bootloader/pureboot/README.md

# pureboot

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 — all 37 —
fitting each chip's smallest boot sector**: 512 bytes everywhere — 488 B on
the tiny13s, 498–502 B on the tiny25/45/85, 466–504 B across the megas
(474 B on the boot-section-less m48s, 498 B on the 644s) — except the
ATmega1284/1284P, whose smallest boot sector is 1 KiB and whose far-flash
machinery (ELPM reads, RAMPZ page commands, word-addressed wire) lands at
558 B in a 1 KiB slot: the 512-byte figure is a hardware boundary those
chips simply do not have, and no implementation of this feature set fits it
there. The device speaks primitives; every composite — verify, erase,
reset-vector surgery, updating the loader itself — lives in the host tool
(`pureboot.py`).

The image is **position-independent**: control flow is PC-relative, the
read/write paths take wire addresses, the write guard protects the slot the
code is *running* in (from the runtime return address), the info block is
addressed from that same anchor, and the application jump is an indirect
call to an absolute entry. The identical binary therefore runs from any
slot with every command intact — which makes pureboot **its own staging
loader**: the host installs the same binary one slot below the resident,
jumps into it, and lets it rewrite the resident. The slot is 512 bytes
(1 KiB on the word-addressed large chips, matching their boot-sector
minimum); on the tinies the budget is 510, not 512: a slot's last word
belongs to the host-managed trampoline (below).

## Link

| Chip | Serial | Baud | Clock assumed |
|---|---|---|---|
| every ATmega | the hardware USART (USART0), RXD/TXD per pinout | 115200 8N1 | 16 MHz crystal |
| ATtiny25/45/85 | software UART, RX = PB0, TX = PB1 | 57600 8N1 | 8 MHz internal RC |
| ATtiny13/13A | software UART, RX = PB0, TX = PB1 | 57600 8N1 | 9.6 MHz internal RC |

The tiny RX pin has its pull-up enabled; TX idles high. All multi-byte
quantities on the wire are little-endian.

## Activation

Reset enters the loader (BOOTRST on the boot-sectioned megas; the patched
reset vector on the tinies and the boot-section-less m48s) — except a
watchdog reset, which hands straight to the application (the application
owns its watchdog; it must clear WDRF itself, which also releases the
WDRF-forced WDE).

The host then has one activation window per awaited byte to knock: `p` then
`b`. Each awaited byte gets a fresh window; any other byte is discarded and
awaited again (line noise cannot lock the loader, only delay it). A window
expiring with an idle line boots the application.

The window length is a compile-time constant — 8 s by default, another value
via the `PUREBOOT_TIMEOUT` CMake cache variable — so the whole EEPROM belongs
to the application; pureboot never uses it for its own state. Re-timing a
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
also the completion ack of the previous command. A session is: await `+`,
send a command, read its 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 and stays
byte-addressed). EEPROM addresses are always bytes, counts always bytes.

| 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) |
| `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 |
| `J` | word address (16-bit) | `+`, then execution continues there |
| 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
512-byte slot the loader is *running* in are drained but never programmed — a
broken host cannot brick the running copy, and a staged copy may rewrite the
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
application (word 0 on the mega, the trampoline word on the tinies — both
known from the info block) and to move between loader copies during a
self-update. A jump to a loader slot's base re-enters that copy's own
startup; it must then be knocked afresh.

The info block (`b`):

| Offset | Content |
|---|---|
| 0–2 | `'P'`, `'B'`, protocol version (1) |
| 3–5 | device signature |
| 6 | SPM page size in bytes (0 means 256) |
| 7–8 | loader base — application flash ends here (a word address when bit 1 is set) |
| 9–10 | EEPROM size |
| 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 =
write `0xff` (per page for flash, per byte for EEPROM).

## Deployment

The build leaves three artifacts per chip. The ELF is a container for the
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 with no addressing, which a programmer would put at address 0.
On a boot-sectioned mega a copy at 0 is dead weight (SPM only executes
from the boot section, so it cannot even heal itself — reflash the .hex);
on the patched-vector chips it *runs* (the image is position-independent
and reset enters word 0), reports its canonical geometry, and the ordinary
`--update-loader` flow re-homes a build into the top slot from any
position — the staging install and the word-0 redirect execute from
copies outside page 0's slot, and a copy sitting in the staging slot
itself is recognized as the installed staging copy and left in place (it
streams the new resident like any staged copy, so an older build installs
a newer one). `pureboot.rehome` is the acceptance test for both
positions. Flashing the application afterwards overwrites the stale copy,
vector surgery included.

