Xenix is Unix -- or at least one flavor of it. In the late 70's,
Microsoft licensed the Unix sources from AT&T and ported them to
a number of platforms. In those days, AT&T would license the Unix
software but not the Unix name, thus each company had to invent
their own name. Microsoft picked Xenix. Microsoft did not sell
Xenix to end users. Instead, they licensed the software to OEMs
(Intel, Tandy, Altos, SCO, etc.) who provided a finished end-user
package. Microsoft no longer supports Xenix, and in fact never
even offered a 286 or 386 version.
Several Unix implementations for the PC architecture have been
tried with varying levels of success. SCO Xenix for the PC/XT
was one. Nearly all of the PC/XT implementations were clunkers,
because the machine lacked the hardware necessary for robust Unix
operations. The PC/AT offered hardware memory protection, and
SCO Xenix/286 took advantage of it. SCO Xenix/386 added demand
paged virtual memory. These added features made multiuser PCs
viable, and SCO Xenix popular.
SCO Xenix starts with a Unix System III base, throws in several
Berkeley enhancements, and adds features to obtain conformance to
the System V Interface Definition (SVID). Today, the bulk of the
code is from System V. Xenix/386 even has capabilities to execute
Unix programs. It differs, however, in many of the SVID `optional'
areas people tend to expect of a full System V. SCO Xenix lacks
a real `inittab', for example. You need to go to a real System V,
such as SCO Unix, for all these features.
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