t a = 0; rcnf() == confirm;) {
+				std::uint8_t count = page;
+				do {
+					eeput(a, rx());
+					++a;
+				} while (--count);
+			}
+			break;
+		case 'c': // read the config page
+		read_config:
+			send_block(false, app_end, page);
+			break;
+		case 'C': // replace the config page, then echo it back to verify
+			if (rcnf() != confirm) {
+				break;
+			}
+			store_flash_page(erase_below(app_end + page));
+			goto read_config;
+		default: // 'q' or any other byte runs the application
+			appjump();
+		}
+	}
+}
+
+} // namespace
+} // namespace tsb
+
+// Reset lands at the boot section base (BOOTRST): the entry stub in .vectors
+// is laid first and does the one line of crt a crt-less image needs.
+template struct avr::startup::entry<tsb::run, avr::startup::stack::hardware>;

tsb/tsb_pure.cpp

@@ -1,10 +1,10 @@
-// TinySafeBoot on libavr — tier 1: pure, idiomatic C++.
+// TinySafeBoot on libavr - tier 1: pure, idiomatic C++.
 //
 // A serial flash bootloader for the ATmega328P boot section, reimplementing the
 // TinySafeBoot native-UART fixed-baud protocol on libavr with the full feature
 // set of the hand-written oracle: a watchdog-reset bail, one-wire half-duplex,
 // a config-page activation timeout, the password gate, emergency erase, and
-// config/flash/EEPROM read-write. This variant is written for clarity —
+// config/flash/EEPROM read-write. This variant is written for clarity  -
 // well-factored functions, no compiler-specific size hacks, no inline assembly.
 // The one-wire wiring, the flash-resident info block and every SPM/EEPROM lock
 // are libavr's to handle; the only attribute is the naked reset entry that
@@ -20,16 +20,29 @@ namespace ee = avr::eeprom;
 
 using dev = avr::device<{.clock = 16_MHz}>;
 // One-wire: RX and TX share the line, exactly as the native-UART TSB expects.
-using serial_t = dev::uart0<{.baud = 115200_Bd, .max_baud_error = 3_pct, .half_duplex = true}>;
+// 115200 at 16 MHz lands +2.1 % off, past the receiver-tolerance table the
+// solver holds rates to - the oracle's own deployment has run there for a
+// decade, so the override states that it is meant.
+using serial_t = dev::uart0<{
+    .baud = 115200_Bd,
+    .allow_baud_error = true,
+    .half_duplex = true,
+}>;
 inline constexpr serial_t serial{};
 
 namespace tsb {
 namespace {
 
-// The loader is purely polled — it never enables interrupts — so every SPM and
+// The loader is purely polled - it never enables interrupts - so every SPM and
 // EEPROM lock folds to nothing under this posture.
 constexpr auto off = avr::irq::guard_policy::unused;
 
+// Strict request/response: every SPM operation is waited out before the next
+// byte moves, so no flash operation is ever in flight at an EEPROM access -
+// the write procedure's step 2 has nothing to guard, the omission the
+// datasheet grants (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
 // The handshake bytes, identical across every TSB host.
 constexpr std::uint8_t confirm = '!';
 constexpr std::uint8_t request = '?';
@@ -50,26 +63,48 @@ constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 20;
 // The 16-byte device-info block the host reads on activation. A flash_table
 // keeps it in progmem with no .data image (there is no crt to copy one).
 // clang-format off
-inline constexpr std::array<std::uint8_t, 16> info_data = {
+inline constexpr auto info_data = std::to_array<std::uint8_t>({
     'T', 'S', 'B',
     build_date & 0xFF, build_date >> 8,
     0xF3,             // status byte (native-UART fixed-baud lineage)
-    0x1E, 0x95, 0x0F, // ATmega328P signature
+    avr::hw::db.signature[0], avr::hw::db.signature[1], avr::hw::db.signature[2],
     page / 2,         // page size in words
     (app_end / 2) & 0xFF, (app_end / 2) >> 8, // app-flash boundary, words
     eeprom_end & 0xFF, eeprom_end >> 8,
     0xAA, 0xAA, // ATmega processor-type marker (bytes 14 == 15)
-};
+});
 // clang-format on
 using info = avr::flash_table<info_data>;
 
