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

See also

• NES CPU