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NES: Reimplement PAL forced OAM refresh according to new research #137

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Merged
Fiskbit merged 4 commits into from
May 24, 2026
+20 −14
Merged

NES: Reimplement PAL forced OAM refresh according to new research #137

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0709af2
NES: Reimplement PAL forced OAM refresh according to new research.
Fiskbit May 23, 2026
a8936a6
Update PR based on feedback.
Fiskbit May 24, 2026
e0ac5f1
NES: Dendy UM6561AF-2 has OAM corruption except on $2003 writes.
Fiskbit May 24, 2026
a39cf9a
Update PR based on more feedback.
Fiskbit May 24, 2026
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34 changes: 20 additions & 14 deletions Core/NES/NesPpu.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -454,17 +454,22 @@ template<class T> void NesPpu<T>::WriteRam(uint16_t addr, uint8_t value)
break;

case PpuRegisters::SpriteData:
if((_scanline >= 240 && (_region != ConsoleRegion::Pal || _scanline < _palSpriteEvalScanline)) || !IsRenderingEnabled()) {
//TODO: The $2003 increment can be eaten if it happens at the same time as another increment (from sprite processing or PAL's forced refresh).
//The increment appears to happen 1.5-2.5 dots after the write ends.
if(_scanline >= 240 || !IsRenderingEnabled()) {
if((_spriteRamAddr & 0x03) == 0x02) {
//"The three unimplemented bits of each sprite's byte 2 do not exist in the PPU and always read back as 0 on PPU revisions that allow reading PPU OAM through OAMDATA ($2004)"
value &= 0xE3;
}
WriteSpriteRam(_spriteRamAddr, value);
_emu->ProcessPpuWrite<CpuType::Nes>(_spriteRamAddr, value, MemoryType::NesSpriteRam);
_spriteRamAddr = (_spriteRamAddr + 1) & 0xFF;
if(_region == ConsoleRegion::Ntsc || _scanline < _palSpriteEvalScanline || ((_cycle & 1) && (_cycle != 339))) {
_spriteRamAddr++;
}
} else {
//"Writes to OAMDATA during rendering (on the pre-render line and the visible lines 0-239, provided either sprite or background rendering is enabled) do not modify values in OAM,
//but do perform a glitchy increment of OAMADDR, bumping only the high 6 bits"
//TODO: This should actually write the contents of the OAM buffer to the current OAM1 or OAM2 byte, 1.0-2.0 dots after the write ends.
_spriteRamAddr = (_spriteRamAddr + 4) & 0xFC;
_emu->BreakIfDebugging(CpuType::Nes, BreakSource::NesInvalidOamWrite);
}
Expand Down Expand Up @@ -1017,7 +1022,7 @@ template<class T> void NesPpu<T>::ProcessSpriteEvaluationEnd()

template<class T> void NesPpu<T>::ProcessSpriteEvaluation()
{
if(_prevRenderingEnabled || (_region == ConsoleRegion::Pal && _scanline >= _palSpriteEvalScanline)) {
if(_prevRenderingEnabled) {
if(_cycle < 65) {
//Clear secondary OAM at between cycle 1 and 64
_oamCopybuffer = 0xFF;
Expand Down Expand Up @@ -1301,14 +1306,15 @@ template<class T> void NesPpu<T>::Exec()
}
_preventVblFlag = false;
} else if(_region == ConsoleRegion::Pal && _scanline >= _palSpriteEvalScanline) {
//"On a PAL machine, because of its extended vertical blank, the PPU begins refreshing OAM roughly 21 scanlines after NMI[2], to prevent it
//from decaying during the longer hiatus of rendering. Additionally, it will continue to refresh during the visible portion of the screen
//even if rendering is disabled. Because of this, OAM DMA must be done near the beginning of vertical blank on PAL, and everywhere else
//it is liable to conflict with the refresh. Since the refresh can't be disabled like on the NTSC hardware, OAM decay does not occur at all on the PAL NES."
if(_cycle <= 256) {
ProcessSpriteEvaluation();
} else if(_cycle >= 257 && _cycle < 320) {
_spriteRamAddr = 0;
//"During scanlines 265-310, the PAL PPU does not perform sprite evaluation at all - instead, it simply increments the OAM address register
//every 2 pixels (at x=2, x=4, x=6, x=8, ..., x=334, x=336, x=338, and x=340, but not at x=0)."
//This code only runs for dots 1-340, so we don't need to check dot 0 explicitly.
if(!(_cycle & 1)) {
_spriteRamAddr++;

if(_enableOamDecay) {
_oamDecayCycles[_spriteRamAddr >> 3] = _console->GetCpu()->GetCycleCount();
}
}
}
} else {
Expand Down Expand Up @@ -1390,9 +1396,9 @@ template<class T> void NesPpu<T>::ProcessScanlineFirstCycle()

template<class T> void NesPpu<T>::CorruptOamRow(uint8_t sourceRow, uint8_t destRow)
{
if(sourceRow != destRow) {
uint8_t sourceByte = (sourceRow << 3) & 0xFF;
uint8_t destByte = (destRow << 3) & 0xFF;
if(sourceRow != destRow && _region != ConsoleRegion::Pal) {
uint8_t sourceByte = sourceRow << 3;
uint8_t destByte = destRow << 3;

memcpy(_spriteRam + destByte, _spriteRam + sourceByte, 8);
_secondarySpriteRam[destRow] = _secondarySpriteRam[sourceRow];
Expand Down