pureboot: the unified autobaud loader, and the hang that settled the decision

Hardware testing found that a lone calibration pulse wedged the autobaud loader:
run() budgeted only the start-edge wait in measure(), and the rx() that read the
knock behind it was unbudgeted, so one stray low pulse held an unattended device
in the loader and the application never ran. 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 an idle line boots the app there.

That fix costs ~22 B, which neither version under review could absorb: the pure
one goes 508 -> 530 on the 1284P and the register one 512 -> 534, both over a
512 B slot. Their margin was never spare capacity, it was the space the missing
fix should have occupied. So the choice between them is moot; both are kept for
the record and no longer built.

pureboot_autobaud_uni.cpp replaces them at 464 B. It is pureboot 5: one read
command and one write command over named spaces (G/g, sel8, addr16, n8) instead
of four per-memory bodies, which collapses four transfer loops into one. The
selector's high nibble carries flash's bank, so the shared cursor stays 16 bits
and no command speaks word addresses. Three things fall out of the freed space:
RAM read/write — the missing feature, and with it arbitrary I/O access, since
AVR maps peripherals into the data space; host-issued SPM, so W's hardcoded
erase/write/RWW tail becomes three writes to a space and any SPM operation is
reachable; and W on the same selector-and-address decode as everything else.

Strictly pure throughout: no inline asm, no global register variable, and no
GPIOR either — the unit lives in a .noinit static, so the loader claims no chip
resource and the chips without GPIOR stop being a special case.

pureboot.py speaks both generations, keyed on the version, so the fixed-baud
path is untouched; --peek/--poke reach the new data space. pbautobaud.py adds a
RAM round-trip and a regression for the hang: a lone pulse must still let the
app boot. All 37 chips plus the 12-preset reflect spot set build and size-test
green, 444-466 B, worst case 46 B under budget. Sim suites 100%: 1284P 17/17,
328P 23/23. Only real-hardware acceptance remains (pureboot/autobaud.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "libavr"]
+	path = libavr
+	url = ../libavr.git

CMakeLists.txt

@@ -8,6 +8,9 @@ include(FetchContent)
 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()
@@ -51,13 +54,18 @@ if(PROJECT_IS_TOP_LEVEL)
 	endif()
 endif()
 
-# avrdude programs Intel-HEX; the ELF is only a container (symbols, section
+# The ELF is only a container (symbols, section headers) and is never flashed —
-# headers) and is never flashed. Every loader image therefore gets a .hex beside
+# and the host tool's load_image() dispatches on extension, so handing it one
-# it at link time. .eeprom is dropped — EEPROM content is its own avrdude update.
+# would silently program the header bytes. Every loader image therefore gets
-function(add_hex_output name)
+# 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.
+function(add_image_outputs name)
 	add_custom_command(TARGET ${name} POST_BUILD
 	                   COMMAND ${CMAKE_OBJCOPY} -O ihex -R .eeprom
-	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.hex)
+	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.hex
+	                   COMMAND ${CMAKE_OBJCOPY} -O binary -R .eeprom
+	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.bin)
 endfunction()
 
 # The TinySafeBoot protocol reimplemented on libavr in three variants that trade
@@ -99,7 +107,7 @@ function(add_tsb_variant name bytes)
 	target_link_options(${name} PRIVATE -nostartfiles -Wl,--section-start=.text=${base_hex}
 	                    -Wl,--defsym=tsb_app=0 -Wl,--pmem-wrap-around=32k)
 	add_custom_command(TARGET ${name} POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:${name}>)
-	add_hex_output(${name})
+	add_image_outputs(${name})
 	if(PROJECT_IS_TOP_LEVEL)
 		add_test(NAME ${name}.size
 		         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:${name}>
@@ -121,70 +129,31 @@ if(LIBAVR_MCU STREQUAL "atmega328p")
 endif()
 
 # pureboot — the pure-constraint port (see pureboot/README.md): one source,
-# no inline assembly, no global register variables, every libavr chip, 512
+# no inline assembly, no global register variables, every libavr chip,
-# bytes each. The loader owns the top 512 bytes of flash on every chip; the
+# fitting each chip's smallest boot sector. The geometry and the
-# application entry symbol is address 0 on the mega (reset re-vectors to the
+# pureboot_add_loader() deployment function live in pureboot/CMakeLists.txt —
-# loader through BOOTRST, so word 0 stays the application's own vector) and
+# the unit a downstream project consumes; everything below is this port's
-# the trampoline word just below the loader on the tinies (host-side vector
+# own build: the stock loaders, their tests, and the size matrix. The
-# surgery points it at the application). --pmem-wrap-around models AVR's
+# distinct binary dir keeps the `pureboot` target's output name free.
-# modulo-flash PC where the flash is big enough to need it.
+add_subdirectory(pureboot pureboot-cmake)
-#
-# The image is position-independent (check_pi.py asserts the two link-time
-# facts that make it so), and on the tinies its budget is 510, not 512: the
-# slot's last word is the trampoline the host composes — the resident slot's
-# holds the application entry, and a staging copy's holds the jump through
-# which it reaches the loader it installed. The activation window is a
-# compile-time constant; a different PUREBOOT_TIMEOUT builds the re-timed
-# binary a self-update then installs.
-set(PUREBOOT_TIMEOUT 8 CACHE STRING "pureboot activation window, seconds")
-if(LIBAVR_MCU STREQUAL "attiny13a")
-	set(_pb_flash 1024)
-	set(_pb_wrap "")
-	set(_pb_page 32)
-	set(_pb_hz 9600000)
-	set(_pb_baud 57600)
-	set(_pb_eeprom 64)
-	set(_pb_limit 510)
-elseif(LIBAVR_MCU STREQUAL "attiny85")
-	set(_pb_flash 8192)
-	set(_pb_wrap -Wl,--pmem-wrap-around=8k)
-	set(_pb_page 64)
-	set(_pb_hz 8000000)
-	set(_pb_baud 57600)
-	set(_pb_eeprom 512)
-	set(_pb_limit 510)
-else()
-	set(_pb_flash 32768)
-	set(_pb_wrap -Wl,--pmem-wrap-around=32k)
-	set(_pb_page 128)
-	set(_pb_hz 16000000)
-	set(_pb_baud 115200)
-	set(_pb_eeprom 1024)
-	set(_pb_limit 512)
-endif()
-math(EXPR _pb_base "${_pb_flash} - 512")
-math(EXPR _pb_base_hex "${_pb_base}" OUTPUT_FORMAT HEXADECIMAL)
-if(LIBAVR_MCU STREQUAL "atmega328p")
-	set(_pb_app 0)
-else()
-	math(EXPR _pb_app "${_pb_base} - 2")
-endif()
 
-add_executable(pureboot pureboot/pureboot.cpp)
+# The stock loader: the family-default deployment (crystal/RC clock, the
-target_link_libraries(pureboot PRIVATE libavr)
+# chip's natural link, default pins). The activation window stays a cache
-target_compile_definitions(pureboot PRIVATE PUREBOOT_TIMEOUT=${PUREBOOT_TIMEOUT})
+# variable — re-timing a deployed loader is a self-update with a re-timed
-target_link_options(pureboot PRIVATE -nostartfiles -Wl,--section-start=.text=${_pb_base_hex}
+# build. pureboot9 is that re-timed build, and what the update test installs.
-                    -Wl,--defsym=pureboot_app=${_pb_app} ${_pb_wrap})
+set(PUREBOOT_TIMEOUT 8 CACHE STRING "pureboot activation window, seconds")
-add_custom_command(TARGET pureboot POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:pureboot>)
+pureboot_add_loader(pureboot TIMEOUT ${PUREBOOT_TIMEOUT})
-add_hex_output(pureboot)
 if(PROJECT_IS_TOP_LEVEL)
+	get_target_property(_pb_stock_hz pureboot PUREBOOT_HZ)
+	get_target_property(_pb_stock_baud pureboot PUREBOOT_BAUD)
+
 	add_test(NAME pureboot.size
 	         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:pureboot>
-	                 -DLIMIT=${_pb_limit} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
+	                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
 	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> ${_pb_base_hex})
+		                 ${CMAKE_OBJDUMP} ${CMAKE_NM} $<TARGET_FILE:pureboot> ${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)
@@ -200,9 +169,9 @@ if(PROJECT_IS_TOP_LEVEL)
 		                   COMMAND ${CMAKE_OBJCOPY} -O binary $<TARGET_FILE:pbapp> $<TARGET_FILE:pbapp>.bin)
 		add_test(NAME pureboot.protocol
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbtest.py
-		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${LIBAVR_MCU} ${_pb_hz} ${_pb_base_hex}
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
-		                 ${_pb_page} ${_pb_baud} ${_pb_eeprom} $<TARGET_FILE:pbapp>.bin
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud} ${PUREBOOT_EEPROM}
-		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 $<TARGET_FILE:pbapp>.bin ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
 		                 ${CMAKE_BINARY_DIR}/pbtest-work)
 		set_tests_properties(pureboot.protocol PROPERTIES TIMEOUT 180)
 
@@ -210,29 +179,247 @@ if(PROJECT_IS_TOP_LEVEL)
 		# installed one slot lower, must serve the full command set.
 		add_test(NAME pureboot.reloc
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbreloc.py
-		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${LIBAVR_MCU} ${_pb_hz} ${_pb_base_hex}
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
-		                 ${_pb_page} ${_pb_baud} ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud}
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
 		                 ${CMAKE_BINARY_DIR}/pbreloc-work)
 		set_tests_properties(pureboot.reloc PROPERTIES TIMEOUT 180
 		                     ENVIRONMENT "PB_OBJCOPY=${CMAKE_OBJCOPY}")
 
