So you can cover about 95% of the library (and 99% of the games people want to play) with about 20 or so mappers. From there it can be a slog to support them al

So you can cover about 95% of the library (and 99% of the games people want to play) with about 20 or so mappers. From there it can be a slog to support them all, but conceptually mappers are really very simple. Almost all of them are just registers that let you page in chunks of memory. The most advanced mapper is the MMC5, that one can take some doing. And the VRC7's OPL audio is also a nightmare (yet used only for one game.) But beyond that, it's all really simple. Usually around 10-30 lines of code per mapper. Mostly very well documented, with the exception of how mappers detect scanlines (important for accurate emulation, not so much for full compatibility. You can just cheat and give them the PPU scanline counter.) But, maybe I'm biased. I've emulated thirteen systems so far, and the NES was one of my favorites to work on due to the documentation and general simplicity. I know the rabbit hole goes on forever even with the NES, though.