The 2A03, short for RP2A03[G], is the common name of the NTSC NES CPU chip. It consists of a MOS Technology 6502 processor (lacking decimal mode) and audio generation logic. The 2A03 is used only in NTSC consoles; PAL systems are equipped with an RP2A07G chip, whose only major changes are a different clock divider (divide by 16 instead of 12) and adjusted PCM playback speeds.
2A03 register map
In addition to the registers in the 6502 core, the 2A03 contains 22 memory-mapped registers for sound generation (see NES APU), joystick reading, and OAM DMA transferring. Unlike the addresses of PPU registers and mapper registers, CPU register addresses are completely decoded, which means that the entire space from the end of CPU registers to the top of address space ($4020 through $FFFF) is available to the Game Pak.
The range $4018-$401F does nothing on a retail NES. It was intended for 2A03 functionality that never made it to production. Various revisions of the 2A03 include test registers (which are disabled in normal operation) or remnants of an incompletely implemented IRQ counter that was disconnected from the rest of the circuit.
|$4000||SQ1_VOL||Pulse 1||Duty cycle and volume||Open bus|
|$4001||SQ1_SWEEP||Sweep control register|
|$4002||SQ1_LO||Low byte of period|
|$4003||SQ1_HI||High byte of period and length counter value|
|$4004||SQ2_VOL||Pulse 2||Duty cycle and volume|
|$4005||SQ2_SWEEP||Sweep control register|
|$4006||SQ2_LO||Low byte of period|
|$4007||SQ2_HI||High byte of period and length counter value|
|$400A||TRI_LO||Low byte of period|
|$400B||TRI_HI||High byte of period and length counter value|
|$400E||NOISE_LO||Period and waveform shape|
|$400F||NOISE_HI||Length counter value|
|$4010||DMC_FREQ||DMC||IRQ flag, loop flag and frequency|
|$4012||DMC_START||Start address = $C000 + $40*$xx|
|$4013||DMC_LEN||Sample length = $10*$xx + 1 bytes (128*$xx + 8 samples)|
|$4014||OAMDMA||OAM DMA: Copy 256 bytes from $xx00-$xxFF into OAM via OAMDATA ($2004)|
|$4015||SND_CHN||Sound channels enable||Sound channel and IRQ status|
|$4016||JOY1||Joystick strobe||Joystick 1 data|
|$4017||JOY2||Frame counter control||Joystick 2 data|
|$4018-$401F||APU and I/O functionality that is normally disabled. See CPU Test Mode.|