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Xenix Screensnap.PNG
Xenix under Bochs
Developer Microsoft, SCO, various resellers
OS family Unix
Working state Historic
Source model Closed source
Initial release 1980
Latest release System V release 2.3.4 / 1989
Kernel type Monolithic kernel
License Proprietary
Succeeded by SCO UNIX


In the late 1980s Xenix was "probably the most widespread version of the UNIX operating system, according to the number of machines on which it runs", according to developers of the BSD version of Unix;[1] Microsoft chairman Bill Gates said in 1996 that for a long time his firm had the highest-volume AT&T Unix license.[2]


Bell Labs, Unix's developer, was part of the regulated Bell System and could not sell Unix. It instead licensed the software to others. Microsoft, which expected that Unix would be its operating system of the future when personal computers became powerful enough,[3] purchased a license for Version 7 Unix from AT&T in 1978,[4] and announced on August 25, 1980 that it would make it available for the 16-bit microcomputer market.[5] Because Microsoft was not able to license the "UNIX" name itself,[6] the company gave it an original name. Microsoft, in turn, did not sell Xenix directly to end users; instead, they licensed it to OEMs such as IBM,[7] Intel,[8] Tandy, Altos, SCO, and Siemens (SINIX) who then ported it to their own proprietary computer architectures.

IBM/Microsoft Xenix 1.00 on 5¼-inch floppy disk

Microsoft Xenix originally ran on the PDP-11. The first port was for the Zilog Z8001 16-bit processor: the first customer ship was January 1981 for Central Data Corporation of Illinois[9]:4, followed in March 1981 by Paradyne Corporation's Z8001 product.[9]:14 The first 8086 port was for the Altos Computer Systems' non-PC-compatible 8600-series computers (first customer ship date Q1 1982).[note 1][9]:3[10][11][12] Intel sold complete computers with Xenix under their Intel System 86 brand (with specific models such as 86/330 or 86/380X); they also offered the individual boards that made these computers under their iSBC brand. This included processor boards like iSBC 86/12 and also MMU boards such as the iSBC 309. The first Intel Xenix systems shipped in July 1982.[9]:9[note 2] Tandy Corporation shipped TRS-XENIX for their 68000-based systems in January 1983. Seattle Computer Products also made (PC-incompatible) 8086 computers bundled with Xenix, like their Gazelle II, which used the S-100 bus and was available in late 1983 or early 1984.[9]:17[13] There was also a port for IBM System 9000.[14]

SCO had initially worked on their own PDP-11 port of V7, called Dynix,[note 3] but then struck an agreement with Microsoft for joint development and technology exchange on Xenix in 1982.[15] In 1984, a port to the 68000-based Apple Lisa was jointly developed by SCO and Microsoft and it was the first shrink-wrapped binary product sold by SCO.[16]

The difficulty in porting to the various 8086 and Z8000-based machines, said Microsoft in their 1983 OEM directory, had been the lack of a standardized memory management unit and protection facilities. Hardware manufacturers compensated by designing their own hardware, but the ensuing complexity made it "extremely difficult if not impossible for the very small manufacturer to develop a computer capable of supporting a system such as XENIX from scratch," and "the XENIX kernel must be custom-tailored to each new hardware environment."[9]:Introduction A generally available port to the unmapped Intel 8086/8088 architecture was done by The Santa Cruz Operation around 1983.[17][18][19] SCO Xenix for the PC XT shipped sometime in 1984 and contained some enhancement from 4.2BSD; it also supported the Micnet local area networking.[20] The later 286 version of Xenix leveraged the integrated MMU present on this chip, by running in 286 protected mode.[21] The 286 Xenix was accompanied by new hardware from Xenix OEMs. For example, the Sperry PC/IT, an IBM PC AT clone, was advertised as capable of supporting eight simultaneous dumb terminal users under this version.

