43 lines
2.2 KiB
Markdown
43 lines
2.2 KiB
Markdown
# RIO board firmware
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- **`RIOv4_2.bin`** — RIO cockpit I/O board firmware **v4.2**, dumped
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2026-07-04 from one of our own boards' EPROM: an **AMD AM27C512-150**
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(64K x 8 UV EPROM, 150ns — the image fills it exactly).
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sha256 `60a88718835c654b6135dbec7721c40ef99dca07df2ad4b57eedeb24037a5f73`.
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For the eventual patched burn: a pin-compatible Winbond W27C512
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(electrically erasable, TL866-friendly) drops straight into the socket;
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the original AMD chip gets labeled and preserved unmodified.
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## First-look analysis (from the image alone, confirmed on hardware)
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- MCU: **Toshiba TMP68HC11** (read off the chip; the code fingerprint
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agrees — 6800-family opcodes with writes into the 68HC11 internal
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register block at `$10xx`).
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- Memory map: image is FF up to **0xC000**; 16KB of code occupies
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`$C000-$FFFF` (EPROM mapped at the top of the HC11 address space).
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- Startup at `$C000`: `SEI; LDS #$8000; STAA $1024 (TMSK2);
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STAA $1022 (TMSK1); ...` then a long `JSR` init chain — textbook HC11
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bring-up.
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- Vector table (`$FFD6-$FFFF`, big-endian):
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- `$FFFE` RESET → `$C000`
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- **`$FFD6` SCI (serial) → `$D630`** — the entry point of the board's
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receive/protocol interrupt handler. The suspected board-side
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DISABLE_AND_DIE-style wedge (see RIO-NOTES.md: the board mirrors the
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game's PCSPAK state machine, and mash-stress leaves the reply path
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dead while the button/event path stays alive) is reachable from here.
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- `$FFE4` → `$C1B2`, `$FFE6` → `$C18E` (timer output-compares); most
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other vectors → `$DB07..$DB3D` stubs.
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## Why this exists
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The remaining RIO reliability issue is board-side: under button-mash
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stress the board's reply/analog state machine wedges (RX dead, TX alive;
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a button press or power cycle revives it), reproduced identically on two
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different USB serial adapters. The game-side half of the protocol was
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binary-patched for tolerance (BTL4OPT patches v2-v4); the board firmware
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is the other half. Plan (RIO-NOTES.md "Board firmware patch plan"):
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disassemble as 68HC11 from `$C000` with the vector entries as roots, find
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the SCI state machine (protocol constants FC=ACK FD=NAK FE=RESTART
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FF=IDLE, idle-reload-4 patterns), patch the early-ACK/error wedge path or
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widen its window, burn a new EPROM, keep this original safe.
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