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NES is a little unusual in this respect because the vast majority of cartridges have "mapper" hardware that must be emulated, and there has been a more-or-less continuous trickle of unlicensed cartridges over the years, some of which have unique mappers. If memory serves you can get to something like 95% compatibility with the few dozen most common mappers, but then there's a pile of oddball mappers that are each used by 1 or 2 games (many of which are just pirate hacks of a licensed game).