Timing

Timing of the SNES hardware.

Master Clock
The SNES master clock:
 * NTSC 21.447 MHz
 * PAL 21.281 MHz

CPU
A 65816 CPU "cycle" can take 6, 8 or 12 master cycles on the SNES, depending on the memory region accessed, and the MEMSEL fast-ROM setting.

This gives some commonly quoted SNES CPU speeds, though none of them tell a complete story:
 * 3.58 MHz fast-ROM (6-cycle)
 * 2.68 MHz slow-ROM (8-cycle)
 * 1.79 MHz other (12-cycle)

The speed of access depends on the memory region:
 * 6-cycles for fast-ROM access, enabled via MEMSEL and accessed at an address of $800000 of higher.
 * 8-cycles for slow-ROM access.
 * 8-cycles for internal S-WRAM.
 * 6-cycles for most MMIO registers.
 * 12-cycles for JOYSER0 and JOYSER1.
 * 6-cycles for "internal" cycles not accessing memory (e.g. 2nd cycle of NOP)

Video
TODO:
 * Cycles per scanline
 * Cycles per frame
 * Cycles per vblank
 * Vblank DMA budgets
 * Scanlines per frame