+		# Entering the loader from a running application with no reset
+		# 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.
+		if(LIBAVR_MCU STREQUAL "atmega328p")
+			add_test(NAME pureboot.dirty
+			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbdirty.py
+			                 ${PB_DEVICE} $<TARGET_FILE:pureboot> ${PUREBOOT_SIM_MCU} ${_pb_stock_hz}
+			                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_pb_stock_baud}
+			                 $<TARGET_FILE:pbapp>.bin
+			                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+			                 ${CMAKE_BINARY_DIR}/pbdirty-work)
+			set_tests_properties(pureboot.dirty PROPERTIES TIMEOUT 180)
+		endif()
+
+		# Re-homing: a loader mistakenly programmed at address 0 (a raw .bin
+		# handed to a programmer) or sitting in the staging slot must heal
+		# into the canonical slot through the ordinary --update-loader flow.
+		# Patched-vector behavior, so one representative chip carries it.
+		if(LIBAVR_MCU STREQUAL "attiny85")
+			add_test(NAME pureboot.rehome
+			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbrehome.py
+			                 ${PB_DEVICE} $<TARGET_FILE:pureboot> $<TARGET_FILE:pureboot9>.bin
+			                 ${PUREBOOT_SIM_MCU} ${_pb_stock_hz} ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE}
+			                 ${_pb_stock_baud} $<TARGET_FILE:pbapp>.bin
+			                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+			                 ${CMAKE_BINARY_DIR}/pbrehome-work)
+			set_tests_properties(pureboot.rehome PROPERTIES TIMEOUT 180)
+		endif()
+
 		# The self-update end-to-end: the re-timed build (same source, only
-		# PUREBOOT_TIMEOUT differs — a byte-different image) replaces the
+		# the timeout differs — a byte-different image) replaces the resident
-		# resident through --update-loader, with every power-fail phase
+		# through --update-loader, with every power-fail phase rehearsed from
-		# rehearsed from the runner's flash dumps.
+		# the runner's flash dumps.
-		add_executable(pureboot9 pureboot/pureboot.cpp)
+		pureboot_add_loader(pureboot9 TIMEOUT 9)
-		target_link_libraries(pureboot9 PRIVATE libavr)
-		target_compile_definitions(pureboot9 PRIVATE PUREBOOT_TIMEOUT=9)
-		target_link_options(pureboot9 PRIVATE -nostartfiles -Wl,--section-start=.text=${_pb_base_hex}
-		                    -Wl,--defsym=pureboot_app=${_pb_app} ${_pb_wrap})
-		add_hex_output(pureboot9)
 		add_test(NAME pureboot.update
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbupdate.py
-		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> $<TARGET_FILE:pureboot9> ${LIBAVR_MCU}
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot> $<TARGET_FILE:pureboot9>
-		                 ${_pb_hz} ${_pb_base_hex} ${_pb_page} ${_pb_baud} $<TARGET_FILE:pbapp>.bin
+		                 ${PUREBOOT_SIM_MCU} ${_pb_stock_hz} ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE}
+		                 ${_pb_stock_baud} $<TARGET_FILE:pbapp>.bin
 		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
 		                 ${CMAKE_BINARY_DIR}/pbupdate-work)
 		set_tests_properties(pureboot.update PROPERTIES TIMEOUT 600
 		                     ENVIRONMENT "PB_OBJCOPY=${CMAKE_OBJCOPY}")
 	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.
+	function(pureboot_size_variant name)
+		pureboot_add_loader(${name} ${ARGN})
+		add_test(NAME ${name}.size
+		         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:${name}>
+		                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
+	endfunction()
+
+	# The autobaud loader (pureboot/autobaud.md): one clock-agnostic image per
+	# chip, no clock × baud axis, size-tested against the same per-chip budget on
+	# every chip.
+	#
+	# Only the unified version is built. Its two predecessors —
+	# pureboot_autobaud_pure.cpp at 508 B and pureboot_autobaud_reg.cpp at 512 —
+	# had 4 B and 0 B of margin on the 1284P, and the fix for the activation hang
+	# costs ~22, which puts them at 530 and 534. Neither can ship, so the choice
+	# the branch existed to offer is settled by measurement rather than taste.
+	# The sources stay for the record; autobaud.md carries the numbers.
+	function(pureboot_autobaud_variant name source)
+		pureboot_add_autobaud(${name} ${source})
+		add_test(NAME ${name}.size
+		         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:${name}>
+		                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
+	endfunction()
+	pureboot_autobaud_variant(pureboot_autobaud_uni pureboot_autobaud_uni.cpp)
+
+	# One point of the exhaustive matrix, named from its resolved parameters
+	# so the enumeration cannot collide with itself. Unreachable rates drop
+	# out here rather than aborting the configure.
+	function(pureboot_matrix_point hz baud link)
+		if(link STREQUAL "software")
+			pureboot_baud_feasible(${hz} ${baud} 1 _ok)
+			set(_args SERIAL software)
+		else()
+			pureboot_baud_feasible(${hz} ${baud} 0 _ok)
+			set(_args USART ${link})
+		endif()
+		if(_ok)
+			pureboot_size_variant(pbm_${hz}_${baud}_${link} 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).
+	if(LIBAVR_MCU MATCHES "^attiny13")
+		set(_matrix_clocks 1200000 4800000 9600000)
+		set(_full_clocks 128000 600000 1200000 4800000 9600000)
+	else()
+		set(_matrix_clocks 1000000 8000000 16000000)
+		set(_full_clocks 128000 1000000 1843200 2000000 3686400 4000000 7372800 8000000
+		                 11059200 12000000 14745600 16000000 18432000 20000000)
+	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
+	# 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.
+	#
+	# 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.
+	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)
+		foreach(_matrix_hz IN LISTS _full_clocks)
+			foreach(_matrix_baud IN LISTS _full_bauds)
+				pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} software)
+				if(PUREBOOT_HAS_USART)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 0)
+				endif()
+				if(PUREBOOT_HAS_USART1)
+					pureboot_matrix_point(${_matrix_hz} ${_matrix_baud} 1)
+				endif()
+			endforeach()
+		endforeach()
+	else()
+		foreach(_matrix_hz IN LISTS _matrix_clocks)
+			math(EXPR _matrix_khz "${_matrix_hz} / 1000")
+			if(PUREBOOT_HAS_USART OR NOT _matrix_hz EQUAL _pb_stock_hz)
+				pureboot_size_variant(pureboot_sw_${_matrix_khz}k CLOCK ${_matrix_hz} SERIAL software)
+			endif()
+			if(PUREBOOT_HAS_USART AND NOT _matrix_hz EQUAL _pb_stock_hz)
+				pureboot_size_variant(pureboot_hw_${_matrix_khz}k CLOCK ${_matrix_hz} SERIAL hardware)
+			endif()
+			if(PUREBOOT_HAS_USART1 AND NOT _matrix_hz EQUAL _pb_stock_hz)
+				pureboot_size_variant(pureboot_usart1_${_matrix_khz}k CLOCK ${_matrix_hz} USART 1)
+			endif()
+		endforeach()
+		list(GET _matrix_clocks -1 _matrix_top_hz)
+		pureboot_size_variant(pureboot_sw_wide CLOCK ${_matrix_top_hz} BAUD 9600 SERIAL software)
+	endif()
+	if(PUREBOOT_HAS_USART1)
+		pureboot_size_variant(pureboot_usart1 USART 1)
+	endif()
+
+	# One configured deployment end to end — a real board's shape rather
+	# than the stock assumption: the ATmega328P on its shipped 1 MHz fuses,
+	# the software UART on hand-picked pins (TX = PB1, RX = PB5), the ladder
+	# baud (9600). The full protocol suite runs against it, fixture
+	# application included, over the runner's GPIO bridge — proving the
+	# configuration plumbing produces a working loader, not just one that
+	# fits.
+	if(LIBAVR_MCU STREQUAL "atmega328p" AND DEFINED PB_DEVICE)
+		pureboot_size_variant(pureboot_custom CLOCK 1000000 SERIAL software RX pb5 TX pb1)
+		get_target_property(_custom_hz pureboot_custom PUREBOOT_HZ)
+		get_target_property(_custom_baud pureboot_custom PUREBOOT_BAUD)
+		get_target_property(_custom_link pureboot_custom PUREBOOT_LINK)
+		add_executable(pbapp_custom test/pbapp.cpp)
+		target_link_libraries(pbapp_custom PRIVATE libavr)
+		target_compile_definitions(pbapp_custom PRIVATE PUREBOOT_CLOCK_HZ=${_custom_hz}
+		                           PUREBOOT_BAUD=${_custom_baud} PUREBOOT_SOFT_SERIAL PUREBOOT_TX=pb1)
+		add_custom_command(TARGET pbapp_custom POST_BUILD
+		                   COMMAND ${CMAKE_OBJCOPY} -O binary
+		                           $<TARGET_FILE:pbapp_custom> $<TARGET_FILE:pbapp_custom>.bin)
+		add_test(NAME pureboot.custom
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbtest.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_custom> ${PUREBOOT_SIM_MCU} ${_custom_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_custom_baud} ${PUREBOOT_EEPROM}
+		                 $<TARGET_FILE:pbapp_custom>.bin ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbcustom-work ${_custom_link})
+		set_tests_properties(pureboot.custom PROPERTIES TIMEOUT 180)
+	endif()
+
+	# The second USART, driven for real on one chip: instance selection is
+	# compile-checked everywhere, but only a live session proves the loader
+	# initialized and polls the USART it claims to. The fixture application
+	# banners on the same instance.
+	if(LIBAVR_MCU STREQUAL "atmega644a" AND DEFINED PB_DEVICE)
+		get_target_property(_usart1_hz pureboot_usart1 PUREBOOT_HZ)
+		get_target_property(_usart1_baud pureboot_usart1 PUREBOOT_BAUD)
+		add_executable(pbapp_usart1 test/pbapp.cpp)
+		target_link_libraries(pbapp_usart1 PRIVATE libavr)
+		target_compile_definitions(pbapp_usart1 PRIVATE PUREBOOT_CLOCK_HZ=${_usart1_hz}
+		                           PUREBOOT_BAUD=${_usart1_baud} PUREBOOT_USART=1)
+		add_custom_command(TARGET pbapp_usart1 POST_BUILD
+		                   COMMAND ${CMAKE_OBJCOPY} -O binary
+		                           $<TARGET_FILE:pbapp_usart1> $<TARGET_FILE:pbapp_usart1>.bin)
+		add_test(NAME pureboot.usart1
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbtest.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_usart1> ${PUREBOOT_SIM_MCU} ${_usart1_hz}
+		                 ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_usart1_baud} ${PUREBOOT_EEPROM}
+		                 $<TARGET_FILE:pbapp_usart1>.bin ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbusart1-work usart1)
+		set_tests_properties(pureboot.usart1 PROPERTIES TIMEOUT 180)
+	endif()
+
+	# The autobaud variants driven end to end over the software-UART bridge (both
+	# under review — pureboot/autobaud.md): the host sends the 0xC0 calibration
+	# pulse, the loader times it, locks, and programs. Run on the near-flash 328P
+	# and the word-addressed 1284P — the two flash-addressing classes — and each
+	# at two clocks with the one binary, which is the clock-agnostic property
+	# autobaud exists for (test/pbautobaud.py). The fixture application banners
+	# over the same software link at the first clock's rate.
+	if(LIBAVR_MCU MATCHES "^atmega(328p|1284p)$" AND DEFINED PB_DEVICE)
+		add_executable(pbapp_autobaud test/pbapp.cpp)
+		target_link_libraries(pbapp_autobaud PRIVATE libavr)
+		target_compile_definitions(pbapp_autobaud PRIVATE PUREBOOT_CLOCK_HZ=1000000
+		                           PUREBOOT_BAUD=9600 PUREBOOT_SOFT_SERIAL PUREBOOT_TX=pb1)
+		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}
+			         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
+			                 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()
+	endif()
 endif()

pureboot/CMakeLists.txt

@@ -0,0 +1,347 @@
+# 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.
+# 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):
+#
+#   add_subdirectory(bootloader/pureboot)
+#   pureboot_add_loader(myboot CLOCK 1000000 SERIAL software TX pb1 RX pb5)
+
+# 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
+# the x4 family's one single-USART die (Atmel-2593).
+set(_pb_has_usart 1)
+set(_pb_has_usart1 0)
+if(LIBAVR_MCU MATCHES "^attiny13a?$")
+	set(_pb_flash 1024)
+	set(_pb_wrap "")
+	set(_pb_page 32)
+	set(_pb_hz 9600000)
+	set(_pb_eeprom 64)
+	set(_pb_has_usart 0)
+elseif(LIBAVR_MCU STREQUAL "attiny25")
+	set(_pb_flash 2048)
+	set(_pb_wrap "")
+	set(_pb_page 32)
+	set(_pb_hz 8000000)
+	set(_pb_eeprom 128)
+	set(_pb_has_usart 0)
+elseif(LIBAVR_MCU STREQUAL "attiny45")
+	set(_pb_flash 4096)
+	set(_pb_wrap "")
+	set(_pb_page 64)
+	set(_pb_hz 8000000)
+	set(_pb_eeprom 256)
+	set(_pb_has_usart 0)
+elseif(LIBAVR_MCU STREQUAL "attiny85")
+	set(_pb_flash 8192)
+	set(_pb_wrap -Wl,--pmem-wrap-around=8k)
+	set(_pb_page 64)
+	set(_pb_hz 8000000)
+	set(_pb_eeprom 512)
+	set(_pb_has_usart 0)
+elseif(LIBAVR_MCU MATCHES "^atmega48(a|p|pa)?$")
+	set(_pb_flash 4096)
+	set(_pb_wrap "")
+	set(_pb_page 64)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 256)
+elseif(LIBAVR_MCU MATCHES "^atmega8a?$" OR LIBAVR_MCU MATCHES "^atmega88(a|p|pa)?$")
+	set(_pb_flash 8192)
+	set(_pb_wrap -Wl,--pmem-wrap-around=8k)
+	set(_pb_page 64)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 512)
+elseif(LIBAVR_MCU MATCHES "^atmega16a?$" OR LIBAVR_MCU MATCHES "^atmega168(a|p|pa)?$")
+	set(_pb_flash 16384)
+	set(_pb_wrap -Wl,--pmem-wrap-around=16k)
+	set(_pb_page 128)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 512)
+elseif(LIBAVR_MCU MATCHES "^atmega164(a|p|pa)$")
+	set(_pb_flash 16384)
+	set(_pb_wrap -Wl,--pmem-wrap-around=16k)
+	set(_pb_page 128)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 512)
+	set(_pb_has_usart1 1)
+elseif(LIBAVR_MCU MATCHES "^atmega32a?$" OR LIBAVR_MCU MATCHES "^atmega328p?$")
+	set(_pb_flash 32768)
+	set(_pb_wrap -Wl,--pmem-wrap-around=32k)
+	set(_pb_page 128)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 1024)
+elseif(LIBAVR_MCU MATCHES "^atmega324(a|p|pa)$")
+	set(_pb_flash 32768)
+	set(_pb_wrap -Wl,--pmem-wrap-around=32k)
+	set(_pb_page 128)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 1024)
+	set(_pb_has_usart1 1)
+elseif(LIBAVR_MCU MATCHES "^atmega644(a|p|pa)?$")
+	# 64 KiB is exactly the 16-bit byte space, so plain LPM still reaches
+	# everything and the wire stays byte-addressed. The plain 644 is the
+	# family's one single-USART die.
+	set(_pb_flash 65536)
+	set(_pb_wrap -Wl,--pmem-wrap-around=64k)
+	set(_pb_page 256)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 2048)
+	if(NOT LIBAVR_MCU STREQUAL "atmega644")
+		set(_pb_has_usart1 1)
+	endif()
+elseif(LIBAVR_MCU MATCHES "^atmega1284p?$")
+	# 128 KiB: wire addresses are words, reads go through ELPM, and the PC's
+	# modulo wrap exceeds what --pmem-wrap-around models.
+	set(_pb_flash 131072)
+	set(_pb_wrap "")
+	set(_pb_page 256)
+	set(_pb_hz 16000000)
+	set(_pb_eeprom 4096)
+	set(_pb_has_usart1 1)
+else()
+	message(FATAL_ERROR "pureboot: no geometry for ${LIBAVR_MCU}")
+endif()
+set(_pb_slot 512)
+math(EXPR _pb_base "${_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.
+if(LIBAVR_MCU MATCHES "^atmega" AND NOT LIBAVR_MCU MATCHES "^atmega48")
+	set(_pb_app 0)
+	set(_pb_limit ${_pb_slot})
+else()
+	math(EXPR _pb_app "${_pb_base} - 2")
+	math(EXPR _pb_limit "${_pb_slot} - 2")
+endif()
+
+# simavr names its cores after the base dies; the A revisions run on them
+# (the 644PA on the 644P core).
+set(_pb_sim_mcu ${LIBAVR_MCU})
+if(LIBAVR_MCU MATCHES "^atmega(8|16|32|48|88|164|168|644)a$")
+	string(REGEX REPLACE "a$" "" _pb_sim_mcu ${LIBAVR_MCU})
+elseif(LIBAVR_MCU STREQUAL "atmega644pa")
+	set(_pb_sim_mcu atmega644p)
+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})
+set_property(GLOBAL PROPERTY PUREBOOT_APP ${_pb_app})
+set_property(GLOBAL PROPERTY PUREBOOT_WRAP "${_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})
+
+# 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.
+set(PUREBOOT_BASE_HEX ${_pb_base_hex} PARENT_SCOPE)
+set(PUREBOOT_PAGE ${_pb_page} PARENT_SCOPE)
+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_HAS_USART ${_pb_has_usart} PARENT_SCOPE)
+set(PUREBOOT_HAS_USART1 ${_pb_has_usart1} PARENT_SCOPE)
+set(PUREBOOT_SIM_MCU ${_pb_sim_mcu} PARENT_SCOPE)
+
+# The rates a default may pick, fastest first.
+set_property(GLOBAL PROPERTY PUREBOOT_BAUD_LADDER 115200 57600 38400 19200 9600)
+
+# Whether <baud> is reachable from <clock> within 2.5 %, by the same
+# best-of-U2X-and-plain divisor search libavr's solve_baud runs, so a build
+# never trips the compile-time error it is checked against. A software build
+# also needs the polled receiver's 100-cycles-a-bit floor: at low clocks the
+# U2X divisor reaches rates the bit-banged sampler cannot.
+function(pureboot_baud_feasible clock baud software outvar)
+	set(${outvar} 0 PARENT_SCOPE)
+	math(EXPR _cycles "${clock} / ${baud}")
+	if(software AND _cycles LESS 100)
+		return()
+	endif()
+	foreach(divisor 8 16)
+		math(EXPR _step "${divisor} * ${baud}")
+		math(EXPR _n "(${clock} + ${_step} / 2) / ${_step}")
+		if(_n LESS 1 OR _n GREATER 4096)
+			continue()
+		endif()
+		math(EXPR _actual "${clock} / (${divisor} * ${_n})")
+		math(EXPR _delta "${_actual} - ${baud}")
+		if(_delta LESS 0)
+			math(EXPR _delta "-(${_delta})")
+		endif()
+		math(EXPR _error_bp "${_delta} * 10000 / ${baud}")
+		if(_error_bp LESS_EQUAL 250)
+			set(${outvar} 1 PARENT_SCOPE)
+			return()
+		endif()
+	endforeach()
+endfunction()
+
+# The fastest ladder rate the clock reaches.
+function(pureboot_default_baud clock software outvar)
+	get_property(_ladder GLOBAL PROPERTY PUREBOOT_BAUD_LADDER)
+	foreach(baud ${_ladder})
+		pureboot_baud_feasible(${clock} ${baud} ${software} _ok)
+		if(_ok)
+			set(${outvar} ${baud} PARENT_SCOPE)
+			return()
+		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")
+endfunction()
+
+# pureboot_add_loader(<name> [CLOCK <hz>] [BAUD <bd>]
+#                     [SERIAL auto|hardware|software] [USART <n>]
+#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>])
+#
+# 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.
+function(pureboot_add_loader name)
+	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT" "" ${ARGN})
+	if(PB_UNPARSED_ARGUMENTS)
+		message(FATAL_ERROR "pureboot_add_loader(${name}): unknown arguments ${PB_UNPARSED_ARGUMENTS}")
+	endif()
+	get_property(_hz GLOBAL PROPERTY PUREBOOT_DEFAULT_HZ)
+	get_property(_base_hex GLOBAL PROPERTY PUREBOOT_BASE_HEX)
+	get_property(_app GLOBAL PROPERTY PUREBOOT_APP)
+	get_property(_wrap GLOBAL PROPERTY PUREBOOT_WRAP)
+	get_property(_usart GLOBAL PROPERTY PUREBOOT_HAS_USART)
+	get_property(_usart1 GLOBAL PROPERTY PUREBOOT_HAS_USART1)
+
+	if(NOT PB_CLOCK)
+		set(PB_CLOCK ${_hz})
+	endif()
+	if(NOT PB_TIMEOUT)
+		set(PB_TIMEOUT 8)
+	endif()
+	if(NOT PB_SERIAL)
+		set(PB_SERIAL auto)
+	endif()
+	if(DEFINED PB_USART AND PB_SERIAL STREQUAL "software")
+		message(FATAL_ERROR "pureboot_add_loader(${name}): USART ${PB_USART} contradicts SERIAL software")
+	endif()
+	if(DEFINED PB_USART)
+		set(PB_SERIAL hardware)
+	elseif(PB_SERIAL STREQUAL "hardware")
+		set(PB_USART 0)
+	endif()
+
+	set(_serial_defines "")
+	if(PB_SERIAL STREQUAL "hardware")
+		if(PB_USART EQUAL 1 AND NOT _usart1)
+			message(FATAL_ERROR "pureboot_add_loader(${name}): ${LIBAVR_MCU} has no USART1")
+		elseif(NOT _usart)
+			message(FATAL_ERROR "pureboot_add_loader(${name}): ${LIBAVR_MCU} has no hardware USART")
+		endif()
+		set(_serial_defines PUREBOOT_USART=${PB_USART})
+		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")
+			endif()
+			if(_usart)
+				set(_link usart0)
+			else()
+				set(PB_SERIAL software)
+			endif()
+		endif()
+		if(PB_SERIAL STREQUAL "software")
+			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_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.
+			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 ${_link})
+		endif()
+	endif()
+	if(NOT PB_BAUD)
+		if(PB_SERIAL STREQUAL "software")
+			pureboot_default_baud(${PB_CLOCK} 1 PB_BAUD)
+		else()
+			pureboot_default_baud(${PB_CLOCK} 0 PB_BAUD)
+		endif()
+	endif()
+
+	set(_defines PUREBOOT_CLOCK_HZ=${PB_CLOCK} PUREBOOT_BAUD=${PB_BAUD} PUREBOOT_TIMEOUT=${PB_TIMEOUT}
+	    ${_serial_defines})
+
+	add_executable(${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)
+	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}>)
+	# The ELF is a container, never flashed: .hex for a programmer, .bin (the
+	# slot's bare bytes) for --update-loader.
+	add_custom_command(TARGET ${name} POST_BUILD
+	                   COMMAND ${CMAKE_OBJCOPY} -O ihex -R .eeprom
+	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.hex
+	                   COMMAND ${CMAKE_OBJCOPY} -O binary -R .eeprom
+	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.bin)
+	set_target_properties(${name} PROPERTIES PUREBOOT_HZ ${PB_CLOCK} PUREBOOT_BAUD ${PB_BAUD}
+	                      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

