Cycle counting: Difference between revisions

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(Converts the "Instruction timings" examples into a table and adds a STA abs,X example.)
 
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You can use a comprehensive
You can use a comprehensive
guide<ref>[http://www.obelisk.me.uk/6502/reference.html Obelisk: 6502 instruction reference]</ref><ref>[http://nesdev.org/6502_cpu.txt 6502_cpu.txt: cycle-by-cycle instruction behaviour]</ref>
guide<ref>[https://www.nesdev.org/obelisk-6502-guide/reference.html Obelisk: 6502 instruction reference]</ref><ref>[http://nesdev.org/6502_cpu.txt 6502_cpu.txt: cycle-by-cycle instruction behaviour]</ref>
as reference for instruction timings,
as reference for instruction timings,
but there are some
but there are some
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Examples:
Examples:


* <code>SEC</code> - 2 cycles: 1 byte opcode, but has to wait for the 2-cycle minimum.
{|class="tabular"
* <code>AND #imm</code> - 2 cycles: opcode + operand = 2 bytes. Only affects registers.
! Instructions || Cycles || Bytes || Details
* <code>LDA zp</code> - 3 cycles: opcode + operand + byte fetched from zp.
|-
* <code>STA abs</code> - 4 cycles: opcode + 2 byte operand + byte written to abs.
| <code>SEC</code> || 2 || 1 || opcode, but has to wait for the 2-cycle minimum.
* <code>LDA abs, X</code> - 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.
|-
* <code>ASL zp</code> - 5 cycles: opcode + operand + read from zp + write to zp, but it takes 1 extra cycle to modify the value.
| <code>AND #imm</code> || 2 || 2 || opcode + operand. Only affects registers.
* <code>LDA (indirect), Y</code> - 5 or 6 cycles: opcode + operand + two reads from zp + read from indirect address. 1 extra cycle if a page is crossed.
|-
* <code>STA (indirect), Y</code> - 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.
| <code>LDA zp</code> || 3 || 2 || opcode + operand + byte fetched from zp.
* <code>PHA</code> - 3 cycles: opcode + stack write, but requires 1 extra cycle to perform the stack operation.
|-
* <code>RTS</code> - 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).
| <code>STA abs</code> || 4 || 3 || opcode + 2 byte operand + byte written to abs.
|-
| <code>LDA abs,X</code> || 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.
|-
| <code>STA abs,X || 5 || 3 || Like LDA abs,X, but assumes the worst case of page crossing and always spends 1 extra read cycle.
|-
| <code>ASL zp</code> || 5 || 2 || opcode + operand + read from zp + write to zp, but it takes 1 extra cycle to modify the value.
|-
| <code>LDA (indirect),Y</code> || 5 or 6 || 2 || opcode + operand + two reads from zp + read from indirect address. 1 extra cycle if a page is crossed.
|-
| <code>STA (indirect),Y</code> || 6 || 2 || Like LDA (indirect),Y, but assumes the worst case of page crossing and always spends 1 extra read cycle.
|-
| <code>PHA</code> || 3 || 1 || opcode + stack write, but requires 1 extra cycle to perform the stack operation.
|-
| <code>RTS</code>|| 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).
|}


== Short delays ==
== Short delays ==
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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.
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.


