private void IncrementOamAddrAndCheckOverflow(int increment)
{
byte temp = (byte)(this.oamAddr + increment);
if (temp < this.oamAddr)
{
this.currentState = SpriteEvaluationState.EvaluationComplete;
}
else
{
this.oamAddr = temp;
}
}
private void DoSpriteEvaluation()
{
if (this.cycle == 0)
{
this.currentState = SpriteEvaluationState.EvaluateYCoord;
this.spritesFound = 0;
this.spriteZeroOnNextScanline = false;
}
else if (this.cycle <= 64)
{
this.overrideOamRead = true;
if ((this.cycle & 0x01) == 0x01)
{
// Odd cycle - write 0xFF to secondary OAM
this.secondaryOam[this.cycle >> 1] = 0xFF;
}
}
else if (this.cycle <= 256)
{
this.overrideOamRead = false;
if ((this.cycle & 0x01) == 0x01)
{
// Odd cycle - read data for the current sprite from primary OAM
// NOTE: This assumes OAMADDR was reset to 0 as expected during ticks 257-320 of the previous
// scanline. If game code modifies OAMADDR afterwards, lots of weird stuff can happen.
this.spriteEvalTemp = this.primaryOam[this.oamAddr];
}
else
{
// Even cycle - write to secondary OAM
switch (this.currentState)
{
case SpriteEvaluationState.EvaluateYCoord:
this.secondaryOam[this.spritesFound * 4] = spriteEvalTemp;
// If the Y coordinate of the sprite is in range, we need to copy the rest of its data
// from primary OAM. Ignore results on line 239, as no sprites are shown on line 0.
if (this.scanline >= this.spriteEvalTemp && this.scanline < (this.spriteEvalTemp + (this.tallSprites ? 16 : 8)) && this.scanline != 239)
{
if (!this.spriteZeroOnNextScanline)
{
this.spriteZeroOnNextScanline = this.cycle == 66;
}
currentState = SpriteEvaluationState.CopyTileIndex;
this.oamAddr++;
}
else
{
// Sprite was not in range, check the next one
this.IncrementOamAddrAndCheckOverflow(4);
}
break;
case SpriteEvaluationState.CopyTileIndex:
this.secondaryOam[(this.spritesFound * 4) + 1] = spriteEvalTemp;
this.currentState = SpriteEvaluationState.CopyAttributes;
this.IncrementOamAddrAndCheckOverflow(1);
break;
case SpriteEvaluationState.CopyAttributes:
this.secondaryOam[(this.spritesFound * 4) + 2] = spriteEvalTemp;
this.currentState = SpriteEvaluationState.CopyXCoord;
this.IncrementOamAddrAndCheckOverflow(1);
break;
case SpriteEvaluationState.CopyXCoord:
this.secondaryOam[(this.spritesFound * 4) + 3] = spriteEvalTemp;
this.spritesFound++;
if (this.spritesFound == 8)
{
// Found 8 sprites for this line - continue processing to check for overflow
this.currentState = SpriteEvaluationState.OverflowCheck;
}
else
{
// Look for another sprite on this line
this.currentState = SpriteEvaluationState.EvaluateYCoord;
}
this.IncrementOamAddrAndCheckOverflow(1);
break;
case SpriteEvaluationState.OverflowCheck:
if (this.scanline >= this.spriteEvalTemp && this.scanline < (this.spriteEvalTemp + 8))
{
// Found another sprite on a full scanline
this.spriteOverflow = true;
this.IncrementOamAddrAndCheckOverflow(4);
}
else
{
// Sprite isn't on this scanline, check the next one
// 2C02 BUG: The OAM address is incremented by 5 in this case, so the Y coordinate
// evaluated for further sprites won't actually be the Y coordinate
this.IncrementOamAddrAndCheckOverflow(5);
}
break;
case SpriteEvaluationState.EvaluationComplete:
// Do nothing
break;
}
}
}
else if (this.cycle <= 320)
{
this.oamAddr = 0;
}
}