-// Blocking byte read/write over the one-wire line: read() releases the line to
-// the receiver, write() takes it and holds it until the frame is out.
+// The lockout-proof floor for the receive window: the oracle's F_CPU/1MHz, so
+// it follows the clock rather than restating it.
+constexpr auto act_min = static_cast<std::uint8_t>(dev::clock.hz / 1'000'000);
+
+// The receive window, pre-floored where it is set. In .noinit: there is no crt
+// to clear a .bss image, and run() stores it before the first receive.
+[[gnu::section(".noinit")]] std::uint8_t window;
+
+// Bounded byte read over the one-wire line - read() releases the line to the
+// receiver - answering 0 on silence. That 0 falls through every compare below:
+// not a knock, not a confirm, not a command, so a silent host unwinds the
+// loader to the application from anywhere and a mid-session cable pull cannot
+// wedge it. The oracle lists that timeout among its own fixes, and a blocking
+// read is how a tier loses it.
 std::uint8_t rx()
 {
-	return serial.read_blocking();
+	std::uint16_t outer = static_cast<std::uint16_t>(window) << 8;
+	do {
+		std::uint8_t fine = 0;
+		do {
+			if (auto byte = serial.read()) {
+				return *byte;
+			}
+		} while (--fine);
+	} while (--outer);
+	return 0;
 }
 
+// write() takes the line and holds it until the frame is out.
 void tx(std::uint8_t byte)
 {
 	serial.write(byte);
@@ -83,14 +118,16 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 // Stream `count` bytes to the host, from flash (LPM) or from EEPROM.
 void send_flash(std::uint16_t addr, std::uint8_t count)
 {
-	while (count--)
+	while (count--) {
 		tx(avr::flash_load(flash_ptr(addr++)));
+	}
 }
 
 void send_eeprom(std::uint16_t addr, std::uint8_t count)
 {
-	while (count--)
-		tx(ee::read(addr++));
+	while (count--) {
+		tx(ee::read<no_spm>(addr++));
+	}
 }
 
 // Prompt the host with '?' and report whether it answered '!'.
@@ -101,32 +138,33 @@ bool request_confirm()
 }
 
 // Stream one page from the host straight into the already-erased flash page at
-// `addr`, filling the SPM word buffer low byte then high — no SRAM staging, so
+// `addr`, filling the SPM word buffer low byte then high - no SRAM staging, so
 // receiving and programming are the same loop.
 void store_flash_page(std::uint16_t addr)
 {
+	const auto open = spm::page::begin<spm::from::boot_section, off>(addr);
 	for (std::uint16_t i = 0; i < page; i += 2) {
 		std::uint8_t lo = rx();
 		std::uint8_t hi = rx();
-		spm::fill<off>(addr + i, static_cast<std::uint16_t>(lo | (hi << 8)));
+		spm::fill<off>(open, addr + i, static_cast<std::uint16_t>(lo | (hi << 8)));
 	}
-	spm::write_page<off>(addr);
-	spm::wait();
+	spm::write_page<spm::from::boot_section, off>(addr); // blocking: waits the write out
 }
 
 // Stream one page from the host straight into EEPROM, byte by byte.
 void store_eeprom_page(std::uint16_t addr)
 {
-	for (std::uint16_t i = 0; i < page; ++i)
-		ee::write<off>(addr + i, rx());
+	for (std::uint16_t i = 0; i < page; ++i) {
+		ee::write<off, no_spm>(addr + i, rx());
+	}
 }
 
-// Erase one flash page and wait it out — the erase step shared by the whole-app
-// erase, the config-page rewrite and the emergency wipe.
+// Erase one flash page, waited out by the blocking spelling - the erase step
+// shared by the whole-app erase, the config-page rewrite and the emergency
+// wipe.
 void erase_page(std::uint16_t addr)
 {
-	spm::erase_page<off>(addr);
-	spm::wait();
+	spm::erase_page<spm::from::boot_section, off>(addr);
 }
 
