$4000=APUPulse1DutyEnvelopeVolume
$4001=APUPulse1Sweep
$4002=APUPulse1TimerLo
$4003=APUPulse1TimerHiAndLengthLoad
$4004=APUPulse2DutyEnvelopeVolume
$4005=APUPulse2Sweep
$4006=APUPulse2TimerLo
$4007=APUPulse2TimerHiAndLength
$4008=APUTriangleLinearCounter
$4009=APUTriangleUnused
$400A=APUTriangleTimerLo
$400B=APUTriangleTimerHiAndLength
$400C=APUNoiseDutyEnvelopeVolume
$400D=APUNoiseUnused
$400E=APUNoiseLoop
$400F=APUNoiseLengthLoad
$4010=APUDMCLoopFrequencyIRQ
$4011=APUDMCLoadCounter
$4012=APUDMCSampleAddress
$4013=APUDMCSampleLength
$4015=APUControlStatus
$4017=APUFrameCounter