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pureboot: the activation countdown in the narrowest type that holds it

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The fixed-baud window counted down a uint32 where almost every window fits
24 bits; the countdown now takes avr::uint24_t when the poll budget allows
(the autobaud budget's own choice), uint32 past 16.7M polls — four bytes
off every fixed-baud image on every chip, the full suites green on the
changed window. The README size table is refreshed — its autobaud column
had also gone stale by the no-assembly pass's measurement-loop win, which
nothing gated: sizes.py check-readme now runs as the gate's final stage,
where every tree is freshly built and the table can actually be held.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-07-28 14:47:29 +02:00
parent 321ff8a4ee
commit c535c4756c
4 changed files with 29 additions and 18 deletions
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32
pureboot/README.md
View File

@@ -28,21 +28,21 @@ it carries the calibration machinery and no clock at all.
| Chip | Flash | Loader at | Link | Stock | Autobaud |
|---|---|---|---|---|---|
| ATtiny13, ATtiny13A † | 1 KiB | 0x0200 | software | 394 B | 464 B |
| ATtiny25 † | 2 KiB | 0x0600 | software | 398 B | 468 B |
| ATtiny45 † | 4 KiB | 0x0e00 | software | 402 B | 472 B |
| ATtiny85 † | 8 KiB | 0x1e00 | software | 402 B | 472 B |
| ATmega8, 8A | 8 KiB | 0x1e00 | USART0 | 364 B | 478 B |
| ATmega16, 16A | 16 KiB | 0x3e00 | USART0 | 366 B | 482 B |
| ATmega32, 32A | 32 KiB | 0x7e00 | USART0 | 366 B | 482 B |
| ATmega48, 48A, 48P, 48PA † | 4 KiB | 0x0e00 | USART0 | 392 B | 468 B |
| ATmega88, 88A, 88P, 88PA | 8 KiB | 0x1e00 | USART0 | 402 B | 478 B |
| ATmega168, 168A, 168P, 168PA | 16 KiB | 0x3e00 | USART0 | 404 B | 482 B |
| ATmega328, 328P | 32 KiB | 0x7e00 | USART0 | 404 B | 482 B |
| ATmega164A, 164P, 164PA | 16 KiB | 0x3e00 | USART0 | 404 B | 482 B |
| ATmega324A, 324P, 324PA | 32 KiB | 0x7e00 | USART0 | 404 B | 482 B |
| ATmega644, 644A, 644P, 644PA | 64 KiB | 0xfe00 | USART0 | 398 B | 476 B |
| ATmega1284, 1284P | 128 KiB | 0x1fe00 | USART0 | 424 B | 502 B |
| ATtiny13, ATtiny13A † | 1 KiB | 0x0200 | software | 390 B | 460 B |
| ATtiny25 † | 2 KiB | 0x0600 | software | 394 B | 464 B |
| ATtiny45 † | 4 KiB | 0x0e00 | software | 398 B | 468 B |
| ATtiny85 † | 8 KiB | 0x1e00 | software | 398 B | 468 B |
| ATmega8, 8A | 8 KiB | 0x1e00 | USART0 | 360 B | 474 B |
| ATmega16, 16A | 16 KiB | 0x3e00 | USART0 | 362 B | 480 B |
| ATmega32, 32A | 32 KiB | 0x7e00 | USART0 | 362 B | 480 B |
| ATmega48, 48A, 48P, 48PA † | 4 KiB | 0x0e00 | USART0 | 388 B | 464 B |
| ATmega88, 88A, 88P, 88PA | 8 KiB | 0x1e00 | USART0 | 398 B | 474 B |
| ATmega168, 168A, 168P, 168PA | 16 KiB | 0x3e00 | USART0 | 400 B | 480 B |
| ATmega328, 328P | 32 KiB | 0x7e00 | USART0 | 400 B | 480 B |
| ATmega164A, 164P, 164PA | 16 KiB | 0x3e00 | USART0 | 400 B | 480 B |
| ATmega324A, 324P, 324PA | 32 KiB | 0x7e00 | USART0 | 400 B | 480 B |
| ATmega644, 644A, 644P, 644PA | 64 KiB | 0xfe00 | USART0 | 394 B | 474 B |
| ATmega1284, 1284P | 128 KiB | 0x1fe00 | USART0 | 420 B | 500 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.
@@ -52,7 +52,7 @@ USART's own pins with the `OSCCAL` trim baked, 510 of its 512 — they alone
carry the far-flash machinery (ELPM reads, RAMPZ page commands), autobaud
alone carries the calibration loop, a bit-banged link on a USART's pins alone
has to release it (below), and the trim adds its one register write. Without
the trim that build is 504; on the default pins, 502. The flash bank riding
the trim that build is 504; on the default pins, 500. The flash bank riding
in a transfer's selector byte keeps even those chips' addressing the same
16-bit form every other chip uses, which is why they are no longer the
outlier they were.

7
pureboot/pureboot.cpp
View File

@@ -351,9 +351,14 @@ consteval std::uint32_t window_polls()
return timeout_seconds * static_cast<std::uint32_t>(dev::clock.hz / link::poll_cycles);
}
// The countdown in the narrowest type that holds it: a fourth byte would
// cost a wider decrement chain at every poll for range most windows never
// use (the autobaud budget makes the same choice).
using window_t = std::conditional_t<window_polls() <= 0xffffff, avr::uint24_t, std::uint32_t>;
bool pending_before_deadline()
{
std::uint32_t polls = window_polls();
window_t polls = window_polls();
do {
if (link::pending())
return true;