 // Erase the whole application, one page at a time, top-down as the reference
@@ -157,8 +195,9 @@ extern "C" [[noreturn]] void tsb_app();
 void read_flash()
 {
 	for (std::uint16_t a = 0; a < app_end; a += page) {
-		if (rx() != confirm)
+		if (rx() != confirm) {
 			return;
+		}
 		send_flash(a, page);
 	}
 }
@@ -167,8 +206,9 @@ void read_flash()
 void read_eeprom()
 {
 	for (std::uint16_t a = 0;; a += page) {
-		if (rx() != confirm)
+		if (rx() != confirm) {
 			return;
+		}
 		send_eeprom(a, page);
 	}
 }
@@ -178,22 +218,25 @@ void read_eeprom()
 void write_flash()
 {
 	erase_application();
-	for (std::uint16_t a = 0; request_confirm(); a += page)
+	for (std::uint16_t a = 0; request_confirm(); a += page) {
 		store_flash_page(a);
+	}
 }
 
 // 'E': take pages the host offers behind '?' into EEPROM.
 void write_eeprom()
 {
-	for (std::uint16_t a = 0; request_confirm(); a += page)
+	for (std::uint16_t a = 0; request_confirm(); a += page) {
 		store_eeprom_page(a);
+	}
 }
 
 // 'C': replace the config page, then echo it back for the host to verify.
 void write_config()
 {
-	if (!request_confirm())
+	if (!request_confirm()) {
 		return;
+	}
 	erase_page(app_end);
 	store_flash_page(app_end);
 	spm::rww_enable<off>();
@@ -206,8 +249,9 @@ void write_config()
 void emergency_erase()
 {
 	erase_application();
-	for (std::uint16_t a = 0; a <= eeprom_end; ++a)
-		ee::write<off>(a, 0xff);
+	for (std::uint16_t a = 0; a <= eeprom_end; ++a) {
+		ee::write<off, no_spm>(a, 0xff);
+	}
 	erase_page(app_end);
 	spm::rww_enable<off>();
 }
@@ -222,14 +266,18 @@ gate password_gate()
 {
 	for (const std::uint8_t *pw = flash_ptr(app_end + 3);; ++pw) {
 		std::uint8_t expected = avr::flash_load(pw);
-		if (expected == 0xff)
+		if (expected == 0xff) {
 			return gate::pass;
+		}
 		std::uint8_t got = rx();
-		if (got == 0)
+		if (got == 0) {
 			return gate::emergency;
-		if (got != expected)
-			for (;;)
+		}
+		if (got != expected) {
+			for (;;) {
 				rx();
+			}
+		}
 	}
 }
 
@@ -237,21 +285,25 @@ gate password_gate()
 {
 	// A watchdog reset hands straight back to the application, as the reference
 	// loader does, rather than re-entering the bootloader.
-	if (avr::hw::mcusr::wdrf.test())
+	if (avr::hw::mcusr::wdrf.test()) {
 		appjump();
+	}
 
 	avr::init<serial_t>();
 
 	// Activation: the host knocks three '@' inside a window whose length is the
-	// config page's timeout byte (floored so a corrupt page can never lock the
-	// loader out). An idle port times out and boots the application.
-	__uint24 idle = static_cast<__uint24>(avr::flash_load(flash_ptr(app_end + 2)) | 16) << 16;
+	// config page's timeout byte, floored so a corrupt page can never lock the
+	// loader out. An idle port times out and boots the application; the same
+	// window then bounds every receive of the session.
+	window = avr::flash_load(flash_ptr(app_end + 2)) | act_min;
+	__uint24 idle = static_cast<__uint24>(window) << 16;
 	std::uint8_t knocks = 0;
 	while (knocks < 3) {
-		if (auto byte = serial.read())
+		if (auto byte = serial.read()) {
 			knocks = *byte == knock ? knocks + 1 : 0;
-		else if (--idle == 0)
+		} else if (--idle == 0) {
 			appjump();
+		}
 	}
 
 	switch (password_gate()) {
@@ -259,8 +311,9 @@ gate password_gate()
 		send_flash(reinterpret_cast<std::uint16_t>(info::storage.data()), info::size());
 		break;
 	case gate::emergency:
-		if (!request_confirm() || !request_confirm())
+		if (!request_confirm() || !request_confirm()) {
 			appjump();
+		}
 		emergency_erase();
 		break;
 	}
@@ -295,14 +348,6 @@ gate password_gate()
 } // namespace
 } // namespace tsb
 
