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.
You can use an instruction reference[1] for instruction timings, but there are some rules-of-thumb[2] 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:
| Instructions | Cycles | Bytes | Details |
|---|---|---|---|
SEC | 2 | 1 | opcode, but has to wait for the 2-cycle minimum. |
AND #imm | 2 | 2 | opcode + operand. Only affects registers. |
LDA zp | 3 | 2 | opcode + operand + byte fetched from zp. |
STA abs | 4 | 3 | opcode + 2 byte operand + byte written to abs. |
LDA abs,X | 4 or 5 | 3 | opcode + 2 byte operand + read from abs, but it delays 1 extra cycle if the addition of the X index causes a page crossing. |
STA abs,X | 5 | 3 | Like LDA abs,X, but assumes the worst case of page crossing and always spends 1 extra read cycle. |
ASL zp | 5 | 2 | opcode + operand + read from zp + write to zp, but it takes 1 extra cycle to modify the value. |
LDA (indirect),Y | 5 or 6 | 2 | opcode + operand + two reads from zp + read from indirect address. 1 extra cycle if a page is crossed. |
STA (indirect),Y | 6 | 2 | Like LDA (indirect),Y, but assumes the worst case of page crossing and always spends 1 extra read cycle. |
PHA | 3 | 1 | opcode + stack write, but requires 1 extra cycle to perform the stack operation. |
RTS | 6 | 1 | 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). |
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.) |
A clockslide[3] 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.
- 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.