Blargg's video game music emulation library, which allows audio applications to easily add playback support for the music of many classic video game consoles.
Occasionally those find their way into NSF files. GME currently doesn't support those, meaning that in order to play in GME-based software the underlying code has to be changed (which is a difficult task on top of the difficulty of making an NSF rip out of a game).
For example, the NES homebrew port of the game "STREEMERZ" uses those, and so does the NSF rip. It plays properly in NES emulators, but not GME-based music players.
Some NES games use 6502 opcodes that, while not officially part of the instruction set, still perform useful operations. Here is the NESDev wiki page about those.
Occasionally those find their way into NSF files. GME currently doesn't support those, meaning that in order to play in GME-based software the underlying code has to be changed (which is a difficult task on top of the difficulty of making an NSF rip out of a game).
For example, the NES homebrew port of the game "STREEMERZ" uses those, and so does the NSF rip. It plays properly in NES emulators, but not GME-based music players.