Timing issues

[jarnobeumer]

Running a few intros from https://intros.c64.org in the C64 emulator, I found several PRGs that seem to have timing issues. Example: https://intros.c64.org/inc_download.php?iid=5, where the sound seems to be way too slow, although it seems the emulator is running at 100% speed.

[floooh]

Yes that's kinda expected unfortunately. I never got the various test suites completely green. While the CPU emulation should be pretty much perfect (it's tested against the visual6502 netlist simulation), the CIA still has minor timing issues (and maybe also the VIC). In some demos this doesn't matter, in some demos it may break spectacularly.

Apart from simple bugs one possible reason might be that the emulation is only running full 1 MHz clock cycles, while some 'events' on the C64 happen at half cycles (I suspect this might be one cause for the problems, because I remember that I had situations where I fixed some timing tests which then broke others).

It could also be even more trivial, like specific demos being written against one hardware revision, but the emulator being written against another.

Not sure yet when I will get around to work on the emulators again, but the bugs have a bit of a fractal nature, the CPC and C64 emulators have most of the obvious issues fixed, but what remains are bugs that take a lot of time to investigate.