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https://github.com/RetroPortingToolKit/psxrecomp
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Matthew Stanley
3d364091e7
This is the bug. Single line. Four days lost.
# The fuck-up
`runtime/src/sio.c` returned `mc_flag` (init 0x08) on every 0x52 (read
command) but NEVER cleared it. Only writes cleared the flag.
Per no$psx spec: real PSX cards return FLAG=0x08 ONCE — first read
after fresh-battery / power-cycle / write — then clear it to 0x00 for
all subsequent reads.
# Why this stalled directory loading
BIOS state-3 chain handler (func_0000579C / 0xBFC152DC) tests bit 3:
andi $t1, $a0, 0x8
beq $t1, $0, BFC15650 # bit 3 NOT set: success (v0=0, advance)
# bit 3 SET: "fresh card" (v0=-1)
Our card returned 0x08 forever, so EVERY read attempt landed in the
"fresh card" path: state-3 returned v0=-1, dispatcher's $v0 cascade
(BFC1499C) reset the chain counter, infinite loop. The directory
literally could not be read.
Beetle's card sim returns 0x00 in steady-state. That was the
canonical first divergence per recomp-template/NES/PRINCIPLES.md #3.
# Why we couldn't find it for four days
We didn't have a transaction-aligned SIO byte diff. The existing
sio_trace dumps were timing-dependent and didn't survive the
press-X window. We were chasing symptoms (dispatch misses, missing
state-12/13 seeds, chain counter reset paths, RAM divergences at
0x74F8 / 0x7560) — all real but all DOWNSTREAM of this one byte.
Per principle "BUILD TOOLS, NOT GUESSWORK" we built
`sio_first_divergence` (transaction-aligned ours-vs-Beetle byte diff,
splits each ring at 0x81 select, compares same-position TX/RX),
which immediately reported byte 1 of the read transaction:
ours: tx=0x52 rx=0x08
beetle: tx=0x52 rx=0x00
Two-line fix in mc_process_byte's MC_CMD case: clear `mc_flag = 0x00`
after returning it.
# Measured impact
Before:
- chain_trace state-3 returned v0=-1 every cycle
- 0x7514 counter cycled 0->1->2->3->4 then reset (BFC1499C path)
- mc_max_state stuck at 10 with mc_aborts ~30
- Both runtimes stuck at MAIN MENU shell selector
After:
- chain_trace states 1..11 all return v0=0, run 14+ times each
- counter no longer resets at state-3
- Both runtimes navigate to MEMORY CARD screen (matching layout)
- byte-1 SIO RX divergence eliminated (both return 0x00)
Memory cards still don't show save data (state-12/13 read-data
handlers don't fire because outer counter never reaches >=5 to enter
the read-chain path at BFC1588C). That's a separate downstream
divergence which the new diff tool will pinpoint next.
# Other artifacts in this commit
- recompiler/seeds/phase2_ghidra_seeds.json: added state-12 (0x5A58)
and state-13 (0x5AB0) chain entries from no$psx-spec read-data
handler ranges. Necessary for state-12/13 to dispatch when the
outer chain finally enters the read-chain path. Dispatch table
4295 -> 4297 entries, no dispatch misses.
- runtime/src/psx_oracle_cmds.c: new sio_first_divergence TCP cmd —
splits each side's SIO ring into card-protocol transactions
(0x81-start, 0x01 or next 0x81 ends), compares the most recent K
transactions byte-by-byte to find first (matching TX, differing RX)
or transaction-length mismatch.
- runtime/src/debug_server.c: chain_trace ring buffer + mc_status
TCP cmd for chain-state visibility.
- runtime/src/memcard.c + memcard.h: memcard_debug_info() accessor.
- runtime/include/sio.h + sio.c: MC_READ_MSB_ECHO/LSB_ECHO states
per no$psx address-handshake spec.
- generated/SCPH1001_*.c: regen with seed additions.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>