-// Reset lands here: BOOTRST vectors to the boot section base and .vectors is
-// laid first, so this is the first instruction executed. No crt ran, so set the
-// stack pointer before anything is called.
-extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
-{
-	SP = RAMEND;
-	// The one line of crt this loader needs: compiled code assumes
-	// __zero_reg__ (r1) is 0, and power-on registers are undefined.
-	asm volatile("clr __zero_reg__");
-	tsb::run();
-}
+// Reset lands at the boot section base (BOOTRST): the entry stub in .vectors
+// is laid first and does the one line of crt a crt-less image needs.
+template struct avr::startup::entry<tsb::run, avr::startup::stack::hardware>;

tsb/tsb_tricks.cpp

@@ -1,16 +1,16 @@
-// TinySafeBoot on libavr — tier 2: C++ with compiler trickery, no assembly.
+// TinySafeBoot on libavr - tier 2: C++ with compiler trickery, no assembly.
 //
-// The full TinySafeBoot feature set — watchdog bail, one-wire half-duplex,
+// The full TinySafeBoot feature set - watchdog bail, one-wire half-duplex,
 // config-page activation timeout, password gate, emergency erase, and
-// config/flash/EEPROM read-write — in pure C++, 526 bytes: 14 over the 512-byte
-// boot section the hand-written oracle fits, from 168 over at this tier's first
-// floor. The structure mirrors the oracle's: a handful of tiny noinline
+// config/flash/EEPROM read-write - in pure C++, a little over the 512-byte boot
+// section the hand-written oracle fits (oracle/README.md holds what each tier
+// measures). The structure mirrors the oracle's: a handful of tiny noinline
 // primitives sharing one whole-loader register allocation, expressed as global
 // register variables so no helper ever saves, spills, or reloads any of it.
 //
 // The register protocol (all call-saved, so calls preserve them by ABI):
-//   Y (r28:r29)  g_addr      the walked flash/EEPROM address — adiw-able
-//   r16          g_cnt       byte countdown of the running block — ldi-able
+//   Y (r28:r29)  g_addr      the walked flash/EEPROM address - adiw-able
+//   r16          g_cnt       byte countdown of the running block - ldi-able
 //   r7           g_window    rx timeout, roughly 30 ms units at 16 MHz
 //   r6           g_receiving one-wire direction latch, cleared at bring-up
 //                            (power-on registers are undefined)
@@ -18,10 +18,10 @@
 // GCC 16.1 miscompiles stores into global register variables: an update whose
 // remaining uses all hide inside callees is deleted whenever a CALL follows it
 // before any jump/ret (the backend's liveness walk lumps fixed registers with
-// call-clobbered ones — minimal repro in libavr's
-// local/scratch/probes/gcc-avr-globalreg-repro.cpp, lessons.md entry). Every
+// call-clobbered ones - minimal repro in libavr's
+// test/upstream/gcc-avr-globalreg-repro.cpp). Every
 // g_* update below therefore sits where a *local* read or a jump/ret follows
-// it — the helpers advance g_addr immediately before returning, and rx()
+// it - the helpers advance g_addr immediately before returning, and rx()
 // re-floors the window on every call instead of storing the floored value
 // once. The layout is load-bearing; do not "simplify" it.
 //
@@ -41,10 +41,16 @@ namespace hw = avr::hw;
 namespace tsb {
 namespace {
 
-// The loader is purely polled — it never enables interrupts — so every SPM and
+// The loader is purely polled - it never enables interrupts - so every SPM and
 // EEPROM lock folds to nothing under this posture.
 constexpr auto off = avr::irq::guard_policy::unused;
 
+// Strict request/response: every SPM operation is waited out before the next
+// byte moves, so no flash operation is ever in flight at an EEPROM access -
+// the write procedure's step 2 has nothing to guard, the omission the
+// datasheet grants (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
 constexpr std::uint8_t confirm = '!';
 constexpr std::uint8_t request = '?';
 constexpr std::uint8_t knock = '@';
@@ -57,28 +63,34 @@ constexpr std::uint16_t boot_bytes = 1024;
 constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
 constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
 