@@ -2,64 +2,146 @@
 
 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 allowed), built for every chip libavr targets, **512 bytes on
+(attributes and compiler flags allowed), **512 bytes on every chip libavr
-each** — 488 B on the ATtiny13A, 502 B on the ATtiny85, 504 B on the
+targets — all 37**. The device speaks primitives; every composite — verify,
-ATmega328P. The device speaks primitives; every composite — verify, erase,
+erase, reset-vector surgery, updating the loader itself — lives in the host
-reset-vector surgery, updating the loader itself — lives in the host tool
+tool (`pureboot.py`).
-(`pureboot.py`).
 
 The image is **position-independent**: control flow is PC-relative, the
-read/write paths take wire addresses, the write guard protects the 512-byte
+read/write paths take wire addresses, the write guard protects the slot the
-slot the code is *running* in (from the runtime return address), the info
+code is *running* in (from the runtime return address), the info block is
-block is addressed from that same anchor, and the application jump is an
+addressed from that same anchor, and the application jump is an indirect call
-indirect call to an absolute entry. The identical binary therefore runs from
+to an absolute entry. The identical binary therefore runs from any slot with
-any 512-byte slot with every command intact — which makes pureboot **its own
+every command intact, which makes pureboot **its own staging loader**: the
-staging loader**: the host installs the same binary one slot below the
+host installs the same binary one slot below the resident, jumps into it, and
-resident, jumps into it, and lets it rewrite the resident. On the tinies the
+lets it rewrite the resident.
-budget is 510, not 512: a slot's last word belongs to the host-managed
-trampoline (below).
 
-## Link
+## Chips
 
-| Chip | Serial | Baud | Clock assumed |
+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
-| ATmega328P | USART0, RXD/TXD = PD0/PD1 | 115200 8N1 | 16 MHz crystal |
+the tinies' RC oscillator (9.6 MHz on the t13s, 8 MHz above). Every axis moves
-| ATtiny85 | software UART, RX = PB0, TX = PB1 | 57600 8N1 | 8 MHz internal RC |
+per build — see *Configuration*; the largest image any of them produces is a
-| ATtiny13A | software UART, RX = PB0, TX = PB1 | 57600 8N1 | 9.6 MHz internal RC |
+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 tiny RX pin has its pull-up enabled; TX idles high. All multi-byte
+| Chip | Flash | Loader at | Link | Size |
-quantities on the wire are little-endian.
+|---|---|---|---|---|
+| 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 |
+
+† 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.
+
+The 1284s are the heaviest because they alone carry the far-flash machinery —
+ELPM reads, RAMPZ page commands, a word-addressed wire.
+
+The software UART enables the RX pull-up; TX idles high. All multi-byte wire
+quantities are little-endian.
+
+## Configuration
+
+Every deployment axis is a build parameter of `pureboot_add_loader()` (in
+`pureboot/CMakeLists.txt`) — the one way a loader target is created, by this
+repo's build and by a downstream project alike:
+
+| Argument | Meaning | Default |
+|---|---|---|
+| `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 |
+| `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 |
+
+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
+solver runs — and on a software build additionally within the polled
+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.
+
+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:
+
+```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)
+
+pureboot_add_loader(myboot CLOCK 1000000 SERIAL software TX pb1 RX pb5)
+```
+
+The function emits the ELF plus `myboot.hex` (the programmer artifact) and
+`myboot.bin` (the self-update image), prints the size, and stamps the resolved
+deployment on the target as the `PUREBOOT_HZ`, `PUREBOOT_BAUD` and
+`PUREBOOT_LINK` properties — what a flashing script or test harness needs to
+speak to the build. This exact deployment runs the full protocol suite in CI
+(`pureboot.custom`).
 
 ## Activation
 
-Reset enters the loader (BOOTRST on the mega, the patched reset vector on the
+Reset enters the loader (BOOTRST on the boot-sectioned megas, the patched
-tinies) — except a watchdog reset, which hands straight to the application
+reset vector elsewhere) — except a watchdog reset, which hands straight to the
-(the application owns its watchdog; it must clear WDRF itself, which also
+application with no activation window, since the application owns its watchdog.
-releases the WDRF-forced WDE).
+This is deliberate: it lets an application reboot itself instantly rather than
+sit through the window. The application must clear WDRF itself (libavr's
+`watchdog::disable()` does). **Gotcha:** WDRF is sticky (cleared only by
+software, not by a later reset), so an application that watchdog-resets and
+never clears it diverts *every* subsequent reset — external ones included —
+past the window too, and the loader becomes reachable only through an external
+programmer until the flag is cleared. A serial recovery path therefore assumes
+the application clears WDRF on its own reset path.
 
-The host then has one activation window per awaited byte to knock: `p` then
+The host then knocks `p` then `b`, each awaited byte under a fresh activation
-`b`. Each awaited byte gets a fresh window; any other byte is discarded and
+window; any other byte is discarded and awaited again, so line noise can delay
-awaited again (line noise cannot lock the loader, only delay it). A window
+the loader but never lock it. A window expiring on an idle line boots the
-expiring with an idle line boots the application.
+application.
 
-The window length is a compile-time constant — 8 s by default, another value
+The window is a compile-time constant (`TIMEOUT`, 8 s by default), so the whole
-via the `PUREBOOT_TIMEOUT` CMake cache variable — so the whole EEPROM belongs
+EEPROM belongs to the application — pureboot keeps no state of its own.
-to the application; pureboot never uses it for its own state. Re-timing a
+Re-timing a deployed loader is a self-update with a re-timed build.
-deployed loader is a self-update with a re-timed build (below).
 
 ## 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
-write to finish and sends the prompt `+` (0x2b) — the prompt is therefore
+write and sends the prompt `+` (0x2b), which is therefore also the previous
-also the completion ack of the previous command. A session is: await `+`,
+command's completion ack. A session is: await `+`, send a command, read its
-send a command, read its reply, repeat.
+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.
+
+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.
 
 | Cmd | Arguments | Reply |
 |---|---|---|
 | `b` | — | the 12-byte info block |
 | `R` | addr16, n8 | n flash bytes (n = 0 means 256) |
-| `W` | addr16, then one page of data | — (completion = next prompt) |
+| `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 |
@@ -67,39 +149,76 @@ send a command, read its reply, repeat.
 | other | — | ignored; the loop re-prompts (send a junk byte, await `+`, to resync) |
 
 `W` streams exactly one SPM page (size from the info block) into the buffer,
-then erases and programs; the address must be page-aligned. Pages inside the
+then erases and programs — except pages inside the 512-byte slot
-512-byte slot the loader is *running* in are drained but never programmed — a
+the loader is *running* in, which are drained and left alone, so a broken host
-broken host cannot brick the running copy, and a staged copy may rewrite the
+cannot brick the running copy and a staged copy may rewrite the resident.
-resident slot. `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 (the ATtiny13A) that slot carries no meaning.
-Fuse *writing* does not exist: SPM reaches flash (and, on the mega, lock
-bits) only — fuse bytes are external-programming territory by hardware.
 
-`J` is the one control-transfer primitive: the host uses it to run the
+The loader never clears the SPM buffer before a fill, so **one `W` may program
-application (word 0 on the mega, the trampoline word on the tinies — both
+the wrong bytes, and the host is what fixes it**. The buffer is write-once per
-known from the info block) and to move between loader copies during a
+word until cleared, and two things leave words in it: a refused page, and —
-self-update. A jump to a loader slot's base re-enters that copy's own
+where SPM runs from anywhere, the tinies and the m48s — an application that
-startup; it must then be knocked afresh.
+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
+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.
+
+`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
+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`):
 
 | Offset | Content |
 |---|---|
-| 0–2 | `'P'`, `'B'`, protocol version (1) |
+| 0–2 | `'P'`, `'B'`, pureboot version (3) |
 | 3–5 | device signature |
-| 6 | SPM page size in bytes |
+| 6 | SPM page size in bytes (0 means 256) |
-| 7–8 | loader base — application flash ends here |
+| 7–8 | loader base — application flash ends here (a word address when bit 1 is set) |
 | 9–10 | EEPROM size |
-| 11 | bit 0 set: host must patch the reset vector (no hardware boot section) |
+| 11 | bit 0: host must patch the reset vector (no hardware boot section); bit 1: flash wire addresses are word addresses |
 
-Composites are the host's job: verify = read back and compare, erase =
+## Version
-write `0xff` (per page for flash, per byte for EEPROM).
+
+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 tool carries its own version, free to drift; `--version` prints it and the
+window.
 
 ## Deployment
 
-**ATmega328P**: program the loader at 0x7e00 with an external programmer.
+The build leaves three artifacts per chip. The ELF is a container for the
-Two fuse profiles, same binary:
+tests and objcopy, never flashed. The **.hex is the programmer artifact**: it
+carries its own addresses and lands the loader in its top slot, touching
+nothing else. The **.bin is the self-update image** — the slot's bare bytes.
+
+**Boot-sectioned megas**: program the loader at `flash − 512` with an external
+programmer. Every such mega has a BOOTSZ step whose boot section is exactly
+the 512-byte slot — the second-smallest step on the 8 KiB and 16 KiB chips,
+the smallest on the 32 KiB ones — so the ATmega328P profiles below apply to
+every one of them with its own addresses; the per-chip BOOTSZ ladders live in
+the host tool (`BOOT_FUSE`).
+
+The **644s and 1284s** are the geometry's sweet spot: their smallest boot
+section (512 words = 1 KiB) is exactly *two* slots, so the resident and its
+staging slot both live inside the minimum section. Self-update needs no fuse
+step up, and the standalone profile does not exist — reset lands one erased
+slot below the loader (0xfc00 / 0x1fc00) and walks up into it.
+
+ATmega328P profiles (addresses for its 32 KiB):
 
 | BOOTSZ | BOOTRST | Behavior |
 |---|---|---|
@@ -107,99 +226,142 @@ Two fuse profiles, same binary:
 | 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) | 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 — word 0 stays the application's own
+Applications are flashed unmodified here — word 0 stays the application's own
 reset vector, and the hand-over jumps to 0.
 
-**Tinies** (no boot section): program the loader at `flash − 512`; erased
+**Patched-vector chips — the tinies and the m48s** (no boot section; the m48s'
-flash below it walks up into the loader, so a virgin chip activates. When
+SPM runs from the entire flash, Atmel-8271 §26): program the loader at
-flashing an application the host performs reset-vector surgery: word 0 is
+`flash − 512`; erased flash below it walks up into the loader, so a virgin
-rewritten to `rjmp` to the loader base, and the application's own entry is
+chip activates. Flashing an application then takes reset-vector surgery: word
+0 becomes an `rjmp` to the loader base, and the application's own entry is
 re-encoded as a trampoline `rjmp` in the word just below the loader
 (`base − 2`, where the hand-over jumps). Every other vector stays the
-application's. The patched page 0 and the trampoline page are written
+application's. The patched page 0 and the trampoline page are written *first*
-*first*, so from the first write on an interrupted flash still resets into
+and an erase runs top-down, so from the first write on an interruption still
-the loader; an erase runs top-down for the same reason.
+resets into the loader.
+
+A .bin programmed at address 0 by mistake is dead weight on a boot-sectioned
+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`).
 
 ## Updating the loader
 
 `pureboot.py --update-loader new_pureboot.bin` replaces the resident loader
-with any pureboot build — a re-timed window, a newer protocol — using the
+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, which
+bytes as a raw binary, or the Intel HEX the build emits beside it.
-links the loader at its base inside an otherwise blank flash image:
 
-1. The staging slot `[base−512, base)` is saved to a host-side state file
+The preflight refuses an image built for another chip: the info block embedded
-   (on the 1 KB tiny13A that is the whole application, vectors included).
+in every pureboot binary (signature, page size, loader base, EEPROM size,
-2. The resident installs the identical update image there. On the tinies the
+flags) must match the device's own, and the error names both. Die revisions
-   host composes the slot's last word — the same address as the resident's
+share their base signature and geometry, so their images are interchangeable —
-   trampoline — as a jump to the resident base, so even an abandoned staging
+as the silicon is.
-   copy times out into a loader, never into garbage.
+
-3. `J` enters the staging copy, which rewrites the resident slot. On the
+1. The staging slot `[base−512, base)` is saved to a host-side state file (on
-   t85 the host first re-aims word 0 at the staging copy, so a power loss
+   the 1 KB tiny13s that is the whole application, vectors included).
-   mid-rewrite still resets into a loader; on the t13a the staging slot
+2. The resident installs the update image there. On the patched-vector chips
-   carries the reset vector itself.
+   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.
+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
+   loader; on the tiny13s the staging slot carries the reset vector itself.
 4. `J` enters the new resident, which restores the staging slot's saved
-   content (word 0 and the trampoline with it) and the state file is
+   content, and the state file is discarded.
-   discarded.
 
-Every phase is idempotent and keyed off the actual flash state: re-running
+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
+the same command after any interruption resumes and completes. The state file
-file carries the only bytes not recoverable from the device; if it is lost
+carries the only bytes not recoverable from the device; losing it mid-update
-mid-update the update still completes, and the staging region is restored by
+still completes the update, and the staging region comes back by reflashing
-reflashing the application. The mega needs its fuses for the preflight
+the application. A boot-sectioned mega needs its fuses for the preflight — read
-(BOOTSZ gate, profile notes) — read from the device, or supplied with
+from the device, or supplied with `--assume-fuses` where reading is impossible
-`--assume-fuses` where reading is impossible (simulators).
+(simulators).
 
