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Xenix is a version of the Unix operating system, licensed by Microsoft from AT&T in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually began distributing it as SCO UNIX.
Xenix was Microsoft's version of Unix intended for use on microcomputers; since Microsoft was not able to license the "UNIX" name itself, they gave it an original name. The -ix ending follows a convention used by many other Unix-like operating systems.
Microsoft purchased a license for Version 7 Unix from AT&T in 1979, and announced on August 25, 1980 that it would make it available for the 16-bit microcomputer market. The initial development of Xenix was done by Human Computing Resources Corporation of Toronto, Canada. The initial port of Xenix to the Intel 8086/8088 architecture was performed by The Santa Cruz Operation.
Xenix varied from its 7th Edition origins by incorporating elements from BSD, and soon possessed the most widely installed base of any Unix flavour due to the popularity of the inexpensive x86 processor.
Microsoft did not sell Xenix directly to end users; instead, they licensed it to software OEMs such as Intel, Tandy, Altos and SCO, who then ported it to their own proprietary computer architectures. Microsoft Xenix originally ran on the PDP-11; the first port was for the Zilog Z8001 16-bit processor. Altos shipped a version for their Intel 8086 based computers early in 1982, Tandy Corporation shipped TRS-XENIX for their 68000-based systems in January 1983, and SCO released their port to the IBM PC in September 1983. A port to the 68000-based Apple Lisa also existed. At the time, Xenix was based on AT&T's UNIX System III.
Version 2.0 of Xenix was released in 1985 and was based on UNIX System V. An update numbered 2.1.1 added support for the Intel 80286 processor. Subsequent releases improved System V compatibility.
In 1986, SCO ported Xenix to the 386 processor, a 32-bit chip. Xenix 2.3.1 introduced support for i386, SCSI and TCP/IP.
When Microsoft entered into an agreement with IBM to develop OS/2, it lost interest in promoting Xenix. In 1987 Microsoft transferred ownership of Xenix to SCO in an agreement that left Microsoft owning 25% of SCO. When Microsoft eventually lost interest in OS/2 as well, it based its further high-end strategy on Windows NT.
Microsoft continued to use Xenix internally, submitting a patch to support functionality in UNIX to AT&T in 1987, which trickled down to the code base of both Xenix and SCO UNIX. Microsoft is said to have used Xenix on Sun workstations and VAX minicomputers extensively within their company as late as 1992.
In the late 1980s, Xenix was, according to The Design and Implementation of the 4.3BSD UNIX Operating System, "probably the most widespread version of the UNIX operating system, according to the number of machines on which it runs".
SCO branched Xenix into SCO UNIX in 1989. In the meantime, AT&T completed its merge of Xenix, BSD, SunOS and System V into System V Release 4. SCO UNIX was still based on System V Release 3, but had most features of Release 4. The last version of Xenix itself was 2.3.4.
Trusted Xenix was a variant developed by Trusted Information Systems which incorporated the Bell-LaPadula model of multilevel security, and had a multilevel secure interface for the STU-III secure communications device (that is, an STU-III connection would only be made available to applications running at the same privilege level as the key loaded in the STU-III). It was evaluated by formal methods and achieved a B2 security rating under the NSA's Trusted Computer System Evaluation Criteria—the second highest rating ever achieved by an evaluated operating system. Version 2.0 was released in January 1991, version 3.0 in April 1992, and version 4.0 in September 1993. It was still in use at least as of 1995.