Many games, for example, Maniac Mansion, or Kissenschlacht, use two screenmodes. Oftentimes, a bitmap mode for the main screen, and a text mode for the lower part which displays scores or whatever. Considering how the raster moves across the screen, it's not so hard to implement. It's basically, set up a raster interrupt, and then change the mode in the interrupt routine. Same way you open up the lower and upper borders.

Now, with careful timing, the left and right borders may be opened up too. So I'm wondering if a similar technique can be used to change the screen mode partway across a line, so that the whole screen is divided into, say, a textmode left half and a bitmapped right half. Has this ever been done or could it be done?

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    Definitely not on all scanlines since the C64 famously has 'bad lines' — lines where it is necessary to fetch tile map contents for that row, in which the CPU isn't actually running. So there's nobody about to change screen mode. On all the other lines I'll bet the answer is 'yes', as it is on almost every other comparable machine, but I don't actually know. So don't take that as an answer. – Tommy Sep 12 at 14:39

I'm going to go out on a limb; after thorough reading of The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64:

  • graphics modes are directly selected by the bits in d011 and d016 (so e.g. they're not just latches that are copied somewhere else upon horizontal or vertical retrace); and
  • most of the pipeline is common, and runs at a fixed clock — the selected mode affects address calculation for pushing to a shift register, which is always some translation of the same mode-independent global counters; and translation of whatever comes out of the register.

No mention is made anywhere in the documentation of anything that would cause mode changes to take effect anything other than instantly, and the idea of raster effects that cause vertical boundaries is well-enough established that you can find the occasional argument about it.

Several vertical raster split demos are available on Youtube but alas I was unable to identify one that I could definitively say was a mode change rather than a palette effect.

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