 ## Host tool
 
 `pureboot.py` — Python 3, standard library only. The port layer is the one
-platform-specific part: termios drives any tty on POSIX (a USB adapter as
+platform-specific part: termios drives any tty on POSIX (a USB adapter as well
-well as a simavr pty), the Win32 serial API through `ctypes` drives a COM
+as a simavr pty), the Win32 serial API through `ctypes` drives a COM port on
-port on Windows (`--port COM6`; the `\\.\` form for two-digit ports is
+Windows (`--port COM6`; the `\\.\` form for two-digit ports is supplied by the
-supplied by the tool). Opening the port asserts DTR and RTS on both, so a
+tool). Opening the port asserts DTR and RTS on both, so a board that wires DTR
-board that wires DTR to reset gets its reset pulse and opens the activation
+to reset gets its reset pulse and opens the activation window by itself.
-window by itself.
 
     pureboot.py --port /dev/ttyUSB0 --baud 57600 \
         --info --fuses --flash app.hex
 
 Operations run in a fixed order within one session: info, fuses, loader
-update, flash (erase / program / read / verify), EEPROM (erase / program /
+update, flash (erase / program / read / verify), EEPROM (the same) — then the
-read / verify) — then the loader hands over to the application; `--stay`
+loader hands over to the application. `--stay` keeps the session alive
-keeps the session alive instead, and a later invocation reconnects into it
+instead, and a later invocation reconnects into it. `--flash` and `--eeprom`
-(the knock converges there too). `--flash` and `--eeprom` verify by
+verify by read-back unless `--no-verify`, and a flash page that reads back
-read-back unless `--no-verify`; images are raw binary, or Intel HEX by
+wrong is rewritten up to three times before the run stops (see `W` above).
-extension. `--force` overrides the refusable safety checks (today: flashing
+`--verify-flash` only reports. Images are raw binary, or Intel HEX by
-application data into a mega's reset walk region).
+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
+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).
 Per chip preset, `ctest` runs:
 
-- `pureboot.size` — the 510-byte (tinies) / 512-byte (mega) budget;
+- `pureboot.size` — the 510-byte (patched-vector) / 512-byte budget;
-- `pureboot.pi` — the position-independence lint: no absolute `jmp`/`call`
+- `pureboot_*.size` — the size matrix: the serial backends × the clock ladder
-  in the image, the info block within its first 256 bytes;
+  (1/8/16 MHz; the t13s' own RC menu), the USART1 instance across that same
-- `pureboot.planner` — the host tool's pure logic: programming orders and
+  ladder on the x4 chips, and `pureboot_sw_wide`, the slowest ladder rate at
-  their recovery properties, the surgery, the staging composition, the
+  the fastest clock — where a software UART's per-bit spin outgrows its
-  boot-fuse decode, and the update preflight's error/warning matrix over
+  one-register delay loop and takes the 16-bit one. That is the largest image
-  synthetic fuse bytes;
+  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;
+- `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.protocol` — end to end against a simavr device
-  (`test/pureboot_device.c` — the mega's USART as a pty; on the tinies a
+  (`test/pureboot_device.c`: a hardware USART as a pty, or a cycle-timed
-  cycle-timed GPIO⇄pty bridge for the software UART, plus the SPM/NVM module
+  GPIO⇄pty bridge for a software-UART build, plus the SPM/NVM module simavr's
-  simavr's tiny cores lack) driven by the real host tool through
+  tiny cores lack) driven by the real host tool through knock-from-reset,
-  knock-from-reset, program + verify of both memories, session reconnect, an
+  program + verify of both memories, session reconnect, an external reset
-  external reset through the patched vector, and the hand-over to a fixture
+  through the patched vector, and the hand-over to a fixture application whose
-  application whose banner proves the launch — cross-checked against the
+  banner proves the launch — cross-checked against the simulator's
-  simulator's ground-truth memory dumps and an independent decode of the
+  ground-truth memory dumps and an independent decode of the surgery;
-  surgery's rjmp words;
+- `pureboot.reloc` — the identical image one slot below the resident serves the
-- `pureboot.reloc` — the identical image installed one slot below the
+  complete command set from there;
-  resident serves the complete command set from there (the
+- `pureboot.rehome` (t85) — a loader programmed at address 0 or in the staging
-  position-independence acceptance test);
+  slot re-homes into the top slot through the ordinary update flow;
-- `pureboot.update` — the full `--update-loader` flow to a re-timed build,
+- `pureboot.custom` (328P) — the configuration example's 1 MHz software-serial
-  then every power-fail phase: the device is killed mid-write, restarted
+  build driving the full protocol suite, proving the plumbing produces a
-  from its flash dump, and a re-run must complete the update with the
+  working loader and not just one that fits;
-  application intact throughout.
+- `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.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
+  one rewrite. Hardware forbids the state here, but simavr dispatches SPM from
+  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.
 
-`size`, `pi`, and `planner` are host logic and run anywhere; the three
+`size`, `pi` and `planner` are host logic and run anywhere; the
-simulator-driven targets need simavr and a pty, so they are POSIX-only —
+simulator-driven targets need simavr and a pty, so they are POSIX-only.
-on Windows the tool is exercised against real hardware.

pureboot/autobaud.md

@@ -0,0 +1,251 @@
+# 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

@@ -1,27 +1,14 @@
-// pureboot — a serial bootloader on libavr, pure by constraint: one C++
+// pureboot — a serial bootloader on libavr: one C++ source, no inline
-// source with no inline assembly and no global register variables, built for
+// assembly, no global register variables, 512 bytes on every chip libavr
-// every chip libavr targets, 512 bytes on each. The device speaks primitives
+// targets. The device speaks primitives; every composite (verify, erase,
-// — read/program flash, read/write EEPROM, fuse bytes, an info block, a jump
+// reset-vector surgery, self-update) lives in the host tool. Protocol,
-// — and everything composite (verify, erase, reset-vector surgery, updating
+// deployment and configuration: README.md next to this file.
-// the loader itself) lives in the host tool. Protocol reference: README.md
-// next to this file.
 //
-// The image is position-independent: control flow is PC-relative, the write
+// The image is position-independent — PC-relative control flow, wire
-// and read paths take wire addresses, the write guard refuses the 512-byte
+// addresses in, the write guard and the info block both anchored on the
-// slot the code is *running* in (taken from the runtime return address), the
+// runtime return address — so the identical binary runs from any slot. That
-// info block is read relative to that same anchor, and the application jump
+// is what makes a copy one slot below able to rewrite the resident one, and
-// is an indirect call to an absolute entry. The identical binary therefore
+// every change here has to keep it (test/check_pi.py).
-// runs from any 512-byte slot with every command intact: flashed one slot
-// below the resident loader it becomes the staging loader that rewrites the
-// resident — how pureboot updates itself, host-driven, with no other
-// firmware involved.
-//
-// Entry: reset lands in avr::startup::entry below (BOOTRST on the mega; the
-// patched reset vector — or erased flash walking up into the loader — on the
-// tinies). A watchdog reset hands straight to the application. Otherwise the
-// host has one activation window per awaited knock byte ("pb"); an idle line
-// boots the application. A session then stays in the command loop until 'J'
-// jumps away or the chip resets.
 
 #include <libavr/libavr.hpp>
 
@@ -32,98 +19,103 @@ namespace ee = avr::eeprom;
 namespace pureboot {
 namespace {
 
-// Purely polled — interrupts stay off, every guard folds to nothing.
+// Purely polled: every interrupt guard folds to nothing.
 constexpr auto off = avr::irq::guard_policy::unused;
 
 constexpr std::uint8_t ack = '+';
 
-// Per-chip personality, from the chip database: the clocks the dogfood
+// Deployment parameters come from the build (pureboot_add_loader()). The
-// boards run (16 MHz crystal on the mega, calibrated RC on the tinies) and
+// signature is not one of them: the chip database is the only universal
-// the device signature (compile-time data — the tiny13A cannot even read its
+// source — a tiny13A cannot read its own signature row from code.
-// signature row from code).
+#if !defined(PUREBOOT_CLOCK_HZ) || !defined(PUREBOOT_BAUD)
-consteval avr::hertz_t clock()
+#error \
+    "PUREBOOT_CLOCK_HZ and PUREBOOT_BAUD select this build's clock and baud — create loader targets with pureboot_add_loader() (README.md)"
+#endif
+
+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()
 {
-	if (avr::hw::db.name == "ATtiny13A")
+	auto reg = std::string_view{avr::hw::db.regs[static_cast<std::size_t>(avr::power::detail::reset_reg())].name};
-		return 9.6_MHz;
+	return avr::hw::db.field_index(reg, "WDRF");
-	if (avr::hw::db.name == "ATtiny85")
-		return 8_MHz;
-	return 16_MHz;
 }
 
-consteval std::array<std::uint8_t, 3> signature()
+// 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
-	if (avr::hw::db.name == "ATtiny13A")
+// runs from anywhere (Atmel-8271 §26) — keep the application's relocated
-		return {0x1e, 0x90, 0x07};
+// reset vector in the word under the slot.
-	if (avr::hw::db.name == "ATtiny85")
+constexpr std::uint16_t slot_bytes = 512;
-		return {0x1e, 0x93, 0x0b};
+constexpr std::uint32_t base = spm::flash_bytes - slot_bytes;
-	return {0x1e, 0x95, 0x0f};
-}
-
-using dev = avr::device<{.clock = clock()}>;
-
-// Geometry: the resident loader owns the top 512 bytes of flash; the word
-// below it is the trampoline (the application's relocated reset vector) on
-// chips without a hardware boot section. The RWWSRE bit marks a separate
-// boot section — on classic AVR the two capabilities coincide.
-constexpr std::uint16_t boot_bytes = 512;
-constexpr std::uint16_t base = static_cast<std::uint16_t>(spm::flash_bytes - boot_bytes);
 constexpr std::uint16_t page = spm::page_bytes;
-constexpr bool boot_section = avr::hw::db.field_index("SPMCSR", "RWWSRE") >= 0;
+constexpr bool boot_section = avr::hw::curated::has_boot_section();
 
-// The activation window, in seconds, is a compile-time constant (the build
+// Past 64 KiB a byte address no longer fits the wire's 16 bits, so flash
-// may override it): the whole EEPROM belongs to the application, and
+// addresses there are word addresses ('J' always was one). A slot is 256 of
-// re-timing the loader is a bootloader self-update with a re-timed binary.
+// 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);
+
+// 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.
 #if !defined(PUREBOOT_TIMEOUT)
 #define PUREBOOT_TIMEOUT 8
 #endif
 constexpr std::uint8_t timeout_seconds = PUREBOOT_TIMEOUT;
 
-// The 12-byte info block the host reads with the 'b' command; flash-resident
+// The loader's one identity number. The protocol carries none of its own —
-// (there is no crt to copy a .data image).
+// a version implies it, and the host tool holds that map (README.md).
-inline constexpr std::array<std::uint8_t, 12> info_data = {
+constexpr std::uint8_t version = 4;
+
+// 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.
+// clang-format off
+inline constexpr avr::flash_table<std::array<std::uint8_t, 12>{
     'P',
     'B',
-    1, // magic, protocol version
+    version,
-    signature()[0],
+    avr::hw::db.signature[0],
-    signature()[1],
+    avr::hw::db.signature[1],
-    signature()[2],
+    avr::hw::db.signature[2],
-    static_cast<std::uint8_t>(page),
+    static_cast<std::uint8_t>(page),                                           // 0 means 256
-    base & 0xff,
+    wire_base & 0xff,
-    base >> 8, // app flash ends here; resident loader base
+    wire_base >> 8,
     avr::hw::db.mem.eeprom_size & 0xff,
     avr::hw::db.mem.eeprom_size >> 8,
-    boot_section ? 0 : 1, // bit 0: host must patch the reset vector (no hardware boot section)
+    static_cast<std::uint8_t>((boot_section ? 0 : 1) | (word_flash ? 2 : 0)),  // patch-vector, word-addressed
-};
+}>
-using info = avr::flash_table<info_data>;
+    info_data;
+// clang-format on
 
-// The serial link: the hardware USART where the chip has one, the polled
+// The serial link, per the build's PUREBOOT_USART / PUREBOOT_SOFT_SERIAL,
-// software UART (no vector — the table belongs to the application) on PB0/PB1
+// defaulting to the chip's USART0 where it has one. The software receiver is
-// elsewhere. Both are class templates on the clock so only the selected
+// the polled one: the vector table belongs to the application. Templates on
-// backend is ever instantiated. pending() is the cheap line test the
+// the clock, so only the selected backend instantiates. pending() is the
-// activation window polls; rx() then picks the byte up; drain() holds until
+// cheap line test the activation window polls; drain() holds until the last
-// the last transmitted frame is fully on the wire (the jump hand-over must
+// frame is off the wire, so a hand-over cannot let the target's re-init clip
-// not let the target's re-init clip the ack).
+// the ack.
-template <avr::hertz_t C>
+#if defined(PUREBOOT_SOFT_SERIAL) && defined(PUREBOOT_USART)
-consteval std::int16_t rxc_field()
+#error "PUREBOOT_SOFT_SERIAL and PUREBOOT_USART select opposing serial backends"
-{
+#endif
-	return avr::hw::db.field_index("UCSR0A", "RXC0");
+#if !defined(PUREBOOT_RX)
-}
+#define PUREBOOT_RX pb0
-
+#endif
-template <avr::hertz_t C>
+#if !defined(PUREBOOT_TX)
-consteval std::int16_t txc_field()
+#define PUREBOOT_TX pb1
-{
+#endif
-	return avr::hw::db.field_index("UCSR0A", "TXC0");
+#if defined(PUREBOOT_USART)
-}
+constexpr char usart_digit = '0' + PUREBOOT_USART;
-
+#else
-template <avr::hertz_t C>
+constexpr char usart_digit = '0';
-consteval std::int16_t status_reg()
+#endif
-{
-	return avr::hw::db.reg_index("UCSR0A");
-}
 
 template <avr::hertz_t C>
 struct hardware_link {
-	using uart = avr::uart::usart0<C, {.baud = 115200_Bd, .max_baud_error = 2.5_pct}>;
+	using uart = avr::uart::usart<usart_digit, C, {.baud = wire_baud, .max_baud_error = 2.5_pct}>;
 
 	// The compiled idle poll: lds UCSR0A (2), sbrc skipping the exit (2),
 	// sbiw + sbci + sbci + brne (6).
@@ -136,7 +128,7 @@ struct hardware_link {
 
 	static bool pending()
 	{
-		return avr::hw::field_impl<rxc_field<C>()>::test();
+		return uart::rx_ready();
 	}
 
 	static std::uint8_t rx()
@@ -151,22 +143,14 @@ struct hardware_link {
 
 	static void drain()
 	{
-		// write() leaves the byte draining behind it. Clear a stale TXC0
+		uart::drain();
-		// first (W1C by writing the sampled status back — the store a hand
-		// assembler writes, keeping U2X0), then wait for the fresh
-		// completion; with a byte still ahead in the shifter TXC0 cannot
-		// re-set until the last pending byte has fully left.
-		using status = avr::hw::reg_impl<status_reg<C>()>;
-		status::write(status::read());
-		while (!avr::hw::field_impl<txc_field<C>()>::test()) {
-		}
 	}
 };
 
 template <avr::hertz_t C>
 struct software_link {
-	using rx_t = avr::uart::software_rx_polled<C, avr::pb0, 57600_Bd>;
+	using rx_t = avr::uart::software_rx_polled<C, avr::PUREBOOT_RX, wire_baud>;
-	using tx_t = avr::uart::software_tx<C, avr::pb1, 57600_Bd>;
+	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, wire_baud>;
 
