APU

I am still polishing this reference. I've gotten most of the main work done. -- Blargg

The NES APU is the audio processing unit in the NES console which generates sound for games. It is implemented in the RP2A03 (NTSC) and RP2A07 (PAL) chips. Its registers are mapped in the range $4000-$4017.

For clarity, this reference describes the abstract operation of the APU, not the exact hardware implementation. As long as the behavior is documented, the exact kind of counter or shift register used to implement a particular behavior is essentially irrelevant; in any case, the exact hardware implementation can only be speculated on without access to very expensive chip-level reverse-engineering tools.

Registers	Channel	Units
$4000-$4003	Pulse 1	Timer, Length Counter, Envelope, Sweep
$4004-$4007	Pulse 2	Timer, Length Counter, Envelope, Sweep
$4008-$400B	Triangle	Timer, Length Counter, linear counter
$400C-$400F	Noise	Timer, Length Counter, Envelope, shift register w/ feedback
$4010-$4013	DMC	Timer, memory reader, sample buffer, output unit
$4015	All	Length Counter enable and status
$4017	All	Update counter

Each channel has a variable-rate timer clocking a waveform generator, and various modulators driven by low-frequency clocks from the update counter. The DMC plays samples while the other channels play waveforms. Each sub-unit of a channel generally runs independently and in parallel to other units, and modification of a channel's parameter usually affects only one sub-unit and doesn't take effect until that unit's next internal cycle begins.

The read/write status register allows channels to be enabled and disabled, and their current length counter status to be queried.

The outputs from all the channels are combined using a non-linear mixing scheme.

The NES APU is also documented in Brad Taylor's 2A03 Technical Reference.

(These web pages are based on Blargg's NES APU Reference.)