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[SOLVED] Donkey Kong Visual Glitches

Posted: Fri Sep 04, 2020 2:02 pm
by dbll
Edit: This has been solved! The issue involved the CPU's ADC instruction.

Hello everyone!

I've been working on an NES emulator the past month, and have been working on the PPU more recently. I seem to have gotten something wrong, but after hours of debugging still can't find the source of the issue. When loading Donkey Kong, the menu looks fine, but when the gameplay demo begins I get the artifacts on the screen shown in the screenshot. When the demo is over and the game goes back to the menu, it looks fine again.

If you look at the top left, you can see the basic versions of the nametables at addresses $2000 and $2400 having the same issues (as seen by the two vertical bars towards the left side on both). So in other words, it is not a rendering issue. My CPU passes nestest as well, which makes me think it isn't a CPU issue either. This has lead me to believe that it's an issue with NMI generation or my PPU registers. I've checked both extensively, but I could be missing something. It also could be due to something I haven't listed, I'm not fully sure.

Has anyone had any similar issues? Or possibly would know how to resolve something like this? If so, I'd love to hear what you did to resolve them.

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Re: [SOLVED] Donkey Kong Visual Glitches

Posted: Sun Oct 11, 2020 6:22 pm
by kingdiesel
Nice -- what was the issue?

I've also found a few bug in my CPU implementation even though NESTest passed.