What makes the UNIX operating system portable?

And by portable I mean what makes it accessible so that it is able to work with a variety of other machines ranging from microcomputers to supercomputers Please help ASAP thank you and god bless.

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Portability of UNIX OS depends upon the proper abstraction of the underlying hardware not only to the applications utilizing that interface, as well as in the operating system's source code to deal with a greater number of distinctive systems with as meager code repetition as could be expected under the circumstances.
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