2A03

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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 transfering. 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 ($4018 through $FFFF) is available to the Game Pak.

$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 Not used, but it's 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 Not used, but it's 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 OAM_DMA Writing $xx copies 256 bytes by reading from $xx00-$xxFF and writing to $2004 (OAM data)
$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)

See also

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