2A03
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 | Duty and volume for square wave 1 |
| $4001 | SQ1_SWEEP | Sweep control register for square wave 1 |
| $4002 | SQ1_LO | Low byte of period for square wave 1 |
| $4003 | SQ1_HI | High byte of period and length counter value for square wave 1 |
| $4004 | SQ2_VOL | Duty and volume for square wave 2 |
| $4005 | SQ2_SWEEP | Sweep control register for square wave 2 |
| $4006 | SQ2_LO | Low byte of period for square wave 2 |
| $4007 | SQ2_HI | High byte of period and length counter value for square wave 2 |
| $4008 | TRI_LINEAR | Triangle wave linear counter |
| $4009 | Unused, but is eventually accessed in memory-clearing loops | |
| $400A | TRI_LO | Low byte of period for triangle wave |
| $400B | TRI_HI | High byte of period and length counter value for triangle wave |
| $400C | NOISE_VOL | Volume for noise generator |
| $400D | Unused, but is eventually accessed in memory-clearing loops | |
| $400E | NOISE_LO | Period and waveform shape for noise generator |
| $400F | NOISE_HI | Length counter value for noise generator |
| $4010 | DMC_FREQ | Play mode and frequency for DMC samples |
| $4011 | DMC_RAW | 7-bit DAC |
| $4012 | DMC_START | Start of DMC waveform is at address $C000 + $40*$xx |
| $4013 | DMC_LEN | Length of DMC waveform is $10*$xx + 1 bytes (128*$xx + 8 samples) |
| $4014 | OAMDMA | Writing $xx copies 256 bytes by reading from $xx00-$xxFF and writing to OAMDATA ($2004) |
| $4015 | SND_CHN | Sound channels enable and status |
| $4016 | JOY1 | Joystick 1 data (R) and joystick strobe (W) |
| $4017 | JOY2 | Joystick 2 data (R) and frame counter control (W) |
| $4018-$401F | Functionality that was planned but later discarded | |
