CPU
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 instructions
- CPU addressing modes
- CPU memory map
- CPU power-up state
- CPU registers
- CPU status flag behavior
- CPU interrupts
- Unofficial opcodes
- CPU pin-out and signals, and other hardware pin-outs
The CPU generates its clock signal by dividing the master clock signal.
| Rate | NTSC NES/Famicom | PAL NES | Dendy |
|---|---|---|---|
| Color subcarrier frequency fsc (exact) | 3579545.45 Hz (315/88 MHz) | 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 (~559 ns per cycle) | 1.662607 MHz (~601 ns per cycle) | 1.773448 MHz (~564 ns per cycle) |
* The vast majority of PAL famiclones use a chipset or NOAC with this timing. A small number have UMC UA6540+6541, which also uses PAL NES timing.[1]
- There are no differences in the execution of illegal 6502 opcodes between the 2A03 and the 2A07.
- Every cycle on 6502 is either a read or a write cycle.
- A printer friendly version covering all section is available here.
- Emulator authors may wish to emulate the NTSC NES/Famicom CPU at 21441960 Hz ((341×262−0.5)×4×60) to ensure a synchronised/stable 60 frames per second.[2]
- Cycle reference chart
- 2A03 technical reference by Brad Taylor. (Pretty old at this point; information on the wiki might be more up-to-date.)