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[GBA] Disable part of SoundDriverVsyncOff() 

The official BIOS executes a copy from the stack pointer data to another
area in the internal working RAM. This is problematic to emulate because
our emulated BIOS does not touch the stack. In addition, the official
BIOS overwrites the first word in the stack with "0". While this is fine
with an official BIOS (the stack points to some internal BIOS data at
this point), this results in a user stack corruption with an emulated
BIOS.

Reproducing the BIOS behavior when doing this copy is near impossible
since the stack and CPU states are very different with the emulated
BIOS. Furthermore, the reason why this copy is done by the official BIOS
is unknown. The most likely explanation is that this a leftover from a
debug attempt and is not actually required for most intents and purposes
so this patch disables the copy.

This fixes Shrek games on trunk.
This commit is contained in:
Fabrice de Gans 2023-08-28 17:09:25 -07:00
parent 0f4ec57557
commit 586e24cc1a
1 changed files with 17 additions and 0 deletions
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17
src/gba/bios.cpp
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@ -1699,12 +1699,29 @@ void BIOS_SndDriverVSyncOff() // 0x1878
CPUWriteHalfWord(base1 + 0x12, 0);
CPUWriteByte(puser1 + 4, 0);
// Actual BIOS executes a copy by calling CpuSet() with the source being
// the stack pointer and the destination a location in memory that is
// an offset from `puser1`. `puser1` is pointing to an area in the
// internal work RAM at this point. In addition, actual BIOS overwrites
// the first word in the stack to be 0. With an actual BIOS, this is
// fine because the stack contains data used by the BIOS at this point,
// and no longer points to the user stack.
// With the emulated BIOS, the stack pointer still points to the user
// stack so overwriting the stack pointer to 0 results in user stack
// corruption and some software (like the Shrek games) fail to boot as
// a result.
// More broadly, why the BIOS writes 318 words from the stack to another
// location in memory is unknown. The `SndDriverVSyncOn()` does not
// attempt to restore this data, only re-writing the IO registers. This
// is why this code is disabled here.
#if 0
uint32_t r1 = puser1 + 0x350;
uint32_t r2 = 0x05000318;
uint32_t sp = reg[13].I; //0x03003c94;
CPUWriteMemory(sp, 0);
BIOS_SndDriver_b4c(sp, r1, r2);
#endif
user1 = CPUReadMemory(puser1); // 0x18aa
CPUWriteMemory(puser1, (--user1)); // this ret is common among funcs