Cycle reference chart

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Clock rates

The clock rate of various components in the NES differs between consoles in the USA and Europe due to the different television standards used (NTSC M vs. PAL B). The color encoding method used by the NES (see NTSC video) requires that the master clock frequency be six times that of the color subcarrier, but this frequency is about 24% higher on PAL than on NTSC. In addition, PAL has more scanlines per field and fewer fields per second than NTSC. Furthermore, the PAL CPU's master clock could have been divided by 15 to preserve the ratio between CPU and PPU speeds, but Nintendo chose to keep the Johnson counter structure, which always has an even period, and divide by 16 instead.

So the main differences between the NTSC and PAL PPUs are depicted in the following table:

Property NTSC (2C02) PAL (2C07) "Dendy" PAL Famiclone RGB (2C03/4/5) Brazil Famiclone Argentina Famiclone
Master clock speed 21.477272 MHz ± 40 Hz
236.25 MHz ÷ 11 by definition 		26.601712 MHz ± 50 Hz
26.6017125 MHz by definition 		Like PAL Like NTSC 21.453671 MHz
3.067875 GHz ÷ 143 by definition 		21.492338 MHz
42.984675 MHz ÷ 2 by definition
CPU Ricoh 2A03 Ricoh 2A07 UMC UA6527P Ricoh 2A03 UMC UA6527
CPU clock speed 21.47~ MHz ÷ 12 = 1.789773 MHz 26.60~ MHz ÷ 16 = 1.662607 MHz 26.60~ MHz ÷ 15 = 1.773448 MHz Like NTSC 21.45~ MHz ÷ 12 = 1.787806 MHz 21.49~ MHz ÷ 12 = 1.791028 MHz
APU Frame Counter rate 60 Hz 50 Hz[1] 59 Hz[2] Like NTSC Like NTSC
PPU Ricoh 2C02 Ricoh 2C07 UMC UA6538 Ricoh 2C03, 2C04, 2C05 UMC UA6548 UMC UA6528P
Master clocks per PPU dot 4 5 5 4 4
PPU dots per CPU cycle 3 3.2 Like NTSC
CPU cycles per scanline 341 × 4÷12 = 1132⁄3 341 × 5÷16 = 1069⁄16 341 × 5÷15 = 1132⁄3 Like NTSC
Height of picture 240 scanlines 239 scanlines Like PAL Like NTSC thought to be like NTSC thought to be like PAL
Nominal visible picture height (see Overscan) 224 scanlines 268 scanlines Like PAL Like NTSC Like NTSC Like PAL
"Post-render" blanking lines between end of picture and NMI 1 scanline 1 scanline 51 scanlines 1 scanline 1 scanline 51 scanlines
Length of vertical blanking after NMI 20 scanlines (≈ 2273 CPU cycles) 70 scanlines (≈ 7459 CPU cycles) Like NTSC
Time during which OAM can be written Vertical or forced blanking Only during first 24 scanlines after NMI (≈2557 CPU cycles) Like NTSC Like NTSC
"Pre-render" lines between vertical blanking and next picture 1 scanline
Total number of dots per frame 341 × 261 + 340.5 = 89341.5
(pre-render line is one dot shorter in every odd frame) 		341 × 312 = 106392 Like PAL 341 × 262 = 89342 unknown
either like NTSC or RGB 		Like PAL
Total number of CPU cycles per frame 89341.5 ÷ 3 = 29780.5 106392 ÷ 3.2 = 33247.5 106392 ÷ 3 = 35464 89342 ÷ 3 = 297802⁄3 unknown
either like NTSC or RGB 		Like PAL famiclone
Frame rate (vertical scan rate) 60.0988 Hz 50.0070 Hz Like PAL 60.0985 Hz 60.03 Hz 50.5027 Hz
Color of top border Always black ($0E)
Side and bottom borders Palette entry at $3F00 Always black ($0E), intruding on left and right 2 pixels and top 1 pixel of picture Like PAL[3] Like NTSC[4] Unknown Unknown, probably like PAL
Color emphasis
(with correlating bit in PPUMASK) 		Blue (D7), green (D6), red (D5) Blue (D7), red (D6), green (D5) Like PAL Blue, green, red (full scale) Unknown
Other quirks Early revisions cannot read back sprite or palette memory Many poorly-researched

Some frequencies in the above table are rounded.

The 2C03, 2C04, and 2C05 PPUs were all found in Nintendo's Vs. System and PlayChoice-10 (a.k.a. PC10 or PC-10) arcade systems. Famicom Titler, Famicom TVs, and RGB-modded NES consoles would use either the 2C03 or a 2C05 with glue logic to unswap $2000 and $2001. (Later RGB mods used a 2C02 in output mode and faked out all palette logic.)

The color emphasis bits on the PAL NES have their red and green bits in PPUMASK swapped

The authentic NES sold in Brazil is an NTSC NES with an adapter board to turn the NTSC video into PAL-M video, a variant of PAL using NTSC frequencies but PAL's color modulation.

Micro Genius is a clone of the Famicom, manufactured by TXC Corporation of Taiwan and sold under various brand names in the 50 Hz market.[5] Among the best known brands is Dendy, distributed in Russia by Steepler, and the attention given by Russian reverse engineers to this clone has led to "Dendy" becoming a common name for all PAL Micro Genius-type famiclones. Its chipset (UA6527P+UA6538) is designed for compatibility with Famicom games, including games with CPU cycle counting mappers (e.g. VRC4) and games that use a cycle-timed NMI handler (e.g. Balloon Fight). This explains the faster CPU divider and longer post-render period vs. the authentic PAL NES.

To compensate for these differences, you can detect the TV system at power-on.

CPU cycle counts

To make things easier for those programming on the NES, the below chart provides the number of CPU cycles that a particular PPU-oriented trait takes.

Property NTSC PAL Dendy
Scanline (341 pixels) 1132⁄3 1069⁄16 1132⁄3
HBlank (85 pixels) 281⁄3 269⁄16 281⁄3
NMI to start of rendering 22731⁄3 74593⁄8 22731⁄3
Frame 29780.5* 33247.5 35464
PPU dots ÷ CPU cycles 3 3.2 3
OAM DMA 513 (+1 if starting on CPU get cycles)

* NTSC frame timing is 29780.5 cycles if rendering is enabled during the 20th scanline; 297802⁄3 otherwise.

See also

References

  1. ↑ nesdev forum post by thefox: http://forums.nesdev.org/viewtopic.php?p=160349#p160349
  2. ↑ nesdev forum post by Eugene.S: http://forums.nesdev.org/viewtopic.php?p=174970#p174970
  3. ↑ nesdev forum post by Eugene.S: https://forums.nesdev.org/viewtopic.php?p=173764#p173764
  4. ↑ nesdev forum post by lidnariq: https://forums.nesdev.org/viewtopic.php?p=179705#p179705
  5. ↑ Post by feos at TASVideos and NESdev
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