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NMI: Prevent NMI being skipped when $4210 is read during the first dot of the NMI scanline (fixes issues in Terranigma)

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Sour 2020-01-06 19:00:18 -05:00
parent 4602ae331f
commit 24beded73d
1 changed files with 10 additions and 4 deletions
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14
Core/InternalRegisters.cpp
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@ -107,11 +107,17 @@ uint8_t InternalRegisters::Read(uint16_t addr)
{
switch(addr) {
case 0x4210: {
uint8_t value =
(_nmiFlag ? 0x80 : 0) |
0x02; //CPU revision
constexpr uint8_t cpuRevision = 0x02;
uint8_t value = (_nmiFlag ? 0x80 : 0) | cpuRevision;
//Reading $4210 on any cycle turns the NMI signal off (except presumably on the first PPU cycle (first 4 master clocks) of the NMI scanline.)
//i.e: reading $4210 at the same it gets set will return it as set, and will keep it set.
//Without this, Terranigma has corrupted sprites on some frames.
if(_memoryManager->GetHClock() >= 4 || _ppu->GetScanline() != _ppu->GetNmiScanline()) {
SetNmiFlag(false);
}
SetNmiFlag(false);
return value | (_memoryManager->GetOpenBus() & 0x70);
}