 	// The compiled idle poll: sbis skipping the exit (2), sbiw + sbci +
 	// sbci + brne (6).
@@ -179,7 +163,7 @@ struct software_link {
 
 	static bool pending()
 	{
-		return !avr::io::input<avr::pb0>::read(); // a start bit has begun
+		return rx_t::start_pending();
 	}
 
 	static std::uint8_t rx()
@@ -198,14 +182,20 @@ struct software_link {
 	}
 };
 
-using link = std::conditional_t<avr::hw::db.has_reg("UDR0"), hardware_link<dev::clock>, software_link<dev::clock>>;
+#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>;
+#elif defined(PUREBOOT_SOFT_SERIAL)
+using link = software_link<dev::clock>;
+#else
+using link =
+    std::conditional_t<avr::uart::has_usart<usart_digit>(), hardware_link<dev::clock>, software_link<dev::clock>>;
+#endif
 
-// The application's entry, an absolute address the linker pins (--defsym in
+// The application's entry, pinned by the linker (--defsym): word 0 on a
-// CMakeLists.txt): 0x0000 on the mega (word 0 stays the application's own
+// boot-sectioned mega, the trampoline at base − 2 elsewhere. Reaching it must
-// vector — BOOTRST re-vectors a reset into the loader in hardware) and the
+// not depend on where this copy runs, so the jump goes through a pointer, and
-// trampoline word at base - 2 on the tinies. Reaching it must not depend on
+// [[gnu::noipa]] keeps the constant from folding back into a relative call.
-// where this copy runs, so the jump goes through a pointer: [[gnu::noipa]]
-// keeps the constant from folding back into a PC-relative call.
 extern "C" [[noreturn]] void pureboot_app();
 
 [[gnu::noipa, noreturn]] void jump(void (*target)())
@@ -219,10 +209,8 @@ extern "C" [[noreturn]] void pureboot_app();
 	jump(pureboot_app);
 }
 
-// One activation window is a single 32-bit poll countdown. The divisor is
+// The window as one 32-bit countdown, divided by the backend's counted
-// the backend's counted poll-loop cycles (its own comment reads them off the
+// poll-loop cycles. Whole seconds is all it promises.
-// compiled loop); whole-second precision is all the window promises, so the
-// nearest cycle count is plenty.
 consteval std::uint32_t window_polls()
 {
 	return timeout_seconds * static_cast<std::uint32_t>(dev::clock.hz / link::poll_cycles);
@@ -238,8 +226,8 @@ bool pending_before_deadline()
 	return false;
 }
 
-// A knock byte under the activation deadline: an idle line means no host is
+// A knock byte under the deadline: an idle window means no host, so the
-// there, and the application runs.
+// application runs.
 std::uint8_t rx_deadline()
 {
 	if (!pending_before_deadline())
@@ -247,27 +235,63 @@ std::uint8_t rx_deadline()
 	return link::rx();
 }
 
-std::uint16_t rx16()
+// 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));
 }
 
-const std::uint8_t *flash_ptr(std::uint16_t address)
+// 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.
+[[gnu::always_inline]] inline std::uint16_t word_of(std::array<std::uint8_t, 2> pair)
 {
-	return reinterpret_cast<const std::uint8_t *>(address);
+	return std::bit_cast<std::uint16_t>(pair);
 }
 
-// The streamers take the count in the wire's 8-bit form: 0 means 256.
+// Counts arrive in the wire's 8-bit form: 0 means 256. Both streamers fold
-// send_flash stays out of line: its two callers ('b' and 'R') otherwise each
+// into the one command that reads flash, which is what lets the far one's
-// inline a private copy of the loop.
+// 24-bit cursor sit in the command loop's own call-saved registers.
-[[gnu::noinline]] void send_flash(std::uint16_t address, std::uint8_t count)
+[[maybe_unused, gnu::always_inline]] inline void send_flash_near(std::uint16_t address, std::uint8_t count)
 {
 	do
-		link::tx(avr::flash_load(flash_ptr(address++)));
+		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.
+[[gnu::noinline]] void tx_ack()
+{
+	link::tx(ack);
+}
+
 void send_eeprom(std::uint16_t address, std::uint8_t count)
 {
 	do
@@ -275,127 +299,156 @@ void send_eeprom(std::uint16_t address, std::uint8_t count)
 	while (--count);
 }
 
-// EEPROM write, host-paced: each ack goes out once the byte's write has
+// Host-paced: the ack goes out once the write has begun, so the next byte
-// begun, so the next byte arrives while it completes and the following
+// arrives while it completes and nothing is missed without a buffer.
-// write's own ready-wait sees an idle line. Nothing is ever missed, on
-// either serial backend, without a buffer.
 void store_eeprom(std::uint16_t address, std::uint8_t count)
 {
 	do {
 		ee::write<off>(address++, link::rx());
-		link::tx(ack);
+		tx_ack();
 	} while (--count);
 }
 
-// One flash page: stream the bytes into the SPM buffer as little-endian
+// One page into the SPM buffer, then erase and program — except the slot
-// words, then erase and program — except the 512-byte slot this code runs
+// this code is running in (`slot_high`, from run()), which is drained and
-// in, which is drained but never programmed, so a copy can never erase
+// left alone. A broken host therefore cannot brick the running loader, and a
-// itself. `slot_high` is the high byte of that running slot's base (run()
+// copy one slot lower may rewrite the resident one.
-// derives it); a broken host thus cannot brick the running loader, and a
+//
-// copy flashed one slot lower may rewrite the slot above it — how pureboot
+// Nothing discards the buffer first: it is write-once per word (§26.2.1), so
-// updates itself. On the mega the RWW section is re-enabled so reads work
+// filling over a refused page or an application's leavings programs stale
-// immediately.
+// words — but a page write auto-erases it (§26.2.1; §19.2 on the tinies), so
-void program_flash(std::uint16_t address, std::uint8_t slot_high)
+// 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)
 {
-	// A buffer word cannot be loaded twice without an erase (§26.2.1), so a
+	// The address names a page, so its in-page bits are dropped and the walk
-	// refused page's drained data must not linger for the next write:
+	// starts at the page base — one induction either way: a byte-addressed
-	// discard the buffer up front — CTPB on the tinies; on the mega writing
+	// wire address walks the page itself (the offset bits wrap back to zero),
-	// RWWSRE aborts a pending load (§26.2.2).
+	// while a word one becomes a byte cursor once. The slot index is the wire
-	if constexpr (boot_section)
+	// address's high byte — on byte-addressed chips the byte address's, with
-		spm::rww_enable<off>();
+	// the low bit dropped, since a slot is two of those.
-	else
+	spm::flash_address_t address;
-		spm::clear_buffer<off>();
+	std::uint8_t page_high;
-	// The address is the loop's only state: pages are aligned, so the walk
+	if constexpr (word_flash) {
-	// ends when the offset bits wrap back to zero.
+		// 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>(address, static_cast<std::uint16_t>(low | (high << 8)));
+			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
-	const std::uint8_t page_high = static_cast<std::uint8_t>(address >> 8) & 0xfe;
+		page_high = static_cast<std::uint8_t>(address >> 8) & 0xfe;
+	}
 	if (page_high != slot_high) {
-		// The tinies halt the CPU through the erase and the write, so only
+		// Only a boot-sectioned mega runs on while its RWW section programs;
-		// the mega — running on while its RWW section programs — waits.
+		// 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) {
+		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/lock bytes in the hardware's own Z order: low, lock,
+// The four fuse and lock bytes in the hardware's own Z order: low, lock,
-// extended, high. Writing fuses is not a thing self-programming can do on
+// extended, high.
-// AVR — SPM reaches flash (and boot lock bits) only.
 void send_fuses()
 {
 	std::uint8_t which = 0;
 	do
 		link::tx(spm::read_fuse<off>(static_cast<spm::fuse>(which)));
-	while (++which & 3);
+	while (++which != 4);
 }
 
 [[noreturn]] void run()
 {
-	// A watchdog reset belongs to the application (whose watchdog stays
+	// A watchdog reset belongs to the application, whose watchdog stays forced
-	// forced on until it clears WDRF) — no activation window in its way.
+	// on until it clears WDRF — no activation window in its way.
-	if (avr::hw::mcusr::wdrf.test())
+	if (avr::hw::field_impl<wdrf_field()>::test())
 		run_app();
 
 	link::init();
 
-	// The high byte of the 512-byte-aligned base this copy runs at: the word
+	// The high byte of the slot this copy runs at, which the write guard and
-	// return address's high byte is the byte address >> 9 (the slot index),
+	// the info block both follow: the return address is a word address, so its
-	// doubled back into address terms. program_flash refuses this one slot
+	// high byte is the 256-word slot index, doubled back into byte terms where
-	// and the info block is addressed from it, so both follow wherever the
+	// the wire counts bytes. Taken as byteswap's low byte — the builtin already
-	// code was flashed.
+	// swaps the two stacked bytes, and the double swap folds away, where `>> 8`
-	const std::uint8_t slot_high =
+	// would leave the swap materialized.
-	    static_cast<std::uint8_t>((reinterpret_cast<std::uint16_t>(__builtin_return_address(0)) >> 8) << 1);
+	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);
 
-	// The knock: 'p' then 'b', each under a fresh window; any other byte is
+	// 'p' then 'b', each under a fresh window; anything else is line noise.
-	// line noise and waits again. Falling out of a window runs the app.
 	while (rx_deadline() != 'p' || rx_deadline() != 'b') {
 	}
 
 	for (;;) {
-		// No prompt while an EEPROM write runs: a pending write blocks SPM
+		// No prompt while an EEPROM write runs: it blocks SPM and fuse reads
-		// and fuse reads (§26.2.1), and the ack tells the host all is done.
+		// (§26.2.1), and the prompt is the previous command's completion ack.
 		ee::wait();
-		link::tx(ack);
+		tx_ack();
 		const std::uint8_t command = link::rx();
 		switch (command) {
-		case 'b': { // info block, read relative to the running slot
-			// The block sits in the image's first 256 bytes (the build lint
-			// asserts it), and slots are 512-aligned — so the low byte of its
-			// link address is its offset in any slot, and the high byte of
-			// its runtime address is the running slot's. Built as a byte
-			// pair so no absolute 16-bit address is ever materialized.
-			const std::uint8_t low = static_cast<std::uint8_t>(reinterpret_cast<std::uint16_t>(info::storage.data()));
-			send_flash(std::bit_cast<std::uint16_t>(std::array{low, slot_high}), info::size());
-			break;
-		}
 		case 'J': { // jump to a wire word address: hand-over and staging transfer
 			auto target = reinterpret_cast<void (*)()>(rx16());
-			link::tx(ack);
+			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
-			std::uint16_t address = rx16();
+			// One address-and-count path for all four: 'b' is a flash read
-			std::uint8_t count = link::rx();
+			// whose arguments the loader already knows, so it joins the
-			if (command == 'R')
+			// wire-argument three rather than streaming from a call site of its
-				send_flash(address, count);
+			// own. That leaves one flash streamer in the image, and lets its
-			else if (command == 'r')
+			// 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 == 'r')
 				send_eeprom(address, count);
-			else
+			else if (command == 'w')
 				store_eeprom(address, count);
+			else
+				send_flash(address, count);
 			break;
 		}
 		case 'W': // program one flash page: addr16, page bytes

pureboot/pureboot_autobaud_pure.cpp

@@ -0,0 +1,396 @@
+// 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

@@ -0,0 +1,371 @@
+// 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

@@ -0,0 +1,411 @@
+// 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

@@ -1,17 +1,11 @@
 #!/usr/bin/env python3
-"""Position-independence lint for the pureboot image.
+"""Position-independence lint: the two link-time facts that let the identical
+image run from any slot, asserted from the built ELF.
 
-The self-staging design lets the identical binary run from any 512-byte
+  1. No absolute jmp/call — -mrelax normally guarantees it, but a branch that
-slot, which holds only if nothing in the image addresses itself absolutely.
+     grows out of relaxation range would break it silently.
-Two link-time facts guarantee it, both asserted here from the built ELF:
+  2. The info block within the image's first 256 bytes: 'b' rebuilds its
-
+     address as (running slot high byte : link address low byte).
-  1. No absolute jmp/call opcodes — all control flow is PC-relative
-     (rjmp/rcall/ijmp/icall). -mrelax normally guarantees this; a code
-     change that grows a branch out of relaxation range would break it
-     silently.
-  2. The info block sits within the image's first 256 bytes: the 'b'
-     command rebuilds its address as (running slot high byte : low byte of
-     the link address), which needs the offset to fit that low byte.
 
 Usage: check_pi.py <objdump> <nm> <elf> <text_start_hex>
 """
@@ -41,7 +35,8 @@ def main():
     if len(info) != 1:
         print(f"FAIL: expected one info-block storage symbol, found {len(info)}")
         sys.exit(1)
-    offset = int(info[0].split()[0], 16) - text_start
+    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)

test/pbapp.cpp

@@ -4,11 +4,16 @@
 // 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 mega it then listens, and an 'L' makes it jump into the resident
+// On the hardware-USART link it then listens, and an 'L' makes it jump into
-// loader — the application-owned loader entry a BOOTRST-unprogrammed mega
+// the resident loader — the application-owned loader entry a
-// relies on (reset always boots the application there), exercised by the
+// BOOTRST-unprogrammed mega relies on (reset always boots the application
-// self-update tests. The tinies idle: reset reaches their loader through
+// there), exercised by the self-update tests. The software link idles:
-// the patched vector, so the application owes it nothing.
+// reset reaches those loaders through the patched vector (or the runner
+// models BOOTRST), so the application owes them nothing.
+//
+// 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).
 #include <libavr/libavr.hpp>
 
 using namespace avr::literals;
@@ -17,33 +22,77 @@ namespace {
 
 consteval avr::hertz_t clock()
 {
-	if (avr::hw::db.name == "ATtiny13A")
+#if defined(PUREBOOT_CLOCK_HZ)
+	return avr::hertz_t{PUREBOOT_CLOCK_HZ};
+#else
+	auto name = std::string_view{avr::hw::db.name};
+	if (name.starts_with("ATtiny13"))
 		return 9.6_MHz;
-	if (avr::hw::db.name == "ATtiny85")
+	if (name.starts_with("ATtiny"))
 		return 8_MHz;
 	return 16_MHz;
+#endif
+}
+
+#if !defined(PUREBOOT_TX)
+#define PUREBOOT_TX pb1
+#endif
+#if !defined(PUREBOOT_USART)
+#define PUREBOOT_USART 0
+#endif
+
+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");
+#endif
 }
 
 using dev = avr::device<{.clock = clock()}>;
 
