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→Emulation Issues
The NES ROM information isn't sufficient to describe the cartridge and emulate it, so emulators have to include the layout and behavior of these mappers in their code, while the ROM header tells the emulator which mapper to choose. So unlike with other consoles, no matter how accurate a given NES emulator will get, it will still never be able to run newly discovered ROM dumps from cartridges that used a so-far unknown mapper. Thus, Unlicensed NES support will be inevitably incomplete and a constant work-in-progress, hence claims some emulators are "inaccurate".
Related to this issue: This is why most emulators won't run unheadered NES ROMs. Newer versions of Nestopia can open those, but they're handled in a slightly different way: the information that would have been included in the iNES header is instead provided in emulator configuration files that get summoned as long as the ROM's hash matches exactly the No-Intro dump of that given game (which is inconvenient for romhacks).
===QD FDS Support===
Games dumped off the Famicom Disc System come into two major types:
* '''.fds format''': Most common format. Ubiquitous in ROM sets (GoodSets, No-Intro). Omits some checksum data.
* '''.qd format''' (stands for QuickDisk): Only ever used in official Nintendo re-releases. Almost identical to fds, but a full dump with checksum data. May omit padding.
The checksum data in question would be checked at BIOS startup to verify the integrity of the image and whether it was tampered with, in which case it will throw an anti-piracy error. As of now, no NES emulators support the alternate more complete dumps, as well as fudging that check's result to always return a negative. To emulate a .qd image, stripping the checksum data with a custom script is needed.
===Overscan===
