Method stepFrameCounter

nes/apu.go:140–168

mode 0: mode 1: function --------- ----------- ----------------------------- - - - f - - - - - IRQ (if bit 6 is clear) - l - l l - l - - Length counter and sweep e e e e e e e e - Envelope and linear counter

()

//  - l - l    l - l - -    Length counter and sweep
//  e e e e    e e e e -    Envelope and linear counter
func (apu *APU) stepFrameCounter() {
	switch apu.framePeriod {
	case 4:
		apu.frameValue = (apu.frameValue + 1) % 4
		switch apu.frameValue {
		case 0, 2:
			apu.stepEnvelope()
		case 1:
			apu.stepEnvelope()
			apu.stepSweep()
			apu.stepLength()
		case 3:
			apu.stepEnvelope()
			apu.stepSweep()
			apu.stepLength()
			apu.fireIRQ()
		}
	case 5:
		apu.frameValue = (apu.frameValue + 1) % 5
		switch apu.frameValue {
		case 0, 2:
			apu.stepEnvelope()
		case 1, 3:
			apu.stepEnvelope()
			apu.stepSweep()
			apu.stepLength()
		}
	}
}

func (apu *APU) stepTimer() {
	if apu.cycle%2 == 0 {