-template <avr::hertz_t C, bool Hardware = avr::hw::db.has_reg("UDR0")>
+template <avr::hertz_t C, bool Hardware = use_hardware()>
 struct link {
-	using tx_t = avr::uart::usart0<C, {.baud = 115200_Bd, .max_baud_error = 2.5_pct}>;
+#if defined(PUREBOOT_BAUD)
+	static constexpr avr::baud_t baud{PUREBOOT_BAUD};
+#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}>;
 	static void tx(char c)
 	{
 		tx_t::write(static_cast<std::uint8_t>(c));
 	}
 	[[noreturn]] static void idle()
 	{
-		for (;;)
+		// 'L' hands back to the loader in the top slot — 512 bytes on every
-			if (tx_t::read_blocking() == 'L')
+		// chip. The jump takes a word address, which is what makes the
-				reinterpret_cast<void (*)()>((avr::hw::db.mem.flash_size - 512) / 2)();
+		// >64 KiB chips' entry reachable through a 16-bit pointer at all.
+		constexpr std::uint32_t slot = 512;
+		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))();
+			// 'D' leaves every word of the SPM page buffer dirty, so that a
+			// following 'L' enters the loader with the buffer it never clears.
+			if (command == 'D') {
+				for (std::uint16_t at = 0; at < avr::spm::page_bytes; at += 2)
+					avr::spm::fill(at, 0xdead);
+				tx('D');
+			}
+		}
 	}
 };
 
 template <avr::hertz_t C>
 struct link<C, false> {
-	using tx_t = avr::uart::software_tx<C, avr::pb1, 57600_Bd>;
+#if defined(PUREBOOT_BAUD)
+	static constexpr avr::baud_t baud{PUREBOOT_BAUD};
+#else
+	static constexpr avr::baud_t baud{57600};
+#endif
+	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, baud>;
 	static void tx(char c)
 	{
 		tx_t::write(static_cast<std::uint8_t>(c));

test/pbautobaud.py

@@ -0,0 +1,154 @@
+#!/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
+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>
+
+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.
+"""
+
+import os
+import sys
+import time
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    (device_bin, elf, mcu, base_hex, page, app_bin, app_hz, app_baud, tool, workdir) = sys.argv[1:]
+    base, page, app_hz, app_baud = int(base_hex, 0), int(page), int(app_hz), int(app_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)
+    ee_image = bytes(range(0xA0, 0xB0))
+    ee_path = os.path.join(workdir, "ee.bin")
+    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
+    # no vector surgery, the tinies and the boot-section-less m48s do, and the
+    # large chips speak word addresses.
+    mega = mcu.startswith("atmega")
+    patch = not mega or mcu.startswith("atmega48")
+    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)
+    ground_truth = pb.Info(bytes([ord("P"), ord("B"), pb.NEWEST_LOADER, 0, 0, 0, page & 0xFF,
+                                  wire_base & 0xFF, wire_base >> 8, 0, 0, flags]))
+
+    def round_trip(hz, baud, label, hand_over):
+        """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")
+        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")
+            for needed in ("version", "signature", "fuses", "verify:", "stays"):
+                if needed not in out:
+                    fail(f"{label}: session output lacks {needed!r}\n{out}")
+            # 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,
+                                 "--verify-eeprom", ee_path, "--read-flash", read_flash,
+                                 "--read-eeprom", read_eeprom, "--stay")
+            if out.count("verify:") != 2:
+                fail(f"{label}: did not verify both memories\n{out}")
+            if open(read_eeprom, "rb").read()[: len(ee_image)] != ee_image:
+                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.
+                # (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)
+                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.
+                    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}")
+                    print(f"  {label}: lone calibration pulse does not wedge the loader")
+                finally:
+                    port.close()
+
+                device.reset()
+                port = pb.Port(device.pty, baud)
+                try:
+                    loader = pb.Loader(port)
+                    live = loader.connect_autobaud(15)
+                    if not pb.OLDEST_LOADER <= live.version <= pb.NEWEST_LOADER:
+                        fail(f"{label}: loader reports pureboot {live.version}")
+                    if loader.unified:
+                        # pureboot 5's data space. 0x0200 is clear of the
+                        # loader's own .noinit unit at the bottom of SRAM and of
+                        # 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:
+                            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)
+                        print(f"  {label}: RAM read/write ok ({verbose_ram.hex()})")
+                    loader.run_application()
+                    banner = port.read_exact(3, 5.0)
+                    if banner != b"APP":
+                        fail(f"{label}: application banner was {banner!r}")
+                finally:
+                    port.close()
+        finally:
+            device.stop()
+
+        # Ground truth (read after the runner exits and writes its dump): what
+        # the tool programmed must be what the simulator actually holds.
+        pages = pb.plan_flash(open(app_bin, "rb").read(), ground_truth)
+        flash_true = open(dump, "rb").read()
+        for address, data in pages.items():
+            if flash_true[address : address + page] != data:
+                fail(f"{label}: simulator flash differs from the programmed image at {address:#06x}")
+        print(f"  {label}: locked at {hz} Hz / {baud} Bd, flash+EEPROM verified"
+              + (", hand-over ok" if hand_over else ""))
+
+    # 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
+    # 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")
+
+
+if __name__ == "__main__":
+    main()

test/pbdirty.py

@@ -0,0 +1,88 @@
+#!/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
+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),
+but simavr dispatches SPM from anywhere, which is what makes it constructible.
+
+Usage: pbdirty.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                  <baud> <app_bin> <tool_py> <workdir>
+"""
+
+import os
+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 = 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
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "dump.bin")
+
+    # Reset boots the application on a BOOTRST-unprogrammed mega; its 'L' is
+    # the loader entry this test needs, reached without a reset.
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, reset_hex="0")
+    try:
+        port = pb.Port(device.pty, baud)
+        loader = pb.Loader(port)
+        loader.connect(25)
+
+        # Install the application and hand over to it.
+        pb.op_flash(loader, app_bin, erase=False, verify=True)
+        loader.run_application()
+        if port.read_exact(3, 5.0) != b"APP":
+            fail("the application did not start")
+
+        port.write(b"D")
+        if port.read_exact(1, 5.0) != b"D":
+            fail("the application did not acknowledge dirtying the page buffer")
+        port.write(b"L")
+        loader = pb.Loader(port)
+        loader.connect(25)
+
+        # Program by hand, so the corruption is observable before anything
+        # repairs it.
+        pages = pb.plan_flash(open(app_bin, "rb").read(), loader.info)
+        for address in sorted(pages):
+            loader.write_page(address, pages[address])
+        try:
+            pb.verify_pages(loader, pages)
+        except pb.Error as error:
+            if "verify failed" not in str(error):
+                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.
+            fail("programming over a dirty page buffer came back clean")
+
+        # What the programming path uses: one rewrite settles it, and it stays
+        # settled.
+        pb.verify_pages(loader, pages, repair=True)
+        pb.verify_pages(loader, pages)
+
+        # Ground truth beyond the loader's own read-back.
+        loader.run_application()
+        if port.read_exact(3, 5.0) != b"APP":
+            fail("the application did not start after the recovered write")
+        port.close()
+    finally:
+        device.stop()
+    print("pbdirty: a dirty page buffer is caught by verify and cleared by the retry")
+
+
+if __name__ == "__main__":
+    main()

test/pbrehome.py

@@ -0,0 +1,96 @@
+#!/usr/bin/env python3
+"""Re-homing acceptance test: an image programmed somewhere other than its
+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.
+
+Usage: pbrehome.py <device_bin> <pureboot_elf> <update_bin> <mcu> <hz>
+                   <base_hex> <page> <baud> <app_bin> <tool_py> <workdir>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"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,
+                mcu, hz):
+    """Place the loader at `place_hex`, heal through --update-loader, flash
+    the application, expect the banner."""
+    dump = os.path.join(workdir, f"dump-{place_hex}.bin")
+    state = os.path.join(workdir, f"rehome-{place_hex}.pbstate")
+    if os.path.exists(state):
+        os.unlink(state)
+    device = pbsim.Device(device_bin, elf, mcu, hz, place_hex, page, baud, dump, reset_hex="0")
+    try:
+        port = pb.Port(device.pty, baud)
+        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")
+
+        # The ordinary update flow puts the build into the top slot.
+        pb.op_update_loader(loader, 25, update_bin, state, None)
+        update = open(update_bin, "rb").read()
+        if loader.read_flash(base, len(update)) != update:
+            fail("the canonical slot does not hold the update image")
+
+        # An application flashed through the healed resident overwrites the
+        # stale copy (surgery included) and launches.
+        pages = pb.plan_flash(open(app_bin, "rb").read(), loader.info)
+        for address in pb.covered(pages, loader.info, skip_blank=False):
+            loader.write_page(address, pages[address])
+        pb.verify_pages(loader, pages)
+        loader.run_application()
+        if port.read_exact(3, 5.0) != b"APP":
+            fail(f"application does not banner after the re-home from {place_hex}")
+        port.close()
+    finally:
+        device.stop()
+
+
+def main():
+    (device_bin, elf, update_bin, mcu, hz, base_hex, page, baud, app_bin, 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)
+
+    # 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)
+    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
+    # 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,
+                mcu, hz)
+    print("re-home from the staging slot: converged")
+
+    print("pbrehome: a misplaced loader re-homes through the ordinary update flow")
+
+
+if __name__ == "__main__":
+    main()

test/pbreloc.py

@@ -1,11 +1,9 @@
 #!/usr/bin/env python3
-"""Position-independence acceptance test: the identical pureboot binary,
+"""Position-independence acceptance test: the identical binary, flashed one
-flashed one slot below the resident loader, must serve the complete command
+slot below the resident, must serve the complete command set from there. The
-set from there. The resident installs it (through-word composed by the host
+info block must come back byte-identical, the write guard must refuse the
-layer), 'J' transfers control, and every command is exercised against the
+staged copy's own slot and permit the resident's, and the staged copy must be
-staged copy — the info block must come back byte-identical, the write guard
+able to rewrite the resident verbatim.
-must protect the staged copy's own slot and permit the resident's, and the
-staged copy must be able to rewrite the resident slot verbatim.
 
 Usage: pbreloc.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
                   <baud> <tool_py> <workdir>
@@ -24,7 +22,7 @@ def fail(message):
 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)
-    stage = base - 512
+    stage = None  # derived from the device's own info (slot-sized) below
     sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
     sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
     import pbsim
@@ -46,6 +44,7 @@ def main():
         resident_info = info.raw
 
         # Install the staging copy exactly as the update flow would.
+        stage = info.stage
         staged = pb.staging_content(image, info)
         pb.write_differing(loader, stage, staged)
 
@@ -65,8 +64,10 @@ 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 guard, both ways: its own slot refused (drained, unchanged), the
-        # the resident slot writable.
+        # 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:
@@ -74,11 +75,13 @@ def main():
         marker = bytes((i * 3) & 0xFF for i in range(page))
         loader.write_page(base, marker)
         if loader.read_flash(base, page) != marker:
-            fail("the staged copy could not write the resident slot")
+            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")
 
         # Restore the resident image through the staged copy, then 'J' back
         # into it and prove it lives.
-        resident = image + b"\xff" * (512 - len(image))
+        resident = image + b"\xff" * (pb.SLOT - len(image))
         pb.write_differing(loader, base, resident)
         back_info = loader.enter_copy(base, 25)
         if back_info.raw != resident_info:

test/pbsim.py

@@ -8,10 +8,17 @@ import subprocess
 
 
 class Device:
-    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None):
+    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None, link=None):
-        cmd = [binary, elf, mcu, hz, base_hex, str(page), str(baud), dump]
+        cmd = [binary]
+        if link:
+            cmd += ["-l", link]
+        cmd += [elf, mcu, hz, base_hex, str(page), str(baud), dump]
         if reset_hex is not None or resume is not None:
-            cmd.append(reset_hex if reset_hex is not None else ("0" if mcu != "atmega328p" else base_hex))
+            # 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))
         if resume is not None:
             cmd.append(resume)
         self.log = open(dump + ".log", "a")

test/pbtest.py

@@ -1,19 +1,17 @@
 #!/usr/bin/env python3
-"""End-to-end pureboot protocol test: spawn the simavr device, then drive it
+"""End-to-end protocol test: drive the simavr device with the real host tool
-with the real host tool (pureboot.py, as a subprocess over the device's pty)
+over its pty through flash, EEPROM, fuse and hand-over scenarios, and
-through flash + EEPROM + fuse + hand-over scenarios, and cross-check
+cross-check the tool's view against the simulator's ground-truth dumps.
-the tool's view against the simulator's ground-truth memory dumps.
 
 Usage: pbtest.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
-                 <baud> <eeprom_size> <app_bin> <tool_py> <workdir>
+                 <baud> <eeprom_size> <app_bin> <tool_py> <workdir> [link]
-Exits 0 if every scenario passes.
+
+The optional link is the runner's -l spec (usart1, sw:B5,B1, ...), for a
+loader built off the chip's natural serial default.
 """
 
 import os
-import signal
-import subprocess
 import sys
-import time
 
 
 def fail(message):
@@ -33,53 +31,14 @@ def rjmp_decode(word, at, flash_words):
     return (at + 1 + offset) % flash_words
 
 
-class Device:
-    def __init__(self, binary, elf, mcu, hz, base, page, baud, dump):
-        self.proc = subprocess.Popen(
-            [binary, elf, mcu, hz, base, str(page), str(baud), dump],
-            stdout=subprocess.PIPE,
-            stderr=subprocess.STDOUT,
-            text=True,
-        )
-        self.dump = dump
-        self.pty = None
-        deadline = time.time() + 5
-        while time.time() < deadline:
-            line = self.proc.stdout.readline()
-            if not line:
-                break
-            if line.startswith("PB_PTY"):
-                self.pty = line.split()[1]
-                break
-        if not self.pty:
-            self.stop()
-            raise RuntimeError("device did not report a pty")
-
-    def stop(self):
-        self.proc.terminate()
-        try:
-            self.proc.wait(timeout=3)
-        except subprocess.TimeoutExpired:
-            self.proc.kill()
-
-
-def run_tool(tool, pty, baud, *args):
-    result = subprocess.run(
-        [sys.executable, tool, "--port", pty, "--baud", str(baud), "--wait", "20", *args],
-        capture_output=True,
-        text=True,
-        timeout=120,
-    )
-    print(result.stdout, end="")
-    if result.returncode != 0:
-        fail(f"tool exited {result.returncode}: {result.stderr.strip()}")
-    return result.stdout
-
-
 def main():
-    (device_bin, elf, mcu, hz, base_hex, page, baud, eeprom_size, app_bin, tool, workdir) = sys.argv[1:]
+    args = sys.argv[1:]
+    link = args.pop() if len(args) == 12 else None
+    (device_bin, elf, mcu, hz, base_hex, page, baud, eeprom_size, app_bin, tool, workdir) = args
     base, page, baud, eeprom_size = int(base_hex, 0), int(page), int(baud), int(eeprom_size)
     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)
@@ -90,25 +49,33 @@ def main():
     read_flash = os.path.join(workdir, "readback_flash.bin")
     read_eeprom = os.path.join(workdir, "readback_eeprom.bin")
 
-    # The geometry the host will discover, for computing the expected image.
+    # The geometry the host will discover, for computing the expected image:
+    # the boot-sectioned megas need no vector surgery (the tinies and the
+    # boot-section-less m48s do), the large chips speak word addresses, and
+    # the page byte is the wire's 0-means-256.
+    mega = mcu.startswith("atmega")
+    patch = not mega or mcu.startswith("atmega48")
+    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)
     info = pb.Info(
-        bytes([ord("P"), ord("B"), 1, 0, 0, 0, page])
+        bytes([ord("P"), ord("B"), pb.NEWEST_LOADER, 0, 0, 0, page & 0xFF])
-        + bytes([base & 0xFF, base >> 8, eeprom_size & 0xFF, eeprom_size >> 8])
+        + bytes([wire_base & 0xFF, wire_base >> 8, eeprom_size & 0xFF, eeprom_size >> 8])
-        + bytes([0 if mcu == "atmega328p" else 1])
+        + bytes([flags])
     )
 
