PPU odd frames should be shorter than even frames.

Author: pherrymason

src/nes/ppu/ppu.go

@@ -170,6 +170,9 @@ func (ppu *P2c02) Tick() {
 			ppu.currentScanline++
 		}
 		ppu.renderCycle = 0
+		if ppu.shouldSkipFirstCycleOnOddFrame() {
+			ppu.renderCycle = 1
+		}
 	} else {
 		ppu.renderCycle++
 	}
@@ -181,6 +184,10 @@ func (ppu *P2c02) Tick() {
 	}
 }
 
+func (ppu *P2c02) shouldSkipFirstCycleOnOddFrame() bool {
+	return ppu.PpuMask.ShowBackground == 1 && ppu.scanlineIsVisibleOrIsPreRender() && ppu.evenFrame == false && ppu.currentScanline == 0 && ppu.renderCycle == 0
+}
+
 func (ppu *P2c02) incrementX() {
 	if ppu.PpuMask.renderingEnabled() {
 		if ppu.vRam.CoarseX() == 31 { // if CoarseX == 31

src/nes/ppu/ppu_test.go

@@ -26,6 +26,26 @@ func aPPU() *P2c02 {
 	return ppu
 }
 
+func TestPpu2c02_odd_frames_are_1_cycle_shorter_when_rendering_is_enabled(t *testing.T) {
+	ppu := aPPU()
+	ppu.PpuMask.ShowBackground = 1
+	for i := 0; i < (341 * (261 + 1)); i++ {
+		ppu.Tick()
+	}
+
+	assert.Equal(t, uint16(1), ppu.renderCycle, "Did not skip cycle 0")
+}
+
+func TestPpu2c02_odd_frames_are_last_341_cycles_when_rendering_is_disabled(t *testing.T) {
+	ppu := aPPU()
+	ppu.PpuMask.ShowBackground = 0
+	for i := 0; i < (341 * (261 + 1)); i++ {
+		ppu.Tick()
+	}
+
+	assert.Equal(t, uint16(0), ppu.renderCycle, "Should not skip cycle 0")
+}
+
 // Render cycles tests
 func TestPPU_Render_Cycles_should_increment_scanline_after_341_cycles(t *testing.T) {
 	ppu := aPPU()
@@ -40,6 +60,7 @@ func TestPPU_Render_Cycles_should_reset_scanline_after_261_scanlines(t *testing.
 	ppu := aPPU()
 	ppu.renderCycle = 340
 	ppu.currentScanline = 261
+	ppu.evenFrame = false
 
 	ppu.Tick()