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.

Instruction timings

You can use a comprehensive guide as reference for instruction timings, but there are some rules-of-thumb that can help remember most of them:

• There is a minimum of 2 cycles per instruction.
• Each byte of memory read or written adds 1 more cycle to the instruction. This includes fetching the instruction, and each byte of its operand, then any memory it references.
• Indexed instructions which cross a page take 1 extra cycle to adjust the high byte of the effective address first.
• Read-modify-write instructions perform a dummy write during the "modify" stage and thus take 1 extra cycle.
• Instructions that push data onto the stack take 1 extra cycle.
• Instructions that pop data from the stack take 2 extra cycles, since they also need to pre-increment the stack pointer.
• "Extra" cycles often include an extra read or write that usually does not affect the outcome.

Examples:

• SEC - 2 cycles: 1 byte opcode, but has to wait for the 2-cycle minimum.
• AND #imm - 2 cycles: opcode + operand = 2 bytes. Only affects registers.
• LDA zp - 3 cycles: opcode + operand + byte fetched from zp.
• STA abs - 4 cycles: opcode + 2 byte operand + byte written to abs.
• LDA abs, X - 4 or 5 cycles: opcode + 2 byte operand + read from abs, but if the addition of the X index causes a page crossing it delays 1 extra cycle.
• ASL zp - 5 cycles: opcode + operand + read from zp + write to zp, but it takes 1 extra cycle to modify the value.
• LDA (indirect), Y - 5 or 6 cycles: opcode + operand + two reads from zp + read from indirect address. 1 extra cycle if a page is crossed.
• STA (indirect), Y - 6 cycles: like LDA (indirect) but assumes the worst case of page crossing, so always spends 1 extra read cycle in case the page correction is being applied.
• PHA - 3 cycles: opcode + stack write, but requires 1 extra cycle to perform the stack operation.
• RTS - 6 cycles: opcode + two stack reads, but requires 2 extra cycles to perform the stack operations, plus 1 cycle to post-increment the program counter (to compensate for the off-by-1 address pushed by JSR).

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 is essential for 2 cycle delays. 3 cycle delays always take 2 bytes, but usually have some compromise. More options become available as delays become longer.

Instructions	Cycles	Bytes	Side effects and notes
NOP	2	1
JMP *+3	3	3
Bxx *+2	3	2	None, but requires a known flag state (e.g. BCC if carry is known to be clear).
BIT zp	3	2	Clobbers NVZ flags. Reads zp.
IGN zp	3	2	Reads zp. (Unofficial instruction.)
NOP, NOP	4	2
NOP, ...	5	3	(... = 3 cycle delay of choice)
CLV, BVC *+2	5	3	Clears V flag. (Can instead use C flag with CLC, BCC or SEC, BCS.)
NOP, NOP, NOP	6	3
PHP, PLP	7	2	Modifies 1 byte of stack.
NOP, NOP, NOP, NOP	8	4
PHP, PLP, NOP	9	3	Modifies 1 byte of stack.
PHP, CMP zp, PLP	10	4	Modifies 1 byte of stack. Reads zp.
PHP, PLP, NOP, NOP	11	4	Modifies 1 byte of stack.
JSR, RTS	12	3	Modifies 2 bytes of stack. (Takes 3 bytes only if reusing an existing RTS; otherwise 4.)

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 you can substitute other instructions with the same size and timing that preserve whichever flags/registers you need at the end of the slide. There are unofficial instructions that can avoid altering any state: replace $C9 (CMP) with $89 or $80, which ignores an immediate operand, and replace $C5 with $04, $44, or $64, which ignore a read from the zero page.

Resources

• Delay code - various variable-cycle delays
• Fixed cycle delay - shortest fixed-cycle delays
• Fixed-cycle delay code vending machine - code for generating shortest-possible delay routines at compile-time.
• 6502 vdelay - code for delaying a variable number of cycles at run-time.