Cycle counting

It is often useful to delay a specific number of CPU cycles. Timing raster effects or generating PCM audio are some examples that might utilize this. This article outlines a few relevant techniques.

Short delays

Here are few ways to create short delays without side effects. As the shortest instruction time is 2 cycles, it is not possible to delay 1 cycle on its own.

• NOP - 2 cycles, 1 byte, no side effects
• JMP *+3 - 3 cycles, 3 bytes, no side effects
• Bxx *+2 - 3 cycles, 2 bytes, no side effects but requires a known flag state (e.g. BCC if carry is known to be clear)
• IGN zp - 3 cycles, 2 bytes, only side effect is a read, unofficial instruction

Clockslide

A clockslide is a sequence of instructions that wastes a small constant amount of cycles plus one cycle per executed byte, no matter whether it's entered on an odd or even address. With official instructions, one can construct a clockslide from CMP instructions: ... C9 C9 C9 C9 C5 EA Disassemble from the start and you get CMP #$C9 CMP #$C9 CMP $EA (6 bytes, 7 cycles). Disassemble one byte in and you get CMP #$C9 CMP #$C5 NOP (5 bytes, 6 cycles). The entry point can be controlled with an indirect jump or the RTS Trick to precisely control raster effect or sample playback timing.

CMP has a side effect of destroying most of the flags, but unofficial instructions that skip one byte can be used to preserve them. For example, replace $C9 (CMP) with $89 or $80, which skips one immediate byte, and replace $C5 with $04, $44, or $64, which reads a byte from zero page and ignores it.