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

Scanline:

• 1364 master cycles = 341 dot cycles

The CPU pauses for 40 master cycles in the middle of each scanline.

TODO:

• Cycles per frame
• Scanlines per frame
• Lines/cycles per vblank
• Vblank DMA budgets

Tools

• SnesInstructionCycleTool - calculates CPU cycles and master clock cycles under different conditions