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docs: correct the size figures for the configured variants

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The headline numbers described the stock deployments but claimed "every
configured variant of them", which the matrix contradicts: choosing the
software UART where the chip has a USART costs 8-46 B, so the megas reach
460-462 rather than 452, and the 1284s' software-serial build is 546 B —
inside their 1 KiB boot sector, but not inside 512.

Also names the actual tightest chip. The 1284 looks like it at 506, but it
deploys in 1 KiB with 478 B spare; against its own budget the ATmega328P
has the least room, 50 B.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-07-22 17:08:24 +02:00
parent 862bc14e1e
commit a39a3c7f46
1 changed files with 10 additions and 5 deletions
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pureboot/README.md
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@@ -4,12 +4,17 @@ A serial bootloader on [libavr](https://git.blackmark.me/avr/libavr), pure by
constraint: one C++ source, no inline assembly, no global register variables
(attributes and compiler flags allowed), built for **every chip libavr
targets — all 37 — in 512 bytes each**: 434 B on the tiny13s, 438–442 B on
the tiny25/45/85, 412–452 B across the megas and every configured variant of
them, and 506 B on the ATmega1284/1284P, whose far-flash machinery (ELPM
reads, RAMPZ page commands, word-addressed wire) is the heaviest. The 1284s
are the tight case: the loop-placement attributes on the byte streamers
the tiny25/45/85, 412–452 B across the megas, and 506 B on the
ATmega1284/1284P, whose far-flash machinery (ELPM reads, RAMPZ page commands,
word-addressed wire) is the heaviest. Those are the stock deployments;
choosing the software UART where the chip has a USART costs 8–46 B more (a
bit-bang against a peripheral), which every chip still absorbs inside its
slot — on the 1284s that means their 1 KiB boot sector, where the
software-serial image lands at 546 B. Bringing the 1284's default build
under 512 at all is what the loop-placement attributes on the byte streamers
(`pureboot.cpp`) and the codegen flags on the loader TU (`CMakeLists.txt`)
are what carry that build under the line. Clock, baud, serial backend and
are for; measured against each chip's own budget the tightest is the
ATmega328P, 50 B spare. Clock, baud, serial backend and
pins are per-build configuration (below); the size matrix in the test suite
holds every combination inside its slot. The device speaks primitives; every
composite — verify, erase, reset-vector surgery, updating the loader itself —