-// Lockout-proof floor for the activation window (the oracle's F_CPU/1MHz).
-constexpr std::uint8_t act_min = 16;
+// Lockout-proof floor for the activation window: the oracle's F_CPU/1MHz, so
+// it follows the clock rather than restating it (rule 41).
+constexpr auto act_min = static_cast<std::uint8_t>((16_MHz).hz / 1'000'000);
 // Post-activation window: the host gets seconds, not milliseconds, mid-session.
 constexpr std::uint8_t comm_window = 200;
 
 constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 20;
 
 // Fixed 115200 8N1; the library solves UBRR + U2X from clock and baud.
-constexpr auto baud = avr::uart::detail::solve_baud(16_MHz, 115200_Bd);
+constexpr auto baud = avr::uart::solve_baud(16_MHz, 115200_Bd, 8, avr::uart::parity::none);
+
+// One bit time on the wire: the turn-around a shared-line peer needs to stop
+// driving before this one starts. Derived from the solved rate, so it follows
+// the link rather than a count measured against one.
+constexpr auto guard_cycles = static_cast<std::uint32_t>((16_MHz).hz / baud.actual);
 
 // The 16-byte device-info block, streamed out on activation.
 // clang-format off
-[[gnu::progmem]] constexpr std::uint8_t info[16] = {
+[[gnu::progmem]] constexpr auto info = std::to_array<std::uint8_t>({
     'T', 'S', 'B',
     build_date & 0xFF, build_date >> 8,
     0xF3,             // status: native-UART fixed-baud lineage
-    0x1E, 0x95, 0x0F, // ATmega328P signature
+    avr::hw::db.signature[0], avr::hw::db.signature[1], avr::hw::db.signature[2],
     page / 2,         // page size in words
     (app_end / 2) & 0xFF, (app_end / 2) >> 8,
     eeprom_end & 0xFF, eeprom_end >> 8,
     0xAA, 0xAA,
-};
+});
 // clang-format on
 
 register std::uint16_t g_addr asm("r28");
@@ -93,8 +105,8 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 
 // Bounded byte receive, the oracle's shape: release the one-wire line on a
 // direction change, poll RXC0 under nested countdowns, 0 on silence. The 0
-// then falls through every compare — not a knock, not a confirm, not a
-// command — so a silent host unwinds the loader to the application from
+// then falls through every compare - not a knock, not a confirm, not a
+// command - so a silent host unwinds the loader to the application from
 // anywhere, and a mid-session cable pull cannot wedge it.
 [[gnu::noinline, gnu::noclone]] std::uint8_t rx()
 {
@@ -102,24 +114,25 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 		g_receiving = 1;
 		hw::ucsr0b::write(hw::ucsr0b::rxen0(1)); // RXEN0 alone: release and listen
 	}
-	// act_min ORs in here, per call, not once into g_window at setup — the
+	// act_min ORs in here, per call, not once into g_window at setup - the
 	// one placement the global-register-store miscompile cannot delete.
 	std::uint16_t outer = static_cast<std::uint16_t>(g_window | act_min) << 8;
 	do {
 		std::uint8_t fine = 0;
 		do {
 			auto status = hw::ucsr0a::read();
-			if (status & hw::ucsr0a::rxc0(1).value)
+			if (status & hw::ucsr0a::rxc0(1).value) {
 				return hw::udr0::read();
+			}
 		} while (--fine);
 	} while (--outer);
 	return 0;
 }
 
-// One-wire transmit: take the line (TXEN0 alone — the receiver must be off
+// One-wire transmit: take the line (TXEN0 alone - the receiver must be off
 // while driving) on a direction change, with a turn-around guard so a shorted
 // peer can switch first; then hold the line until the whole frame is out
-// (TXC0, not UDRE0 — the stop bit must be on the wire before a caller may
+// (TXC0, not UDRE0 - the stop bit must be on the wire before a caller may
 // release the line), and W1C TXC0 by storing the sampled status back, which
 // keeps U2X0.
 [[gnu::noinline, gnu::noclone]] void tx(std::uint8_t byte)
@@ -127,8 +140,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	if (g_receiving) {
 		g_receiving = 0;
 		hw::ucsr0b::write(hw::ucsr0b::txen0(1));
-		for (std::uint8_t guard = 46; guard; --guard)
-			;
+		avr::delay::cycles<guard_cycles>();
 	}
 	hw::udr0::write(byte);
 	std::uint8_t status;
@@ -145,7 +157,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	return rx();
 }
 
