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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