A 6502 delay routine for delaying a number of cycles specified at runtime.
You might call this arbitrary delay, procedural delay, programmatic delay, variable delay, dial-a-delay etc. It might be handy in situations where you have a 6502 without effective timer hardware (e.g. Apple II, NES).
Uses ca65 (cc65) assembly syntax.
Authors:
- Brad Smith
- Fiskbit (source)
- Eric Anderson (source)
- Joel Yliluoma (source)
Version 10
- vdelay.s - normal version (29-65535 cycles, 55 bytes)
- vdelay_short.s - short version (27-255 cycles, 30 bytes)
Assemble and include the source code in your project. It exports the vdelay subroutine, which you call with a 16-bit value for the number of cycles to delay. Low bits in A, high bits in X.
The minimum amount of delay is currently 29 cycles. If the given parameter is less than that it will still delay that minimum number of cycles. The cycle count includes the JSR/RTS of the subroutine call, though you will probably need to account for a few extra cycles to load A/X before calling.
This code must be placed in a 64-byte-aligned segment. Add align=64 to your CODE segment CFG or add a .segment directive of your own to place it in a custom segment that is appropriately aligned.
The short version only permits delays only up to 255, with A as its parameter. The minimum is slightly lower, and the code is smaller. Since X is ignored, there may be less calling overhead.
This code should be 65C02 compatible, as it does not use instructions that have different timings from 6502.
The byte alignment requirements were chosen for ease of maintenance/use. If you remove the .align directive, it will still ensure correct branch timing with asserts, so if you are extremely cramped for space and willing to experiment with a few bytes of internal padding you might be able to get away with much smaller alignment.
If you need hard-coded delays of specific lengths (i.e. decided at compile-time, not run-time), or want to investigate other alternative, you may find these articles and tools written by Bisqwit useful:
- NESDev Wiki: Fixed cycle delay (minimal-size fixed-cycle delays)
- NESDev Wiki: Delay code (collection of variable-delay methods)
- NES 6502 / RP2A03 / RP2A07 fixed-cycle delay code vending machine
Place cc65 in test/cc65 and create a test/temp folder.
Run test/compile.bat to build the test binaries.
The python3 program test/test.py will use sim65 to simulate the program with all possible parameters and logs the cycle count of each. (This takes a few minutes.) Once finished it will analyze the log to verify the relative measured cycles is correct.
The NES ROM compiled to test/temp/test_nes.nes can be used to test the code in an NES debugging emulator. In FCEUX or Mesen emulators you can set a read breakpoint on $FE to quickly find the function entry, then step over it to count its cycles, or step into it for debugging. Use the gamepad to set the parameter (the default is set at 64 = $0040), and press A to run the test.
- Version 1
- vdelay - 78 cycles, 176 bytes.
- Version 2
- vdelay - 64, 174.
- Version 3
- vdelay - reduced alignment requirement.
- vdelay_short - 57-255, 135.
- vdelay_compact - 72, 144.
- vdelay_extreme - 40, 896.
- Version 4
- vdelay - 63, 113.
- vdelay_short - 56, 80.
- vdelay_compact - obsoleted.
- vdelay_extreme - 40, 837.
- Version 5
- vdelay - 63, 98.
- vdelay_short - 56, 71.
- vdelay_extreme - 40, 827.
- Version 6
- vdelay - 61, 96.
- vdelay_clockslide - 60, 89.
- vdelay_modify - 44, 72/27+52.
- vdelay_extreme - 40, 826.
- vdelay_short - 56, 70.
- vdelay_short_clockslide - 51, 48.
- Version 7
- vdelay - 48, 62.
- vdelay_clockslide - obsoleted: vdelay is now a clockslide technique.
- vdelay_modify - 35, 54.
- vdelay_extreme - obsoleted.
- vdelay_short - 46, 36.
- vdelay_short_clockslide - obsoleted.
- vdelay_short_modify - 33, 28.
- Version 8
- vdelay - 37, 55. Y not clobbered.
- vdelay_modify - Y not clobbered.
- vdelay_short - 35, 30. X not clobbered.
- vdelay_short_modify - X not clobbered.
- Version 9
- vdelay - 36, 62.
- vdelay_short - 34, 37.
- vdelay_extreme - 31, 297/23+283.
- vdelay_short_extreme - 27, 254/16+240.
- Version 10
- vdelay - 29, 55.
- vdelay_modify - obsoleted.
- vdelay_extreme - obsoleted.
- vdelay_short - 27, 30.
- vdelay_short_modify - obsoleted.
- vdelay_short_extreme - obsoleted.
This library may be used, reused, and modified for any purpose, commercial or non-commercial. If distributing source code, do not remove the attribution to its original authors, and document any modifications with attribution to their new author as well.
Attribution in released binaries or documentation is appreciated but not required.