-// One flash byte ← [g_addr++] (the advance right before ret — see header).
+// One flash byte <- [g_addr++] (the advance right before ret - see header).
 [[gnu::noinline, gnu::noclone]] std::uint8_t sflash()
 {
 	std::uint8_t byte = avr::flash_load(flash_ptr(g_addr));
@@ -153,18 +165,18 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	return byte;
 }
 
-// One EEPROM byte ← [g_addr++].
+// One EEPROM byte <- [g_addr++].
 [[gnu::noinline, gnu::noclone]] std::uint8_t eerd()
 {
-	std::uint8_t byte = ee::read(g_addr);
+	std::uint8_t byte = ee::read<no_spm>(g_addr);
 	++g_addr;
 	return byte;
 }
 
-// One EEPROM byte → [g_addr++].
+// One EEPROM byte -> [g_addr++].
 [[gnu::noinline, gnu::noclone]] void eewr(std::uint8_t byte)
 {
-	ee::write<off>(g_addr, byte);
+	ee::write<off, no_spm>(g_addr, byte);
 	++g_addr;
 }
 
@@ -176,7 +188,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 	} while (--g_cnt);
 }
 
-// Wait out a running SPM op, then re-open the RWW section — after every page
+// Wait out a running SPM op, then re-open the RWW section - after every page
 // op and before handing over, as the oracle does.
 [[gnu::noinline, gnu::noclone]] void settle()
 {
@@ -198,12 +210,12 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 [[gnu::noinline, gnu::noclone]] void erase_below()
 {
 	g_addr -= page;
-	spm::erase_page<off>(g_addr);
+	spm::command<off>(spm::op::erase, g_addr);
 	settle();
 }
 
 // Erase the whole application, top-down like the oracle: the loop bound is a
-// compare with zero, and g_addr = 0 — the value every caller wants next — is
+// compare with zero, and g_addr = 0 - the value every caller wants next - is
 // handed back for free.
 [[gnu::noinline, gnu::noclone]] void erase_application()
 {
@@ -214,18 +226,19 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 }
 
 // Stream one host page into the erased flash page at g_addr (SPM word buffer,
-// low byte then high) — no SRAM staging, receive and program are one loop.
+// low byte then high) - no SRAM staging, receive and program are one loop.
 // g_addr is left at the next page base.
 [[gnu::noinline, gnu::noclone]] void store_flash()
 {
+	const auto open = spm::page::begin<spm::from::boot_section, off>(g_addr);
 	g_cnt = page / 2;
 	do {
 		std::uint16_t word = rx();
 		word |= static_cast<std::uint16_t>(rx()) << 8;
-		spm::fill<off>(g_addr, word);
+		spm::fill<off>(open, g_addr, word);
 		g_addr += 2;
 	} while (--g_cnt);
-	spm::write_page<off>(g_addr - page);
+	spm::command<off>(spm::op::write, g_addr - page);
 	settle();
 }
 
@@ -233,14 +246,15 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 {
 	// A watchdog reset hands straight back to the application, as the
 	// reference loader does, rather than re-entering the bootloader.
-	if (hw::mcusr::wdrf.test())
+	if (hw::mcusr::wdrf.test()) {
 		appjump();
+	}
 
 	// Lean bring-up from reset state: UCSR0C already reads 8N1, UBRR0H reads
-	// 0, and rx()/tx() raise RXEN0/TXEN0 on first use — only the divisor low
+	// 0, and rx()/tx() raise RXEN0/TXEN0 on first use - only the divisor low
 	// byte and U2X0 need a store. The library still does the datasheet work.
 	static_assert(baud.u2x && baud.ubrr < 256, "lean bring-up writes UBRR0L only, with U2X0");
-	hw::reg<"UBRR0">::write(static_cast<std::uint8_t>(baud.ubrr));
+	hw::ubrr0::write(static_cast<std::uint8_t>(baud.ubrr));
 	hw::ucsr0a::write(hw::ucsr0a::u2x0(1));
 	// General-purpose registers are undefined at power-on (no crt zeroes them);
 	// the direction latch must start "not receiving" so the first rx() enables
@@ -248,13 +262,15 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 	// same reason.
 	g_receiving = 0;
 
