[anyfoo]

> was to change the scroll registers and I think the character look up location at the moment the screen refresh cycle reached the appropriate point on the display. These registers would then need to be reset during the vertical blanking interval. So all in all either 100 or 120 times a second depending on the TV system.

That's a cool way to do it on the NES. While the NES did not have a raster interrupt, requiring such tricks (though you mentioned the later cartridges with the extra chip that added one), the Game Boy did have a raster interrupt, and the SNES essentially went all in on it. Switching the mode mid-frame became a very common method then.

By the way, PAL/NTSC is 50/60Hz, but a frame consists of two fields (odd/even lines), so 25 or 30 frames per second for PAL/NTSC respectively. So I guess this means you probably did the trick 50/60 times per second? (Unless you used more than two configs per frame maybe.)

[thristian]

NTSC draws each frame as two fields; first the 240 odd rows, then the 240 even rows, for a total of 480 interlaced rows, or "480i".

However, older video game consoles (such as the NES and SNES) typically output a malformed NTSC signal, designed to trick the TV into drawing just the odd lines, over and over again, sometimes referred to as "240p". As a result, it's effectively 60 independent frames per second, not 30 frames of two fields.

[anyfoo]

Yes. Even if that weren't the case, the NES would have needed to separately buffer the whole frame (or worse) if you did not want to repeat the scroll register dance per field, which is where I went wrong.

[janzer]

My guess is the trick is needed four times per frame. For every field you need it once to start the static portion drawing, then once more to stop.

[anyfoo]

Ah, duh, you're completely right. I somehow hallucinated another buffer between PPU-generated frames and video output. Which would have been rather expensive at the time...

Also I only just learned that because the NES does not actually output the half scanlines that make interlacing work, both fields are drawn on top of each other, effectively making it 50/60 actual frames per second anyway, instead of interlaced fields! (https://wiki.nesdev.com/w/index.php/NTSC_video)

[yosser]

Ah yes, in retrospect I think the NES was refreshing at 25/30 times a second, so double that for the number of scroll register updates we needed to make.

It's been a long time!