Thanks, that'll help a lot. I get some title screens to load now (implemented very basic graphics support), but most ROMs inevitably crash (except the overflow test rom, that works fine :-))..so I'll have to take a closer look at the cpu core :-) I don't think the flags are getting set properly.
Also, I'm starting to wonder what state the PPU really is in on power up, as some ROMs (demos on this site) seemingly crash because the VBlank comes too soon. In addition, the NES test cart says the PPU RAM has failed.. What could cause this? Other ROM's seem to have no problems loading stuff into VRAM (and that's about the only part that seems to work, except when the cpu crashes first..).