-	// Activation: 3×'@', each inside the config page's timeout window (rx
-	// floors it so a corrupt page cannot lock the loader out); anything else —
-	// including silence — hands over.
+	// Activation: 3x'@', each inside the config page's timeout window (rx
+	// floors it so a corrupt page cannot lock the loader out); anything else  -
+	// including silence - hands over.
 	g_window = avr::flash_load(flash_ptr(app_end + 2));
-	for (std::uint8_t k = 3; k; --k)
-		if (rx() != knock)
+	for (std::uint8_t k = 3; k; --k) {
+		if (rx() != knock) {
 			appjump();
+		}
+	}
 	g_window = comm_window;
 
 	// Password gate (config page from app_end+3, 0xff-terminated; a blank
@@ -268,17 +284,19 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 		std::uint8_t expected = avr::flash_load(flash_ptr(g_addr)) & mask;
 		++g_addr;
 		if (expected == 0xff) {
-			g_addr = reinterpret_cast<std::uint16_t>(&info[0]);
+			g_addr = reinterpret_cast<std::uint16_t>(info.data());
 			g_cnt = sizeof(info);
 			sendf();
 			break;
 		}
 		std::uint8_t got = rx();
 		if (got == 0) {
-			if (mask == 0)
+			if (mask == 0) {
 				continue;
-			if (rcnf() != confirm || rcnf() != confirm)
+			}
+			if (rcnf() != confirm || rcnf() != confirm) {
 				appjump();
+			}
 			erase_application(); // leaves g_addr = 0 for the EEPROM walk
 			do {
 				eewr(0xff);
@@ -287,8 +305,9 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 			erase_below();
 			break;
 		}
-		if (got != expected)
+		if (got != expected) {
 			mask = 0;
+		}
 	}
 
 	for (;;) {
@@ -297,23 +316,27 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 		switch (rx()) {
 		case 'f': // read application flash, one page per host '!'
 			for (;;) {
-				if (rx() != confirm)
+				if (rx() != confirm) {
 					break;
+				}
 				g_cnt = page;
 				sendf();
-				if (g_addr >= app_end)
+				if (g_addr >= app_end) {
 					break;
+				}
 			}
 			break;
 		case 'F':                // erase the application, then take pages behind '?'
 			erase_application(); // leaves g_addr = 0, the write start
-			while (rcnf() == confirm)
+			while (rcnf() == confirm) {
 				store_flash();
+			}
 			break;
 		case 'e': // read EEPROM, one page per host '!', until the host stops
 			for (;;) {
-				if (rx() != confirm)
+				if (rx() != confirm) {
 					break;
+				}
 				g_cnt = page;
 				do {
 					tx(eerd());
@@ -335,8 +358,9 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 			sendf();
 			break;
 		case 'C': // replace the config page, then echo it back to verify
-			if (rcnf() != confirm)
+			if (rcnf() != confirm) {
 				break;
+			}
 			g_addr = app_end + page;
 			erase_below(); // leaves g_addr = app_end, the store target
 			store_flash();
@@ -350,14 +374,6 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 } // namespace
 } // namespace tsb
 
-// Reset lands here: BOOTRST vectors to the boot section base and .vectors is
-// laid first, so this is the first instruction executed. No crt ran, so set
-// the stack pointer before anything is called.
-extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
-{
-	SP = RAMEND;
-	// The one line of crt this loader needs: compiled code assumes
-	// __zero_reg__ (r1) is 0, and power-on registers are undefined.
-	asm volatile("clr __zero_reg__");
-	tsb::run();
-}
+// Reset lands at the boot section base (BOOTRST): the entry stub in .vectors
+// is laid first and does the one line of crt a crt-less image needs.
+template struct avr::startup::entry<tsb::run, avr::startup::stack::hardware>;