-    device = Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump)
+    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 = run_tool(tool, device.pty, baud, "--info", "--fuses", "--flash", app_bin,
+        out = pbsim.run_tool(tool, device.pty, baud, "--info", "--fuses", "--flash", app_bin,
                              "--eeprom", ee_path, "--stay")
-        for needed in ("device: signature", "fuses:", "verify:", "stays"):
+        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 = run_tool(tool, device.pty, baud, "--verify-flash", app_bin, "--verify-eeprom", ee_path,
+        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")
         if out.count("verify:") != 2:
             fail("session 2 did not verify both memories")
@@ -128,11 +95,30 @@ def main():
         # runner resets them to address 0 like silicon) or BOOTRST (mega).
         # The loader must answer a fresh knock, and the 'J' hand-over must
         # land in the application, which banners on the same link.
-        device.proc.send_signal(signal.SIGUSR1)
+        device.reset()
         port = pb.Port(device.pty, baud)
         try:
             loader = pb.Loader(port)
-            loader.connect(15)
+            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
+            # 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.
+            if not pb.OLDEST_LOADER <= live.version <= pb.NEWEST_LOADER:
+                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)
+            if port.read_exact(1, 5.0) != pb.PROMPT:
+                fail("unaligned W did not return to the prompt")
+
             loader.run_application()
             banner = port.read_exact(3, 5.0)
             if banner != b"APP":
@@ -150,9 +136,10 @@ def main():
         fail("loader region looks erased in the ground-truth dump")
 
     # The surgery, decoded independently: the patched vector must land on the
-    # loader, the trampoline on the application's own entry.
+    # loader, the trampoline on the application's own entry (patched-vector
-    if mcu != "atmega328p":
+    # chips only — a boot-sectioned mega's word 0 stays the application's).
-        flash_words = (base + 512) // 2
+    if patch:
+        flash_words = (base + pb.SLOT) // 2
         app = open(app_bin, "rb").read()
         word0 = flash_true[0] | (flash_true[1] << 8)
         if rjmp_decode(word0, 0, flash_words) != base // 2:

test/pbupdate.py

@@ -1,16 +1,12 @@
 #!/usr/bin/env python3
-"""Self-update end-to-end: an application is flashed, then the loader
+"""Self-update end-to-end: an application is flashed, the loader replaces
-replaces itself with a re-timed build through the host tool's
+itself with a re-timed build, and every power-fail phase is rehearsed by
---update-loader — and the power-fail phases of that update are rehearsed by
+killing the device mid-write, restarting it from its flash dump, and letting
-killing the simulated device mid-write, restarting it from its flash dump,
+a re-run complete the update.
-and letting a re-run complete the update.
 
-The mega runs the BOOTRST-unprogrammed profile (reset boots the application;
+The boot-sectioned megas run the BOOTRST-unprogrammed profile — reset boots
-the fixture application's 'L' jump is the application-owned loader entry),
+the application, whose 'L' is the application-owned loader entry — with
-with --assume-fuses standing in for the fuse read simavr cannot model. The
+--assume-fuses standing in for the fuse read simavr cannot model.
-tinies reset into a loader at every phase by construction — the t13a because
-its staging slot carries the reset vector itself, the t85 through the word-0
-redirect the tool plants around the resident rewrite.
 
 Usage: pbupdate.py <device_bin> <pureboot_elf> <update_elf> <mcu> <hz>
                    <base_hex> <page> <baud> <app_bin> <tool_py> <workdir>
@@ -41,10 +37,21 @@ class PowerFail(Exception):
     pass
 
 
-MEGA_FUSES = "ffffffdd"  # high 0xdd: BOOTSZ = 1 KB, BOOTRST unprogrammed
+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
+    and fuse byte come from the tool's own table, keyed by the update
+    image's embedded signature."""
+    info = pb.image_info(image)
+    which, ladder = pb.BOOT_FUSE[bytes(info.signature[1:3])]
+    bits = min((b for b in ladder if ladder[b] * 2 >= 2 * pb.SLOT), key=lambda b: ladder[b])
+    fuses = bytearray((0xFF, 0xFF, 0xFF, 0xFF))
+    fuses[which] = 0xF8 | (bits << 1) | 1
+    return bytes(fuses)
 
 
-def make_fault_loader(pb, base, kill_region, kill_hits, device):
+def make_fault_loader(pb, base, slot, kill_region, kill_hits, device):
     """A Loader whose write_page kills the device (or, with device=None,
     just the host) at the Nth write into a region; the sequence
     stage->resident->stage distinguishes the install from the restore."""
@@ -59,7 +66,7 @@ def make_fault_loader(pb, base, kill_region, kill_hits, device):
             if address >= base:
                 phase = "resident"
                 self.seen_resident = True
-            elif address >= base - 512:
+            elif address >= base - slot:
                 phase = "stage_restore" if self.seen_resident else "stage"
             else:
                 phase = "app"
@@ -77,13 +84,17 @@ def make_fault_loader(pb, base, kill_region, kill_hits, device):
 def main():
     (device_bin, elf, update_elf, mcu, hz, base_hex, page, baud, app_bin, tool, workdir) = sys.argv[1:]
     base, page, baud = int(base_hex, 0), int(page), int(baud)
-    mega = mcu == "atmega328p"
+    mega = mcu.startswith("atmega")
-    reset_hex = "0" if mega else None  # the mega runs BOOTRST-unprogrammed here
+    # The m48s are megas without a boot section: patched vector, no fuse
+    # preflight, and the same reset-to-0 the tinies get.
+    patch = not mega or mcu.startswith("atmega48")
+    reset_hex = "0" if mega else None  # the boot-sectioned mega runs BOOTRST-unprogrammed here
     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
 
+    slot = pb.SLOT
     os.makedirs(workdir, exist_ok=True)
     objcopy = os.environ.get("PB_OBJCOPY", "avr-objcopy")
     images = {}
@@ -95,7 +106,7 @@ def main():
         fail("the update image is byte-identical to the resident build")
     dump = os.path.join(workdir, "dump.bin")
     state = os.path.join(workdir, "update.pbstate")
-    fuses = bytes.fromhex(MEGA_FUSES) if mega else None
+    fuses = assumed_fuses(pb, images["v0"]) if mega and not patch else None
 
     def connect(device):
         port = pb.Port(device.pty, baud)
@@ -109,16 +120,16 @@ def main():
         return port, loader
 
     def padded(image):
-        return image + b"\xff" * (512 - len(image))
+        return image + b"\xff" * (slot - len(image))
 
     def resident_bytes(loader):
-        return loader.read_flash(base, 256) + loader.read_flash(base + 256, 256)
+        return loader.read_flash(base, slot)
 
     def assert_state(loader, image, app_pages):
         if resident_bytes(loader) != padded(image):
             fail("resident loader does not match the update image")
-        stage = base - 512
+        stage = base - slot
-        got = loader.read_flash(stage, 256) + loader.read_flash(stage + 256, 256)
+        got = loader.read_flash(stage, slot)
         for address, data in app_pages.items():
             if stage <= address < base:
                 if got[address - stage : address - stage + page] != data:
@@ -135,8 +146,8 @@ def main():
 
         # A clean CLI update, resident -> v9.
         args = ["--update-loader", os.path.join(workdir, "v9.bin"), "--state", state, "--stay"]
-        if mega:
+        if fuses:
-            args += ["--assume-fuses", MEGA_FUSES]
+            args += ["--assume-fuses", fuses.hex()]
         out = pbsim.run_tool(tool, device.pty, baud, *args)
         if "loader updated" not in out:
             fail("update did not report success")
@@ -160,14 +171,14 @@ def main():
         # cost of that profile (README).
         for kill_region, kill_hits, kill_device in (
             ("stage", 2, True),
-            ("resident", 1, not mega),
+            ("resident", 1, patch),
             ("stage_restore", 2, True),
         ):
             device.reset()  # the previous round left the application running
             port, loader = connect(device)
             target = "v9" if resident_bytes(loader) == padded(images["v0"]) else "v0"
             image_path = os.path.join(workdir, target + ".bin")
-            injected = make_fault_loader(pb, base, kill_region, kill_hits, device if kill_device else None)(port)
+            injected = make_fault_loader(pb, base, slot, kill_region, kill_hits, device if kill_device else None)(port)
             injected.info = loader.info
             try:
                 pb.op_update_loader(injected, 25, image_path, state, fuses)
@@ -191,12 +202,12 @@ def main():
         device.stop()
 
     # Ground truth: the simulator's own flash against the final state, and
-    # on the tinies an independent decode of the reset routing.
+    # on the patched-vector chips an independent decode of the reset routing.
     flash = open(dump, "rb").read()
-    if flash[base : base + 512] != padded(images[final]):
+    if flash[base : base + slot] != padded(images[final]):
         fail("ground-truth resident region does not match the final image")
-    if not mega:
+    if patch:
-        flash_words = (base + 512) // 2
+        flash_words = (base + slot) // 2
         word0 = flash[0] | (flash[1] << 8)
         if rjmp_decode(word0, 0, flash_words) != base // 2:
             fail("ground-truth reset vector does not land on the loader")

test/pureboot_device.c

@@ -1,12 +1,16 @@
-// simavr "device" for the pureboot protocol tests, all three chips. Loads
+// simavr "device" for the pureboot protocol tests, every chip. Loads the
-// the boot-linked ELF at the loader base, starts execution there (BOOTRST /
+// boot-linked ELF at the loader base, starts execution there (BOOTRST / the
-// the patched vector are not what is under test), and exposes the loader's
+// patched vector are not what is under test), and exposes the loader's
 // serial link as a pty for the real host tool:
 //
-//   - ATmega328P: the hardware USART0 through simavr's uart_pty.
+//   - Hardware USART builds: simavr's uart_pty on the selected instance.
-//   - Tinies: an 8N1 bridge between a pty and the GPIO software UART
+//   - Software UART builds: an 8N1 bridge between a pty and the GPIO pins,
-//     (drives PB0, the loader's RX; decodes PB1, its TX), timed against the
+//     timed against the simulated cycle counter (drives the loader's RX,
-//     simulated cycle counter.
+//     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
@@ -38,17 +42,44 @@
 
 static avr_t *avr;
 static uart_pty_t uart_pty;
-static int use_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);
 
@@ -64,6 +95,15 @@ static int fixed_flash_ioctl(avr_io_t *io, uint32_t ctl, void *param)
 		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);
 }
 
@@ -257,29 +297,42 @@ static void finish(int sig)
 			}
 		}
 	}
-	if (use_uart_pty)
+	if (!link_software)
 		uart_pty_stop(&uart_pty);
 	_exit(0);
 }
 
 int main(int argc, char *argv[])
 {
-	if (argc < 8 || argc > 10) {
+	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 <pureboot.elf> <mcu> <hz> <base_hex> <page> <baud> <flash_dump>"
+		        "usage: %s [-l link] <pureboot.elf> <mcu> <hz> <base_hex> <page> <baud> <flash_dump>"
 		        " [reset_hex] [resume_flash]\n"
-		        "  reset_hex:    reset vector (default: base on the mega, 0 on the tinies)\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];
-	use_uart_pty = strcmp(mcu_name, "atmega328p") == 0;
+	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) {
@@ -290,7 +343,7 @@ int main(int argc, char *argv[])
 	avr->frequency = (uint32_t)strtoul(argv[3], NULL, 0);
 	memset(avr->flash, 0xff, avr->flashend + 1); // real flash powers up erased
 
-	if (argc > 9) {
+	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) {
@@ -306,11 +359,13 @@ int main(int argc, char *argv[])
 		}
 		memcpy(avr->flash + base, fw.flash, fw.flashsize);
 	}
-	// The mega enters the loader in hardware (BOOTRST, not modeled — the
+	// The boot-sectioned megas enter the loader in hardware (BOOTRST, not
-	// argument picks the modeled fuse's target); the tinies reset to word 0
+	// modeled — the argument picks the modeled fuse's target); the tinies
-	// like silicon — erased flash walks up into the loader, and after the
+	// and the boot-section-less m48s reset to word 0 like silicon — erased
-	// host's surgery the patched vector routes there.
+	// flash walks up into the loader, and after the host's surgery the
-	reset_pc = argc > 8 ? (uint32_t)strtoul(argv[8], NULL, 0) : (use_uart_pty ? base : 0);
+	// 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;
 
@@ -323,27 +378,34 @@ int main(int argc, char *argv[])
 		avr_ioctl(avr, AVR_IOCTL_EEPROM_SET, &seed);
 	}
 
-	if (use_uart_pty) {
+	// 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();
-		// 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('0'), &flags);
-		flags &= ~AVR_UART_FLAG_POLL_SLEEP;
-		avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS('0'), &flags);
-		uart_pty_init(avr, &uart_pty);
-		uart_pty_connect(&uart_pty, '0');
-		printf("PB_PTY %s\n", uart_pty.pty.slavename);
 	} 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('B'), 0);
+		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('B'), 1), tx_hook, NULL);
+		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;
@@ -371,18 +433,27 @@ int main(int argc, char *argv[])
 			reset_requested = 0;
 			avr_reset(avr);
 			avr->pc = reset_pc;
-			if (use_uart_pty) { // reset restores the pacing hack; re-clear it
+			if (!link_software) { // reset restores the pacing hack; re-clear it
 				uint32_t flags = 0;
-				avr_ioctl(avr, AVR_IOCTL_UART_GET_FLAGS('0'), &flags);
+				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('0'), &flags);
+				avr_ioctl(avr, AVR_IOCTL_UART_SET_FLAGS(uart_digit), &flags);
 			} else {
 				bridge_reset();
 			}
 		}
-		if (!use_uart_pty && ++since_poll >= 2000) {
+		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);

test/test_planner.py

@@ -1,9 +1,8 @@
 #!/usr/bin/env python3
-"""Host-tool unit tests — the pure planning and policy logic, no simulator:
+"""Host-tool unit tests — the planning and policy logic, no simulator:
-the flash-programming orders and their recovery properties, the reset-vector
+programming orders and their recovery properties, the reset-vector surgery,
-surgery, the staging-slot composition, the mega boot-fuse decode, and the
+the staging composition, the boot-fuse decode, and the update preflight over
-update preflight's error/warning matrix (fuse combinations simavr cannot
+fuse combinations simavr cannot model.
-model reach it here as synthetic bytes).
 
