[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.