Ehm, now I feel really old. I just did a comment about this in some other thread. It's one of those timing things where it is hard to use a debugger. The scanlines scan in whatever frequency they are set to. You can influence the frequency and what color it should hit but you don't have exact control of the speed and have to set a timer or interrupt to fire when the electron beam is on the location you want to paint red. There are many programmers still alive that knows how this stuff works and now I feel like a dinosaur. :-)