BlackMarkandClaude Opus 5.5
44db419c4b
.clangd said the repository rides as a submodule in its consumers, which none does, and removed three codegen flags only pureboot's loaders carry; its em dashes are ASCII now. The settings.json comment built on the same premise goes, and the oracle README states the variant rather than who chose it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
62 lines
3.0 KiB
Markdown
62 lines
3.0 KiB
Markdown
# Oracle — the hand-written TinySafeBoot assembly
|
||
|
||
`tsb-fixedbaud.asm` is the reference implementation this port is measured
|
||
against: the **native-UART, fixed-baud** TinySafeBoot bootloader, hand-written
|
||
in AVR assembly. It is the size-and-feature bar for the port's `tsb_asm` tier.
|
||
|
||
- **Source**: <https://github.com/seedrobotics/tinysafeboot>
|
||
(`firmware_ASM/latest_stable_release/20200727-fixedbaud/main.asm`), the Seed
|
||
Robotics fixed-baud fork of Julien Thomas' TinySafeBoot.
|
||
- **License**: GPLv3 (see the header in the file). It is vendored here **only as
|
||
a reference oracle** — it is not compiled, linked, or distributed as part of
|
||
the MIT-licensed port. Mere aggregation.
|
||
|
||
## Why this variant
|
||
|
||
The fixed-baud, hardware-UART variant is the one whose feature set the port must
|
||
match. It fits the **complete** TSB feature set into the 512-byte ATmega boot
|
||
section:
|
||
|
||
| Feature | Oracle routine |
|
||
|---|---|
|
||
| Watchdog-reset bail straight to the app | `RESET` (WDRF check) |
|
||
| One-wire half-duplex (RX/TX shorted): RXEN/TXEN toggled per direction, TX turnaround guard | `SetRX` / `SetTX` / `TransmitByte` |
|
||
| Activation timeout read from the config page, with a lockout-proof minimum | `WRX1To` (uses `utimeoutH`) |
|
||
| 3×`@` activation knock | `ActCharRcvd` |
|
||
| Password gate; wrong byte hangs (still draining the UART) | `CheckPassword` |
|
||
| Emergency erase on password `\0` + double-confirm — wipes flash, EEPROM and the config page | `EmergencyErase` |
|
||
| Device-info block (16 bytes) | `SendDeviceInfo` / `DEVICEINFO` |
|
||
| App-flash read/write (`f`/`F`), EEPROM read/write (`e`/`E`), config read/write (`c`/`C`) | `CheckCommands` |
|
||
|
||
## Assembled size (the bar)
|
||
|
||
Assembled for the ATmega328P with `avra`:
|
||
|
||
```
|
||
avra -I /usr/share/avra tsb-fixedbaud.asm # after uncommenting .include "m328Pdef.inc"
|
||
# Code : 250 words (500 bytes) — the whole loader, all features, in the 512 B section
|
||
```
|
||
|
||
**500 bytes with every feature** — the proof that ≤512 B and full feature parity
|
||
are simultaneously reachable. The port's four tiers reach it from the other
|
||
side, and the gradient between them is the cost of the mechanisms each is
|
||
allowed:
|
||
|
||
| tier | bytes | section | what it is allowed |
|
||
|---|---|---|---|
|
||
| oracle | 500 | 512 B | hand-written assembly, the reference |
|
||
| `tsb_asm` | 512 | 512 B | C++ on libavr, two routines in asm |
|
||
| `tsb_tricks` | 528 | 1 KB | no asm; global register variables |
|
||
| `tsb_policy` | 630 | 1 KB | pureboot's rules: no asm, no register variables |
|
||
| `tsb_pure` | 776 | 1 KB | idiomatic libavr throughout |
|
||
|
||
The two routines `tsb_asm` keeps are the ones whose remaining cost is the
|
||
calling convention itself: the bounded rx and the page-store loop. It fills
|
||
its section exactly, with the same one-bit-time turn-around guard the oracle
|
||
spends six bytes on - every tier implements the whole feature set, which is
|
||
what makes the column a gradient rather than four different loaders.
|
||
|
||
The oracle targets 20 MHz / 33333 baud; the port targets 16 MHz / 115200 baud
|
||
(what the simavr protocol test drives). Baud and geometry differ, code size and
|
||
feature set do not.
|