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Comment by nonane | Hacker News Reader

Here is a video where Martin Piper uses a C64 debug tools to figure out how Nine was created: https://m.youtube.com/watch?v=Ik1vsMM2EuY .

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This is probably a really basic question but why does the wizard need to stop being drawn with characters when the sprites move out past the left/right borders? Is this a hardware restriction where those lines aren't able to have text on them when there are sprites on the edges?

I think it's probably about the timing. I'm not an expert, but I believe that opening the side borders requires cycle exact timing at both edges of the screen, and when you have characters enabled on the screen, the VIC-II graphics chip ends up 'stealing' memory cycles from the CPU every 8 scanlines to fetch character and font data from RAM. These lines are known as 'badlines'.

Across an entire scanline I think you normally get something like 63 6510 cpu cycles to do 'work' in, but only 23 if you hit a badline - keeping in mind that some instructions take multiple cycles to execute. This probably makes the timing difficult or impossible to manage with the characters turned on.

It's not just that the 'badlines' steal 40 cycles. They steal a solid block of 40 cycles that cover most of the screen, from end of hblank until just before the right border starts. This blog post [1] has a nice interactive demo showing badline timings.

During a badline it's simply impossible write to the VIC-II's registers during the left border. Though, this seems to indicate it's still possible to open the right border during a bad line, but it's a 1 cycle window (maybe 2).

[1] https://nurpax.github.io/posts/2018-06-19-bintris-on-c64-par...

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