* <code>NOP</code> - 2 cycles, 1 byte, no side effects
{|class="wikitable sortable"
* <code>JMP *+3</code> - 3 cycles, 3 bytes, no side effects
! Instructions !! Cycles !! Bytes !!class="unsortable"| Side effects and notes
* <code>Bxx *+2</code> - 3 cycles, 2 bytes, no side effects but requires a known flag state (e.g. BCC if carry is known to be clear)
|-
* <code>BIT zp</code> - 3 cycles, 2 bytes, clobbers NVZ but preserves C, reads zp
| <code>NOP</code> || 2 || 1 ||
* <code>[[Programming with unofficial opcodes#NOPs|IGN zp]]</code> - 3 cycles, 2 bytes, only side effect is a read, unofficial instruction
|-
* <code>NOP, NOP</code> - 4 cycles, 2 bytes
| <code>JMP *+3</code> || 3 || 3 ||
* <code>NOP, ...</code> - 5 cycles, 3 bytes (... = 3 cycle delay of choice)
|-
* <code>CLV, BVC *+2</code> - 5 cycles, 3 bytes, clears V flag (can instead use C flag with CLC, BCC or SEC, BCS)
| <code>Bxx *+2</code> || 3 || 2 || None, but requires a known flag state (e.g. BCC if carry is known to be clear).
* <code>NOP, NOP, NOP</code> - 6 cycles, 4 bytes
|-
* <code>PHP, PLP</code> - 7 cycles, 2 bytes, modifies 1 byte of stack
| <code>BIT zp</code> || 3 || 2 || Clobbers NVZ flags. Reads zp.
* <code>NOP, NOP, NOP, NOP</code> - 8 cycles, 4 bytes
|-
* <code>PHP, PLP, NOP</code> - 9 cycles, 3 bytes, modifies 1 byte of stack
| <code>[[Programming with unofficial opcodes#NOPs|IGN zp]]</code> || 3 || 2 || Reads zp. (Unofficial instruction.)
* <code>PHP, CMP zp, PLP</code> - 10 cycles, 4 byes, modifies 1 byte of stack, reads zp
|-
* <code>PHP, PLP, NOP, NOP</code> - 11 cycles, 4 bytes, modifies 1 byte of stack
| <code>NOP, NOP</code> || 4 || 2  
* <code>JSR, RTS</code> - 12 cycles, 3 bytes (if taking advantage of an existing RTS elsewhere), modifies 2 bytes of stack
|-
| <code>NOP, ...</code> || 5 || 3 || (... = 3 cycle delay of choice)
|-
| <code>CLV, BVC *+2</code> || 5 || 3 || Clears V flag. (Can instead use C flag with CLC, BCC or SEC, BCS.)
|-
| <code>NOP, NOP, NOP</code> || 6 || 3
|-
| <code>PHP, PLP</code> || 7 || 2 || Modifies 1 byte of stack.
|-
| <code>NOP, NOP, NOP, NOP</code> || 8 || 4
|-
| <code>PHP, PLP, NOP</code> || 9 || 3 || Modifies 1 byte of stack.
|-
| <code>PHP, CMP zp, PLP</code> || 10 || 4 || Modifies 1 byte of stack. Reads zp.
|-
| <code>PHP, PLP, NOP, NOP</code> || 11 || 4 || Modifies 1 byte of stack.
|-
| <code>JSR, RTS</code> || 12 || 3 || Modifies 2 bytes of stack. (Takes 3 bytes only if reusing an existing RTS; otherwise 4.)
|}


== Clockslide ==
== Clockslide ==
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The entry point can be controlled with an indirect jump or the [[RTS Trick]] to precisely control raster effect or sample playback timing.
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 [[CPU unofficial opcodes|unofficial instructions]] that skip one byte can be used to preserve them.
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.
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.
There are [[CPU unofficial opcodes|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 ==
== Resources ==


* [[Delay code]] - various variable-cycle delays
* [[Fixed cycle delay]] - shortest fixed-cycle delays
* [https://bisqwit.iki.fi/utils/nesdelay.php Fixed-cycle delay code vending machine] - code for generating shortest-possible delay routines at compile-time.
* [https://bisqwit.iki.fi/utils/nesdelay.php Fixed-cycle delay code vending machine] - code for generating shortest-possible delay routines at compile-time.
* [https://github.com/bbbradsmith/6502vdelay 6502 vdelay] - code for delaying a variable number of cycles at run-time.
* [https://github.com/bbbradsmith/6502vdelay 6502 vdelay] - code for delaying a variable number of cycles at run-time.

Latest revision as of 03:22, 3 June 2024

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[1][2] as reference for instruction timings, but there are some rules-of-thumb[3] 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).

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[4] 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

References

  1. ↑ Obelisk: 6502 instruction reference
  2. ↑ 6502_cpu.txt: cycle-by-cycle instruction behaviour
  3. ↑ Forum: Is there a logic to instruction timings?
  4. ↑ Clockslide: How to waste an exact number of clock cycles on the 6502
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