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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.

Address Name Write Read
$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
$4008 TRI_LINEAR Triangle Linear counter
$4009 Unused
$400A TRI_LO Low byte of period
$400B TRI_HI High byte of period and length counter value
$400C NOISE_VOL Noise Volume
$400D Unused
$400E NOISE_LO Period and waveform shape
$400F NOISE_HI Length counter value
$4010 DMC_FREQ DMC IRQ flag, loop flag and frequency
$4011 DMC_RAW 7-bit DAC
$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.

See also