 Usage: test_planner.py <tool_py>
 """
@@ -27,11 +26,15 @@ def expect_error(what, fn, *needles):
     fail(f"{what}: no error raised")
 
 
-def info_of(pb, base, page, patch, flash):
+def info_of(pb, base, page, patch, flash, signature=(0x1E, 0x93, 0x0B), word_flash=False, version=None):
-    raw = bytes((0x50, 0x42, 1, 0x1E, 0x93, 0x0B, page, base & 0xFF, base >> 8,
+    scale = 2 if word_flash else 1
-                 0, 2, 1 if patch else 0))
+    wire_base = base // scale
+    flags = (1 if patch else 0) | (2 if word_flash else 0)
+    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))
     info = pb.Info(raw)
-    assert info.flash_size == flash
+    if info.flash_size != flash:
+        fail(f"info_of({base:#x}) decodes to {info.flash_size:#x} of flash, not {flash:#x}")
     return info
 
 
@@ -50,16 +53,67 @@ def main():
     import pureboot as pb
 
     tiny = info_of(pb, 0x1E00, 64, True, 0x2000)
-    mega = info_of(pb, 0x7E00, 128, False, 0x8000)
+    mega = info_of(pb, 0x7E00, 128, False, 0x8000, signature=(0x1E, 0x95, 0x0F))
 
-    # mega_boot: BOOTSZ words and the BOOTRST sense, DS40002061B §27.
+    # Versioning: the block's third byte is the loader's version, and the tool
-    for bits, start in ((0b11, 0x7E00), (0b10, 0x7C00), (0b01, 0x7800), (0b00, 0x7000)):
+    # speaks a window of them. Every version in the window decodes, so an older
-        prog, at = pb.mega_boot((0xF8 | (bits << 1)) & ~1)
+    # deployed loader stays usable; one above the window is refused by name,
+    # since which version changed the protocol is knowledge only the tool
+    # holds, and it holds none about a version it has never heard of.
+    for version in range(pb.OLDEST_LOADER, pb.NEWEST_LOADER + 1):
+        if info_of(pb, 0x1E00, 64, True, 0x2000, version=version).version != version:
+            fail(f"pureboot {version} does not decode")
+    expect_error(
+        "unknown loader version",
+        lambda: info_of(pb, 0x1E00, 64, True, 0x2000, version=pb.NEWEST_LOADER + 1),
+        f"pureboot {pb.NEWEST_LOADER + 1}",
+        "newer tool",
+    )
+
+    # 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
+    # carries meaning.
+    cases = (
+        ((0x1E, 0x93, 0x07), 0x2000, 3, {0b11: 0x1F00, 0b10: 0x1E00, 0b01: 0x1C00, 0b00: 0x1800}),  # m8
+        ((0x1E, 0x94, 0x03), 0x4000, 3, {0b11: 0x3F00, 0b10: 0x3E00, 0b01: 0x3C00, 0b00: 0x3800}),  # m16
+        ((0x1E, 0x95, 0x02), 0x8000, 3, {0b11: 0x7E00, 0b10: 0x7C00, 0b01: 0x7800, 0b00: 0x7000}),  # m32
+        ((0x1E, 0x93, 0x0A), 0x2000, 2, {0b11: 0x1F00, 0b10: 0x1E00, 0b01: 0x1C00, 0b00: 0x1800}),  # m88
+        ((0x1E, 0x93, 0x0F), 0x2000, 2, {0b11: 0x1F00, 0b10: 0x1E00, 0b01: 0x1C00, 0b00: 0x1800}),  # m88P
+        ((0x1E, 0x94, 0x06), 0x4000, 2, {0b11: 0x3F00, 0b10: 0x3E00, 0b01: 0x3C00, 0b00: 0x3800}),  # m168/168A
+        ((0x1E, 0x94, 0x0B), 0x4000, 2, {0b11: 0x3F00, 0b10: 0x3E00, 0b01: 0x3C00, 0b00: 0x3800}),  # m168P
+        ((0x1E, 0x95, 0x14), 0x8000, 3, {0b11: 0x7E00, 0b10: 0x7C00, 0b01: 0x7800, 0b00: 0x7000}),  # m328
+        ((0x1E, 0x95, 0x0F), 0x8000, 3, {0b11: 0x7E00, 0b10: 0x7C00, 0b01: 0x7800, 0b00: 0x7000}),  # m328P
+        ((0x1E, 0x94, 0x0F), 0x4000, 3, {0b11: 0x3F00, 0b10: 0x3E00, 0b01: 0x3C00, 0b00: 0x3800}),  # m164A
+        ((0x1E, 0x94, 0x0A), 0x4000, 3, {0b11: 0x3F00, 0b10: 0x3E00, 0b01: 0x3C00, 0b00: 0x3800}),  # m164P
+        ((0x1E, 0x95, 0x15), 0x8000, 3, {0b11: 0x7E00, 0b10: 0x7C00, 0b01: 0x7800, 0b00: 0x7000}),  # m324A
+        ((0x1E, 0x96, 0x09), 0x10000, 3, {0b11: 0xFC00, 0b10: 0xF800, 0b01: 0xF000, 0b00: 0xE000}),  # m644
+        ((0x1E, 0x96, 0x0A), 0x10000, 3, {0b11: 0xFC00, 0b10: 0xF800, 0b01: 0xF000, 0b00: 0xE000}),  # m644P
+        ((0x1E, 0x97, 0x06), 0x20000, 3, {0b11: 0x1FC00, 0b10: 0x1F800, 0b01: 0x1F000, 0b00: 0x1E000}),  # 1284
+        ((0x1E, 0x97, 0x05), 0x20000, 3, {0b11: 0x1FC00, 0b10: 0x1F800, 0b01: 0x1F000, 0b00: 0x1E000}),  # 1284P
+    )
+    for signature, flash, which, ladder in cases:
+        chip = info_of(pb, flash - pb.SLOT, 128 if flash < 0x20000 else 0, False, flash,
+                       signature=signature, word_flash=flash > 0x10000)
+        for bits, start in ladder.items():
+            fuses = bytearray((0xFF, 0xFF, 0xFF, 0xFF))
+            fuses[which] = (0xF8 | (bits << 1)) & ~1
+            prog, at = pb.mega_boot(chip, bytes(fuses))
             if not prog or at != start:
-            fail(f"mega_boot BOOTSZ={bits:02b} programmed: {prog} {at:#06x}")
+                fail(f"mega_boot {signature[1]:02x}{signature[2]:02x} BOOTSZ={bits:02b} programmed: {prog} {at:#07x}")
-        prog, at = pb.mega_boot(0xF8 | (bits << 1) | 1)
+            fuses[which] |= 1
+            prog, at = pb.mega_boot(chip, bytes(fuses))
             if prog or at != start:
-            fail(f"mega_boot BOOTSZ={bits:02b} unprogrammed: {prog} {at:#06x}")
+                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
+    # 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.
@@ -115,6 +169,27 @@ def main():
         fail("image_info misses the embedded block")
     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
+    # identified and installed like any other.
+    old = info_of(pb, 0x1E00, 64, True, 0x2000, version=pb.OLDEST_LOADER)
+    found_old = pb.image_info(bytes((0xAA,)) * 10 + old.raw)
+    if found_old is None or found_old.version != pb.OLDEST_LOADER:
+        fail("image_info misses an older loader's block")
+
+    # loader_image must peel a padded image down to the slot content: a raw
+    # .bin padded from address 0 (or a whole-flash read-back with the loader
+    # resident at base) yields the same bytes as the bare slot image.
+    import tempfile
+    slot_image = bytes((0xAA,)) * 10 + tiny.raw + bytes((0xCC,)) * 40
+    padded = bytes((0xFF,)) * tiny.base + slot_image
+    with tempfile.NamedTemporaryFile(suffix=".bin", delete=False) as f:
+        f.write(padded)
+        padded_path = f.name
+    try:
+        if pb.loader_image(padded_path) != slot_image:
+            fail("loader_image does not peel a padded image to the slot content")
+    finally:
+        os.unlink(padded_path)
 
     # Update preflight: the full fuse matrix, plus target mismatch.
     other = info_of(pb, 0x1E00, 32, True, 0x2000)
@@ -140,6 +215,15 @@ def main():
     if pb.update_preflight(bytes((0xAA,)) * 8 + tiny.raw, tiny, None) != []:
         fail("tiny preflight should pass without fuses")
 
+    # 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
+    # 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))
+    if not any("staging slot" in n for n in notes):
+        fail(f"1284 minimum-BOOTSZ notes: {notes}")
+
     # The walk-region refusal: BOOTRST aimed below the loader plus app data
     # in the walk span errors without --force; erased spans and unprogrammed
     # BOOTRST pass.
@@ -150,6 +234,114 @@ def main():
     pb.check_walk_region({0x7800: bytes((0xFF,)) * 128}, mega, fuses(0xFA), False)
     pb.check_walk_region(deep, mega, None, False)  # fuses unknown: no check
 
+    # The repairing verify: a mismatched page is rewritten rather than raised,
+    # bounded so a fault that is not self-clearing cannot spin.
+    class FakeLoader:
+        """A device whose first `bad` writes of any page land wrong."""
+
+        def __init__(self, info, bad):
+            self.info = info
+            self.bad = bad
+            self.flash = {}
+            self.writes = 0
+
+        def write_page(self, address, data):
+            self.writes += 1
+            self.flash[address] = bytes(len(data)) if self.bad > 0 else bytes(data)
+            self.bad -= 1
+
+        def read_flash(self, address, count):
+            return self.flash.get(address, bytes(count))
+
+    want = {0: bytes((i * 5) & 0xFF for i in range(128))}
+
+    # One bad write, then good: repaired in place, and the caller never sees
+    # an error. The rewrite is counted, so a silent no-op cannot pass.
+    device = FakeLoader(info_of(pb, 0x7E00, 128, False, 0x8000), bad=1)
+    device.write_page(0, want[0])
+    pb.verify_pages(device, want, repair=True)
+    if device.writes != 2:
+        fail(f"repairing verify made {device.writes} writes, expected 2")
+
+    # Without repair the same state raises, so the repair is what fixed it.
+    device = FakeLoader(info_of(pb, 0x7E00, 128, False, 0x8000), bad=1)
+    device.write_page(0, want[0])
+    expect_error("verify without repair", lambda: pb.verify_pages(device, want), "verify failed")
+
+    # A page that never comes good stops after RETRIES rewrites, and says so.
+    device = FakeLoader(info_of(pb, 0x7E00, 128, False, 0x8000), bad=99)
+    device.write_page(0, want[0])
+    expect_error(
+        "unrepairable page",
+        lambda: pb.verify_pages(device, want, repair=True),
+        "verify failed",
+        f"after {pb.RETRIES} retries",
+    )
+    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
+    # 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.
+    class FakePort:
+        """A loader in its activation window, plus `lost` leading writes the
+        reset swallows and one stale prompt still on the wire."""
+
+        def __init__(self, info_raw, lost=0, stale=b"", active=False):
+            self.info_raw = info_raw
+            self.lost = lost
+            self.inflight = bytearray(stale)
+            self.rx = bytearray()
+            self.active = active
+            self.last = None
+
+        def flush_input(self):
+            self.rx.clear()
+
+        def write(self, data):
+            if self.lost:
+                self.lost -= 1
+                return
+            for byte in bytes(data):
+                if not self.active:
+                    self.active = self.last == ord("p") and byte == ord("b")
+                    self.last = byte
+                    if self.active:
+                        self.rx += pb.PROMPT
+                elif byte == ord("b"):
+                    self.rx += self.info_raw + pb.PROMPT
+                else:
+                    self.rx += pb.PROMPT
+
+        def read_available(self, wait):
+            self.rx = self.inflight + self.rx  # the stale prompt lands late
+            self.inflight.clear()
+            out, self.rx = bytes(self.rx), bytearray()
+            return out
+
+        def read_exact(self, count, timeout):
+            if len(self.rx) < count:
+                raise pb.Error(f"timeout: got {len(self.rx)} of {count} bytes")
+            out, self.rx = bytes(self.rx[:count]), self.rx[count:]
+            return out
+
+    raw = info_of(pb, 0x7E00, 128, False, 0x8000).raw
+    for what, port in (
+        ("clean window", FakePort(raw)),
+        ("stale prompt over a lost knock", FakePort(raw, lost=1, stale=pb.PROMPT)),
+        ("live session", FakePort(raw, active=True)),
+    ):
+        info = pb.Loader(port).connect(5)
+        if info.raw != raw:
+            fail(f"connect ({what}) returned {info.raw.hex()}")
+
+    # A device that never answers still says so, and a version the tool cannot
+    # speak is reported as such rather than retried into a timeout.
+    expect_error("dead device", lambda: pb.Loader(FakePort(raw, lost=99)).connect(0), "no answer")
+    old = bytes(raw[:2]) + bytes((pb.NEWEST_LOADER + 1,)) + bytes(raw[3:])
+    expect_error("unspeakable version", lambda: pb.Loader(FakePort(old)).connect(5), "needs a newer tool")
+
     print("test_planner: all planner and policy checks pass")
 
 

tools/check.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+# 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.
+set -e
+cd "$(dirname "$0")/.."
+
+full=0
+[[ "$1" == "--full" ]] && { full=1; shift; export PUREBOOT_FULL_MATRIX=1; }
+
+CHIPS=(attiny13 attiny13a attiny25 attiny45 attiny85
+       atmega8 atmega8a atmega16 atmega16a atmega32 atmega32a
+       atmega48 atmega48a atmega48p atmega48pa
+       atmega88 atmega88a atmega88p atmega88pa
+       atmega168 atmega168a atmega168p atmega168pa
+       atmega328 atmega328p
+       atmega164a atmega164p atmega164pa
+       atmega324a atmega324p atmega324pa
+       atmega644 atmega644a atmega644p atmega644pa
+       atmega1284 atmega1284p)
+REFLECT_SPOT=(attiny13a attiny85 atmega8 atmega16a atmega32a atmega48pa
+              atmega88 atmega168pa atmega328p atmega164a atmega644p atmega1284)
+
+for chip in "${CHIPS[@]}"; do
+	echo "==== $chip ===="
+	cmake --workflow --preset "$chip-generated" "$@"
+done
+
+if ((full)); then
+	for chip in "${REFLECT_SPOT[@]}"; do
+		echo "==== $chip reflect ===="
+		cmake --workflow --preset "$chip-reflect" "$@"
+	done
+fi
+
+echo "check: every chip green"

tools/make_presets.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""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
+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
+"""
+
+import json
+import os
+
+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",
+]
+
+# libavr's REFLECT_SPOT (tools/check.sh): one chip per hardware class and
+# pack vintage.
+REFLECT_SPOT = [
+    "attiny13a", "attiny85", "atmega8", "atmega16a", "atmega32a",
+    "atmega48pa", "atmega88", "atmega168pa", "atmega328p", "atmega164a",
+    "atmega644p", "atmega1284",
+]
+
+
+def main():
+    configure = [{
+        "name": "base",
+        "hidden": True,
+        "generator": "Ninja",
+        "binaryDir": "${sourceDir}/build/${presetName}",
+        "toolchainFile": "$env{LIBAVR_ROOT}/cmake/avr-toolchain.cmake",
+        "cacheVariables": {
+            "CMAKE_BUILD_TYPE": "Release",
+            "CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
+            "CMAKE_COLOR_DIAGNOSTICS": "ON",
+        },
+    }]
+    build, test, workflows = [], [], []
+
+    def add(chip, mode):
+        name = f"{chip}-{mode}"
+        configure.append({
+            "name": name,
+            "inherits": "base",
+            "cacheVariables": {
+                "LIBAVR_MCU": chip,
+                "LIBAVR_REFLECT": "ON" if mode == "reflect" else "OFF",
+            },
+        })
+        build.append({"name": name, "configurePreset": name})
+        steps = [{"type": "configure", "name": name}, {"type": "build", "name": name}]
+        if mode == "generated":
+            test.append({"name": name, "configurePreset": name, "output": {"outputOnFailure": True}})
+            steps.append({"type": "test", "name": name})
+        workflows.append({"name": name, "steps": steps})
+
+    for chip in CHIPS:
+        add(chip, "generated")
+    for chip in REFLECT_SPOT:
+        add(chip, "reflect")
+
+    presets = {
+        "version": 8,
+        "configurePresets": configure,
+        "buildPresets": build,
+        "testPresets": test,
+        "workflowPresets": workflows,
+    }
+    path = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "CMakePresets.json")
+    with open(path, "w") as f:
+        json.dump(presets, f, indent=1)
+        f.write("\n")
+    print(f"{len(CHIPS)} chips, {len(REFLECT_SPOT)} reflect: {os.path.normpath(path)}")
+
+
+if __name__ == "__main__":
+    main()