> I've built a disassembler for the NES roms to test the 6502 cpu of my nes
> emulator. However the disassembler showed many FF opcodes (mostly chained in
> long groups of FF op's). I used other NES disassemblers to compare the output so
> to rule out errors and I also looked at the binary data directly. So I would
> like to know why there are so many FF opcodes in Nes roms (e.g. zelda, SMB2,
> Ninja Gaiden)?
It's not an opcode -- it's just meaningless data you're looking at. Long runs of 0xFF usually mean that part of the ROM has not been programmed with anything.