An emulator, even a paid one, would be totally legal in the US as long as:
It does not use any patented technologies. I’m not sure if Nintendo has any patents in the emulation space, but regardless the GBA is so simple that it wouldn’t require patented techniques to emulate.
It does not contain any proprietary (copyrighted) code. On more modern consoles, this would include the BIOS or Firmware files. Does the GBA even need something like that?
Number 1 is a non-issue for a GBA emulator. Number 2 is more tricky, but it’s always possible to reverse engineer and reimplement the firmware. That’s protected by the Compaq v. IBM case.
The recent drama with the Switch emulator is that they violated the second principle.
Which is probably not copyrightable. A key is just a number, and copyright only applies to creative works. In a court, Nintendo would have to argue that copyright does apply because the key was created via some artistic or creative process by a human. It likely is just the output of a random number generator. Also, we’ve already been through this when people figure out how to decrypt DVDs.
An emulator, even a paid one, would be totally legal in the US as long as:
It does not use any patented technologies. I’m not sure if Nintendo has any patents in the emulation space, but regardless the GBA is so simple that it wouldn’t require patented techniques to emulate.
It does not contain any proprietary (copyrighted) code. On more modern consoles, this would include the BIOS or Firmware files. Does the GBA even need something like that?
Number 1 is a non-issue for a GBA emulator. Number 2 is more tricky, but it’s always possible to reverse engineer and reimplement the firmware. That’s protected by the Compaq v. IBM case.
The recent drama with the Switch emulator is that they violated the second principle.
What proprietary code did they have?
Stupidly, the decryption key.
Which is probably not copyrightable. A key is just a number, and copyright only applies to creative works. In a court, Nintendo would have to argue that copyright does apply because the key was created via some artistic or creative process by a human. It likely is just the output of a random number generator. Also, we’ve already been through this when people figure out how to decrypt DVDs.
You’re forgetting about the DMCA.