If you want an emulator PC for MAC, it's virtual pc (
http://www.connectix.com)
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<br />If you want an emulator MAC for PC, these emulators don't work very good and it's often AppleII emulators.
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<br />If you want to develop an emulator Mac (or PC) it's a very long and difficult work but very interesting because the competences required are electronic and operating system and programmation asm and c/c++.
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<br />Bye and good luck. :)