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The NES CPU core is based on the 6502 processor and runs at approximately 1.79 MHz (1.66 MHz in a PAL NES). It is made by Ricoh and lacks the MOS6502's decimal mode. In the NTSC NES, the RP2A03 chip contains the CPU and APU; in the PAL NES, the CPU and APU are contained within the RP2A07 chip.


CPU signals and frequencies

The CPU generates its clock signal by dividing the master clock signal.

Rate NTSC NES/Famicom PAL NES PAL Famiclone
Color subcarrier frequency fsc (exact) 39375000/11 Hz 4433618.75 Hz 4433618.75 Hz
Color subcarrier frequency fsc (approx.) 3.579545 MHz 4.433619 MHz 4.433619 MHz
Master clock frequency 6fsc 21.477272 MHz 26.601712 MHz 26.601712 MHz
Clock divisor d 12 16 15
CPU clock frequency 6fsc/d 1.789773 MHz 1.662607 MHz 1.773448 MHz


  • Kevtris has confirmed that all illegal 6502 opcodes execute identically on the 2A03/2A07. He has even went as far as to integrate them into the CopyNES BIOS.
  • A printer friendly version covering all section is available here.