CPU: Difference between revisions

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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.  
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 [http://en.wikipedia.org/wiki/Ricoh Ricoh] and lacks the MOS6502's decimal mode. In the NTSC NES, the [http://en.wikipedia.org/wiki/Ricoh_2A03 RP2A03] chip contains the CPU and APU; in the PAL NES, the CPU and APU are contained within the [http://en.wikipedia.org/wiki/Ricoh_2A03 RP2A07] chip.  


<noinclude>
== Sections ==
* [[6502 instructions|CPU instructions]]
* [[CPU addressing modes]]
* [[CPU memory map]]
* [[CPU power up state|CPU power-up state]]
* [[CPU registers]]
* [[CPU status flag behavior]]
* [[CPU interrupts]]
* [[CPU unofficial opcodes|Unofficial opcodes]]
* [[CPU pinout|CPU pin-out and signals]], and other [[hardware pinout|hardware pin-outs]]


* [[CPU_MEMORY_MAP|Memory map]]
== Frequencies ==
* [[CPU_PINOUT_AND_SIGNAL_DESCRIPTION|Pinout]]
The CPU generates its clock signal by dividing the master clock signal.
* [[CPU_POWERUP_STATE|Powerup state]]
{| class="tabular"
* [[CPU_STATUS_FLAG_BEHAVIOR|status flag behavior]]
|-
! Rate || NTSC NES/Famicom || PAL NES || Dendy
|-
| Color subcarrier frequency ''f{{sub|sc}}'' (exact) || 3579545.<span style="text-decoration:overline;">45</span> Hz (315/88 MHz) || 4433618.75 Hz || 4433618.75 Hz
|-
| Color subcarrier frequency ''f{{sub|sc}}'' (approx.) || 3.579545 MHz || 4.433619 MHz || 4.433619 MHz
|-
| Master clock frequency 6''f{{sub|sc}}'' || 21.477272 MHz || 26.601712 MHz || 26.601712 MHz
|-
| Clock divisor ''d'' || 12 || 16 || 15
|-
| CPU clock frequency 6''f{{sub|sc}}''/''d'' || 1.789773 MHz (~559 ns per cycle) || 1.662607 MHz (~601 ns per cycle) || 1.773448 MHz (~564 ns per cycle)
|}


</noinclude>
<nowiki>*</nowiki> 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.<ref>[https://forums.nesdev.org/viewtopic.php?f=3&t=17213#p216082 nesdev forum: Eugene.S provides a list of famiclones]</ref>


== Note ==  
== Notes ==
* The CPU's clock is obtained by dividing a 21.477272 MHz clock source by 12 (26.601712 MHz divided by 16 for PAL).
* All illegal 6502 opcodes execute identically on the 2A03/2A07.
* 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.
* Every cycle on 6502 is either a read or a write cycle.
* For the CPU instruction list, please refer to this [[CPU_INSTRUCTION_LIST|section]].
* A printer friendly version covering all section is available [[CPU ALL|here]].
* A printer friendly version covering all section is available [[CPU_ALL|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.<ref>[http://forums.nesdev.org/viewtopic.php?p=223679#p223679 nesdev forum: Mesen - NES Emulator]</ref>
 
== See also ==
* [[Cycle reference chart]]
* [http://nesdev.org/2A03%20technical%20reference.txt 2A03 technical reference] by Brad Taylor. (Pretty old at this point; information on the wiki might be more up-to-date.)
 
== References ==
<references/>

Latest revision as of 05:35, 11 January 2023

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.

Sections

Frequencies

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]

Notes

  • All illegal 6502 opcodes execute identically on the 2A03/2A07.
  • 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]

See also

References