**Boot-sectioned megas**: program the loader at `flash − slot` with an
external programmer. Every such mega has a BOOTSZ step whose boot section
is exactly the loader slot — 512 B, the second-smallest step on the 8 KiB
and 16 KiB chips (m8, m88, m16, m168, m164), the smallest on the 32 KiB
ones (m32, m328, m324); on the 1284s that step is the smallest, 512 words,
which is why their slot is 1 KiB — so the ATmega328P profiles below apply
to every one of them with its own addresses and slot size; the per-chip
BOOTSZ ladders live in the host tool (`BOOT_FUSE`). The 1284s' numbers:
standalone = BOOTSZ 512 words (reset at the loader base 0x1fc00);
self-update = 1024 words, covering both 1 KiB slots, the loader-first
reset landing at 0x1f800 — the staging slot, walked across when erased.

The **644s** are the geometry's sweet spot: their smallest boot section
(512 words = 1 KiB) is exactly *two* 512-byte 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 at
0xfc00, one erased slot below the loader: the loader-first walk built in).

ATmega328P profiles (addresses for its 32 KiB):

| BOOTSZ | BOOTRST | Behavior |
|---|---|---|
| 256 words (512 B) | programmed | *Standalone*: reset always enters the loader; **self-update impossible** (the staging slot lies outside the boot section, where SPM is disabled). |
| 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
reset vector, and the hand-over jumps to 0.

**Patched-vector chips — the tinies and the m48s** (no boot section; the
m48s' SPM runs from the entire flash, Atmel-8271 §26): program the loader
at `flash − 512`; erased flash below it walks up into the loader, so a
virgin chip activates. When flashing an application the host performs
reset-vector surgery: word 0 is rewritten to `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 *first*, so from the first write on an interrupted flash
still resets into the loader; an erase runs top-down for the same reason.
The m48s speak this profile over their hardware USART — no fuse preflight,
BOOTRST does not exist there.

## 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
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
links the loader at its base inside an otherwise blank flash image:

The preflight refuses an image built for another chip: the info block
embedded in every pureboot binary (signature, page size, loader base,
EEPROM size, flags) must match the device's own, and the error names both.
Die revisions share their base signature and geometry, so their images are
interchangeable — as the silicon is. `loader_image()` also accepts a
padded image (a raw .bin padded from 0, or a whole-flash read-back with
the loader resident) and peels it to the slot content by the embedded base.

1. The staging slot `[base−slot, base)` is saved to a host-side state file
   (on the 1 KB tiny13s that is the whole application, vectors included).
2. The resident installs the identical update image there. On the
   patched-vector chips the host composes the slot's last word — the same
   address as the resident's trampoline — as a jump to the resident base,
   so even an abandoned staging copy times out into a loader, never into
   garbage. A loader already sitting whole in the staging slot (its info
   block in place, the slot unchanged since the update began) 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. On the
   patched-vector chips whose staging slot sits away from page 0 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
   discarded.

Every phase is idempotent and keyed off the actual flash state: re-running
the same command after any interruption resumes and completes. The state
file carries the only bytes not recoverable from the device; if it is lost
mid-update the update still completes, and the staging region is restored by
reflashing the application. A boot-sectioned mega needs its fuses for the
preflight (BOOTSZ gate, profile notes) — read from the device, or supplied
with `--assume-fuses` where reading is impossible (simulators); the
patched-vector chips need none.

## 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
well as a simavr pty), the Win32 serial API through `ctypes` drives a COM
port on Windows (`--port COM6`; the `\\.\` form for two-digit ports is
supplied by the tool). Opening the port asserts DTR and RTS on both, so a
board that wires DTR to reset gets its reset pulse and opens the activation
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 /
read / verify) — then the loader hands over to the application; `--stay`
keeps the session alive instead, and a later invocation reconnects into it
(the knock converges there too). `--flash` and `--eeprom` verify by
read-back unless `--no-verify`; images are raw binary, or Intel HEX by
extension. `--force` overrides the refusable safety checks (today: flashing
application data into a mega's reset walk region).

## Tests

Per chip preset, `ctest` runs:

- `pureboot.size` — the 510-byte (tinies) / 512-byte (mega) budget;
- `pureboot.pi` — the position-independence lint: no absolute `jmp`/`call`
  in the image, the info block within its 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, and the update preflight's error/warning matrix over
  synthetic fuse bytes;
- `pureboot.protocol` — end to end against a simavr device
  (`test/pureboot_device.c` — the mega's USART as a pty; on the tinies a
  cycle-timed GPIO⇄pty bridge for the software UART, plus the SPM/NVM module
  simavr's tiny cores lack) driven by the real host tool through
  knock-from-reset, program + verify of both memories, session reconnect, an
  external reset through the patched vector, and the hand-over to a fixture
  application whose banner proves the launch — cross-checked against the
  simulator's ground-truth memory dumps and an independent decode of the
  surgery's rjmp words;
- `pureboot.reloc` — the identical image installed one slot below the
  resident serves the complete command set from there (the
  position-independence acceptance test);
- `pureboot.update` — the full `--update-loader` flow to a re-timed build,
  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 throughout.

`size`, `pi`, and `planner` are host logic and run anywhere; the three
simulator-driven targets need simavr and a pty, so they are POSIX-only —
on Windows the tool is exercised against real hardware.