[hannob]

> - the graphics "driver" reads values out of certain registers (AL and AH?) at a set interrupt (maybe every X clock cycles?) and writes one pixel to the screen of whatever color those registers had in them

It's actually much simpler than that. After you set the right graphics mode (which for most simple dos demos is usually mode 13h, 256 color on 320x200) then there's an area of the memory that you can write to and it will show up as a pixel.

The "flow control" is usually just that you run your effect n times in these simple demos. Which means it will run faster on a faster CPU, but you usually wouldn't bother implement any form of timing in 256 byte.