[MarkS]

It isn’t illegal. Super Nintendo, Gameboy, and other such emulators are illegal because they need to include software copies of the physical game system hardware ROMs to work. That is where it gets legally murky.

Running an OS is not, in and of itself, illegal. Now, if your emulator is trying to emulate the hardware of a physical device, say an iPad, then it would be illegal. If the OS, such as iOS, requires certain firmware to run, firmware that isn’t normally contained in your PC, then it would require a copy of the device’s firmware to run. It is that copy that makes the emulator illegal.