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pureboot 7: the same features in fewer words on every chip

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Four cuts, none touching what the loader can do. The entry stub stops
re-doing the reset logic's own SP write where the datasheet guarantees
RAMEND (stack::hardware — the classic megas keep theirs). The autobaud
unit moves into GPIOR2:GPIOR1 wherever the chip has the pair: one-word
accesses, no RAM object, and the host's measured-clock peek follows it
by version and geometry. 'J' rides the unified decode, carrying a
selector it ignores so its address is the same two reads as every other
command — the tool sends the bare form to older residents. run_app stops
insisting on a body of its own. The fleet lands at 358–410 B stock and
438–474 B autobaud; the tightest image in the space — the 1284s'
autobaud on a USART's own pins with the OSCCAL trim — drops from 510 to
484 of its 512. Every chip's suite is green on the wire that changed,
and the README's table is machine-checked against the built images.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-07-28 17:02:38 +02:00
parent a3ea099105
commit 8f6319c068
9 changed files with 164 additions and 103 deletions
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38
pureboot/pureboot.cpp
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@@ -75,7 +75,7 @@ static_assert(PUREBOOT_OSCCAL >= 0 && PUREBOOT_OSCCAL <= 0xff, "PUREBOOT_OSCCAL
// The loader's one identity number. The protocol carries none of its own —
// a version implies it, and the host tool holds that map (README.md).
constexpr std::uint8_t version = 6;
constexpr std::uint8_t version = 7;
// The image's identity stamp, for the host tool rather than for the wire: an
// update image is a bare 512-byte slot, and without this nothing in it says
@@ -241,7 +241,10 @@ struct software_link {
// every rate. Activation differs in kind from the other two — there is no
// clock to time a window against — so this backend brings its own, below.
struct autobaud_link {
using uart = avr::uart::software_autobaud<avr::PUREBOOT_RX, avr::PUREBOOT_TX>;
// The unit in GPIOR2:GPIOR1 where the chip has them: the loader owns the
// whole chip while it runs, and the pair is four instructions cheaper per
// session than a RAM word.
using uart = avr::uart::software_autobaud<avr::PUREBOOT_RX, avr::PUREBOOT_TX, avr::uart::unit_home::gpior>;
static void init()
{
@@ -288,7 +291,7 @@ extern "C" [[noreturn]] void pureboot_app();
__builtin_unreachable();
}
[[gnu::noinline, noreturn]] void run_app()
[[noreturn]] void run_app()
{
jump(pureboot_app);
}
@@ -488,12 +491,6 @@ void fill_page(std::uint8_t bank, std::uint16_t at)
tx_ack();
const std::uint8_t command = link::rx();
switch (command) {
case 'J': { // jump to a wire word address: hand-over and staging transfer
auto target = reinterpret_cast<void (*)()>(rx16());
tx_ack();
link::drain();
jump(target);
}
case 'b': // identity: the version, then the three signature bytes
// Straight out of the stamp, so the wire and the image can never
// disagree about what this loader is. The indices are constant and
@@ -502,18 +499,26 @@ void fill_page(std::uint8_t bank, std::uint16_t at)
for (std::uint8_t at = stamp_identity; at != sizeof identity_stamp; ++at)
link::tx(identity_stamp[at]);
break;
case 'J': // jump: sel8 (reserved), addr16 as a wire word address
case 'W': // fill one flash page buffer: sel8, addr16, then page bytes
case 'G': // read: sel8, addr16, n8 (0 = 256)
case 'g': { // write: sel8, addr16, n8, then n bytes, each acked
// One decode, one cursor and one loop for every space and both
// directions: a command per memory would carry a copy of all three
// each. 'W' joins the same decode rather than keeping an address
// form of its own, so flash addressing is uniform across every
// command that names it.
// One decode, one cursor and one loop for every space, both
// directions and the jump: a command per memory would carry a copy
// of all three each. 'J' — the hand-over and staging transfer —
// carries a selector it ignores so its address rides the same two
// reads as everything else; 'W' joins the same decode rather than
// keeping an address form of its own, so flash addressing is
// uniform across every command that names it.
const std::uint8_t selector = link::rx();
const std::uint8_t space = space_of(selector);
const std::uint8_t bank = bank_of(selector);
std::uint16_t at = rx16();
if (command == 'J') {
tx_ack();
link::drain();
jump(reinterpret_cast<void (*)()>(at));
}
if (command == 'W') {
fill_page(bank, at);
break;
@@ -540,4 +545,7 @@ void fill_page(std::uint8_t bank, std::uint16_t at)
} // namespace
} // namespace pureboot
template struct avr::startup::entry<pureboot::run>;
// stack::hardware: activation is reset-only, so the reset logic's own
// SP = RAMEND stands wherever the datasheet guarantees it (the classic
// megas still get the write); a 'J' entry runs on the caller's live stack.
template struct avr::startup::entry<pureboot::run, avr::startup::stack::hardware>;