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Matthew Stanley
2aeb4db4e5
Tomba's `ResetGraph` (FUN_8005E694 at 0x8005E694) calls
`FUN_80061620` to autodetect the GPU video mode. That function
writes `GP1(0x10000007)` ("Get GPU info", subcommand 7 = GPU
version) and reads back GPUREAD; if the low 24 bits equal 2 it
returns 3 (the byte stored at `0x80090C9C`).
Our `gp1_get_info` was treating subcommand 7 as "leave latch
unchanged" (a misleading DuckStation-derived note in the source).
So GPUREAD held stale junk, the `== 2` check failed, and
`0x80090C9C` ended up 0 instead of 3 — landing the OPTIONS draw
code on a no-draw config table entry that produces 11 GP0 commands
per frame with zero draw primitives.
Mednafen-psx (`beetle-psx/mednafen/psx/gpu.cpp:1212`) returns 2
hardcoded for subcommand 7 — that's the GPU-version constant real
hardware returns. We now do the same, plus subcommand 8 = 0 (also
matches mednafen), and mask the subcommand with `& 0x0F` instead of
`& 0x07` to match mednafen's decode.
Verified: runtime's `0x80090C9C` now reads `0x03` at boot,
matching beetle. OPTIONS UI is still black so more downstream
divergences remain — but this is a real root cause and a step
forward.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>