While Xenix 2.0 was still based on Version 7 Unix,[22] version 3.0 was upgraded to a Unix System III code base,[9]:9[23][24] A 1984 Intel manual for Xenix 286 noted that the Xenix kernel had about 10,000 lines at this time.[8]:1–7 It was followed by a System V.2 codebase in Xenix 5.0 (aka Xenix System V).[25]

After the breakup of the Bell System AT&T started selling Unix. Microsoft, believing that it could not compete with Unix's developer, decided to abandon Xenix. The decision was not immediately transparent, and so Xenix gave birth to the term vaporware.[26] An agreement was signed with IBM to develop OS/2,[3] and the Xenix team (together with the best MS-DOS developers) was assigned to that project. 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.

In 1987, SCO ported Xenix to the 386 processor, a 32-bit chip, this after securing knowledge from Microsoft insiders that Microsoft was no longer developing Xenix.[26] Xenix System V release 2.3.1 introduced support for i386, SCSI and TCP/IP. SCO's Xenix System V/386 was the first 32-bit operating system available on the market for the x86 CPU Architecture.




Aside from its AT&T Unix base, Xenix incorporated elements from BSD, notably the vi text editor and its supporting libraries (termcap and curses).[7] Its kernel featured some original extensions by Microsoft, notably file locking and semaphores,[7][8]:1.12 while to the userland Microsoft added a "visual shell" for menu-driven operation instead of the traditional Unix shell.[7] A limited form of local networking over serial lines (RS-232 ports) was possible through the "micnet" software, which supported file transfer and electronic mail, although UUCP was still used for networking via modems.[7]

OEMs added further modifications to the Xenix system.[7][8]

Trusted Xenix[edit]

Trusted Xenix was a variant developed by Trusted Information Systems which incorporated the Bell-La Padula 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).[30] It was evaluated by formal methods and achieved a B2 security rating under the DoD's Trusted Computer System Evaluation Criteria—the second highest rating ever achieved by an evaluated operating system.[31] Version 2.0 was released in January 1991, version 3.0 in April 1992, and version 4.0 in September 1993.[32] It was still in use as late as 1995.

See also[edit]


  3. ^ Unrelated to the later Dynix from Sequent Computer Systems


  3. ^ a b Letwin, Gordon (1995-08-17). "What's happening to OS/2". comp.os.ms-windows.misc,comp.os.os2.advocacy. Web link. Retrieved 6 November 2013.
  4. ^ Steve D. Pate (1996). Unix Internals: A Practical Approach. Addison Wesley Professional. p. 9. ISBN 978-0-201-87721-2. "Microsoft licensed Seventh Edition UNIX from AT&T in 1978 to produce the XENIX operating system initially for the PDP-11." 
  6. ^ "Xenix variant information". 2010-02-26. "In the late 1970s Microsoft licensed Unix source code from AT&T, which at the time was not licensing the name Unix." [dead link]
  8. ^ a b c d Overview of the XENIX 286 Operating System. Intel Corporation. November 1984. "XENIX 286 is Intel's value-added version of the XENIX operating system released by Microsoft Corporation." 
  11. ^ John Halamka (7 November 1983). Review: Altos 586. InfoWorld. p. 89. ISSN 01996649. 
  12. ^ IDG Enterprise (26 October 1987). Computerworld. IDG Enterprise. pp. 77–. ISSN 00104841. 
  14. ^ BYTE Guide to the IBM PC, fall 1984, p.61
  21. ^ Microsoft Xenix 3.0 ready for 286
  25. ^ J. E. Lapin (1987). Portable C and Unix System Programming. Pearson Education. p. 106. ISBN 978-0-13-686494-3. "The XENIX 2.3 version generally resembles V7's [ABI]; the XENIX 3.0 version resembles SIII's, and the XENIX 5.0 version resembles SV2's." 
  27. ^ Terry Lambert (2000-11-07). "Re: Microsoft Source (fwd)". sol.lists.freebsd.chat. Web link. Retrieved 2006-10-25.

Further reading[edit]

External links[edit]