According to what I know, you are supposed to update the V counter during cycle 252 and the horizontal counter during cycle 256. Also, doing this does not seem to be enough for Battletoads and a few other demos: you must include a delay between the time when the VBlank flag is set and when an NMI is generated. I've tried emulating games without such a delay, and many will work. The only exceptions, though, were Battletoads (level 2) and FAU.

Apparently the scanline demo does not run on the real deal the way it's supposed to. FCEUltra comes the closest to emulating it correctly.

http://www12.brinkster.com/hydesprojects/IM001700.JPG

There also seems to be some sort of flickering going on, but that's pretty much what it looks like (thanks to Memblers for the picture).



hydesprojects.cjb.net