A table, or a giant switch() statement, or what have you. It just has to make a decision between a lot of instructions at once. [edit] I think I misunderstood your question.

I'm not sure I understand it, for that matter. Put the data in a format so the O.S can run it? Run data?

Are you asking how would you get useful data in/out of the emulator? Up to you I suppose. If you're expecting it to run programs meant for a different kind of computer, you'd probably need to emulate more hardware too, like DOSbox does.

DOSbox for example does what it says on the tin. It emulates a DOS machine well enough to run a good number of old games and applications. (On any system -- it doesn't depend on a real x86 cpu.) The list of hardware it emulates ends up being a bit daunting, including but probably not limited to:DOSbox even has several different CPU emulators -- a fast simplified one, a slower 32-bit one with protected mode, etc -- and switches between them depending on what CPU features are needed.

So there's a lot more to emulating a whole computer than emulating its CPU.