BlackMark
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R/r/w/F collapse into G and g over a selector byte naming the space — flash,
EEPROM, data, fuse, SPM — with the flash bank in its high nibble. Four command
bodies, four transfer loops and four argument decodes become one of each, and
W joins the same decode instead of keeping an address form of its own. The
loader shrinks while gaining everything below: on the 1284P the stock build
goes 480 -> 432 B and the software one 496 -> 450.
What the freed space buys:
- Data space. On AVR one pointer spans SRAM, the register file and the whole
I/O space, so G over space 2 reads all three. pureboot keeps zero static
RAM and pushes no register, so at loader entry an application's SRAM is
still what the application left there — this is a post-mortem, not just a
poke hole. As its own command it needed a dispatch arm and a loop; as one
more space it is a single ld/st.
- Host-issued SPM. W fills the page buffer and stops; erase, write and RWW
re-enable are writes to space 4, which reach the same fused store-and-SPM
pair through the transfer's own address and data. Any SPM operation, lock
bits included, is now reachable and the loader carries no page-commit logic.
The four-cycle SPMCSR-to-SPM window is why that primitive stays fused: no
host can hit it across a serial link, and that — not the byte count — is
the floor on how low-level a bootloader's primitives can go.
- Byte addresses everywhere. The bank in the selector retires the
word-addressed wire the >64 KiB parts needed, so the 1284s stop being the
outlier.
SERIAL autobaud is a third backend on the same loader, over libavr's
software_autobaud: no clock, no baud, one binary per chip for every F_CPU and
every rate. Activation counts poll iterations rather than seconds and bounds
every wait, so a stray pulse cannot hold an unattended device.
b answers with the version and signature only; the host derives geometry from
the signature, which is what an autobaud build requires anyway. An update image
is a bare slot with no device to ask, so every image carries a six-byte stamp —
the same bytes b answers with, and the source of both — that the loader never
reads from flash and the host refuses to install a mismatch against. The
running-slot write guard moved onto the SPM commit, which covers erase and
write both where guarding W covered neither directly.
The position-independence lint now proves the property instead of a proxy for
it: the image must come out byte-identical linked at a different base.
-fno-move-loop-invariants left the tuned flag set — it was fitted to a command
loop carrying four transfer bodies and costs bytes now that it carries one.
Verified: the exhaustive matrix on all 37 chips (every clock x every baud x
every backend, non-standard rates included, plus the autobaud build) —
8174 size checks, no failures, tightest fit the 1284s' autobaud at 510 of 512.
Behavioral suites green on every chip class: t13a 10/10, t85 11/11, m8 13/13,
m16a 13/13, m48pa 13/13, 328P 23/23, 644A 17/17, 1284P 17/17. Data-space
round trip through --peek/--poke and the autobaud handshake are both red-green
proven.
The two prototype sources and their findings file go; the README carries the
protocol and dev/done.md in libavr carries the reasoning.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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