pureboot: the classic megas and the word-addressed 1284P groundwork

Device: boot-section detection probes SPMCR beside SPMCSR, the link
picks any hardware USART through the instance-aware lookups (URSEL
chips included), WDRF reads MCUSR-or-MCUCSR, and the >64 KiB shape
lands — word-addressed wire flash (info flag bit 1, page byte 0 means
256, base as a word address), far reads through flash_load_far, a
single 32-bit byte-cursor page walk (the 256-byte page wraps its low
byte exactly), and slot arithmetic in words (the return address
already is one). Host: addresses stay bytes internally and scale at
the wire, the boot-fuse decode becomes a per-signature table (byte
index + BOOTSZ ladder — the m168A's lives in EXTENDED), and the
planner tests pin every chip's ladder plus the word-addressed info
decode. Tests: the device runner serves every mega over the USART pty,
pbapp banners over the right link, the update rehearsal synthesizes
its assumed fuses from the tool's own table, and the PI lint tracks
the renamed info symbol. All six classic-mega/168A targets pass the
full suite (size, PI, planner, protocol, reloc, self-update) at
466–504 B; the 1284P builds await a libavr far-path slimming to make
its 512.

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

test/pbapp.cpp

@@ -26,7 +26,8 @@ consteval avr::hertz_t clock()
 
 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 = avr::hw::db.has_instance("USART0") || avr::hw::db.has_instance("USART")>
 struct link {
 	using tx_t = avr::uart::usart0<C, {.baud = 115200_Bd, .max_baud_error = 2.5_pct}>;
 	static void tx(char c)