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All previous OS's ran on a subsystem that Apple created, it was far more integrated with the GUI, and added layers to the equasion that Apple didn't want to deal with. The MacOS itself actually was the subsystem, the Finder (with a version number, like 7.5 or 8) was the GUI. That's kind of a bad explanation but it works. Apple made dramatic changes to this subsystem on a few occasions, usually that was marked with an entire number change, like System 6 to System 7.

pnw
 
Like the question, like the answer paulwhannel!
I have had this question asked to be a couple of times aswell. Windows (non-NT) users asked this:" OK, so Windows uses DOS, what does Apple use?"
I try to explain that Mac OS is GUI from startup right away, this being the ROM file. ROM goes to RAM >> System boots (wtih extensions, control panels) >> Finder boots. Viola! Mac OS

BTW, Mac OS X doesn't run on Unix. It IS a Unix. What you probably refer to is the Mac OS X GUI (Aqua) which runs on Apple's own implementation of Unix BSD, called Darwin.
 
Well I am thinking about going the unix route. Aka trying to learn it. I notice the pay is quite nice. What type of thing would a unix career be like? sorry if this is a bit off topic.
 
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