Interstingly they talk about the "change palette in the middle of the screen" tricks that we used in Imphobia. The precision timing was certainly tricky, especially when playing a MOD-file while drawing ! Ahhh memories...
Argh ! was that possible ? I actually stop playing with that with my last overscan "320 x +/- 240" attempt during which, for some reason, the screen beamer concentrated itself on exactly on scan line of the screen, rendering it super bright and emitting a super scary sound. My screen always had a darker line in the middle of it since that experiment :-( You could actually damage things by playing with hardware...
Sure, if you were prepared to give up a few scanlines for the register changes. The monitor will happily continue to scan as long as the basics (vertical resolution, frame rate) don't change and you make the the coils are still being swept.
That's why you ended up with that darker scanline, for a brief time the vertical deflection was turned off and that caused that one scanline to be hit by the electron beam in rapid succession at an intensity that it normally would not receive.
It's like looking into the sun.
Scanning is the hard part, so you don't need to worry too much if you keep the timing steady but you can change things like colours, palette contents, horizontal resolution without too much trouble.
If you're going to mess with the vertical resolution then you'll have to have write access to the register that counts the scanlines (and you'll need to set it to what it would have been had the whole screen be that resolution).
And of course at the end of the frame you have to switch it all back.
