Method Step

nes/ppu.go:676–744

Step executes a single PPU cycle

()

// Step executes a single PPU cycle
func (ppu *PPU) Step() {
	ppu.tick()

	renderingEnabled := ppu.flagShowBackground != 0 || ppu.flagShowSprites != 0
	preLine := ppu.ScanLine == 261
	visibleLine := ppu.ScanLine < 240
	// postLine := ppu.ScanLine == 240
	renderLine := preLine || visibleLine
	preFetchCycle := ppu.Cycle >= 321 && ppu.Cycle <= 336
	visibleCycle := ppu.Cycle >= 1 && ppu.Cycle <= 256
	fetchCycle := preFetchCycle || visibleCycle

	// background logic
	if renderingEnabled {
		if visibleLine && visibleCycle {
			ppu.renderPixel()
		}
		if renderLine && fetchCycle {
			ppu.tileData <<= 4
			switch ppu.Cycle % 8 {
			case 1:
				ppu.fetchNameTableByte()
			case 3:
				ppu.fetchAttributeTableByte()
			case 5:
				ppu.fetchLowTileByte()
			case 7:
				ppu.fetchHighTileByte()
			case 0:
				ppu.storeTileData()
			}
		}
		if preLine && ppu.Cycle >= 280 && ppu.Cycle <= 304 {
			ppu.copyY()
		}
		if renderLine {
			if fetchCycle && ppu.Cycle%8 == 0 {
				ppu.incrementX()
			}
			if ppu.Cycle == 256 {
				ppu.incrementY()
			}
			if ppu.Cycle == 257 {
				ppu.copyX()
			}
		}
	}

	// sprite logic
	if renderingEnabled {
		if ppu.Cycle == 257 {
			if visibleLine {
				ppu.evaluateSprites()
			} else {
				ppu.spriteCount = 0
			}
		}
	}