Cycle reference chart
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||RGB (2C03)||RGB (Vs. 4)||RGB (2C05)|
|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|
|CPU||Ricoh 2A03||Ricoh 2A07||UMC UA6527P||Ricoh 2A03|
|CPU clock speed|| 21.47~ MHz ÷ 12 = 1.789773 MHz
Same as NTSC Amiga clock ÷ 4
|26.60~ MHz ÷ 16 = 1.662607 MHz|| 26.60~ MHz ÷ 15 = 1.773448 MHz
Same as PAL Amiga clock ÷ 4
|APU Frame Counter rate||60 Hz||50 Hz||59 Hz||Like NTSC|
|PPU||Ricoh 2C02||Ricoh 2C07||UMC UA6538||Ricoh 2C03||Ricoh 2C04-0001 through 2C04-0004||Ricoh 2C05|
|PPU clock speed||21.477272 MHz ÷ 4||26.601712 MHz ÷ 5||Like PAL||Like NTSC|
|PPU dots per CPU cycle||3||3.2||3||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|
|Nominal visible picture height (see Overscan)||224 scanlines||268 scanlines||Like PAL||Like NTSC|
|"Post-render" blanking lines between end of picture and NMI||1 scanline||1 scanline||51 scanlines||Like NTSC|
|Length of vertical blanking after NMI||20 scanlines (≈ 2273 CPU cycles)||70 scanlines (≈ 7459 CPU cycles)||Like NTSC||Like NTSC|
|Time during which OAM can be written||Vertical or forced blanking||Only during first 20 scanlines after NMI (≈2131 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 other frame)
|341 × 312 = 106392||Like PAL||341 × 262 = 89342|
|Total number of CPU cycles per frame||89341.5 ÷ 3 = 29780.5||106392 ÷ 3.2 = 33247.5||106392 ÷ 3 = 35464||89342 ÷ 3 = 297802⁄3|
|Vertical scan rate||60.0988 Hz||50.0070 Hz||Like PAL||60.0985 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||Like NTSC|
| 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)|
|Other quirks||Early revisions cannot read back sprite or palette memory||Missing $2D and $3D greys||Permutated palette||PPUCTRL and PPUMASK are swapped; revision ID in PPUSTATUS (D4-D0)|
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. 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.
|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|
|PPU dots ÷ CPU cycles||3||3.2||3|
|OAM DMA||513 (+1 on odd-numbered CPU cycles)|
* NTSC frame timing is 29780.5 cycles if rendering is enabled during the 20th scanline; 297802⁄3 otherwise.
- nesdev forum post by thefox: http://forums.nesdev.com/viewtopic.php?p=160349#p160349
- nesdev forum post by Eugene.S: http://forums.nesdev.com/viewtopic.php?p=174970#p174970
- nesdev forum post by Eugene.S: https://forums.nesdev.com/viewtopic.php?p=173764#p173764
- nesdev forum post by lidnariq: https://forums.nesdev.com/viewtopic.php?p=179705#p179705
- Post by feos at TASVideos and NESdev