|Developer||Microsoft, SCO, HCR, Logica|
|OS family||Unix (SysV)|
|Source model||Closed source|
|Latest release||System V Release 2.3.4 / 1989|
|Platforms||PC/XT, x86, PDP-11, Z8001, 68k|
|Kernel type||Monolithic kernel|
|Succeeded by||SCO UNIX, OS/2|
Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually replaced it with SCO UNIX (now known as SCO OpenServer).
In the mid-to-late 1980s, Xenix was the most common Unix variant, measured according to the number of machines on which it was installed. Microsoft chairman Bill Gates said at Unix Expo in 1996 that, for a long time, Microsoft had the highest-volume AT&T Unix license.
Bell Labs, the developer of Unix, was part of the regulated Bell System and could not sell Unix directly to most end users (academic and research institutions excepted); it could, however, license it to software vendors who would then resell it to end users (or their own resellers), combined with their own added features. Microsoft, which expected that Unix would be its operating system of the future when personal computers became powerful enough, purchased a license for Version 7 Unix from AT&T in 1978, and announced on August 25, 1980, that it would make the software available for the 16-bit microcomputer market. Because Microsoft was not able to license the "Unix" name itself, the company gave it an original name.
Microsoft called Xenix "a universal operating environment". It did not sell Xenix directly to end users, but licensed the software to OEMs such as IBM, Intel, Management Systems Development, Tandy, Altos Computer, SCO, and Siemens (SINIX) which then ported it to their own proprietary computer architectures.
In 1981, Microsoft said the first version of Xenix was "very close to the original Unix version 7 source" on the PDP-11, and later versions were to incorporate its own fixes and improvements. The company stated that it intended to port the operating system to the Zilog Z8000 series, Digital LSI-11, Intel 8086 and 80286, Motorola 68000, and possibly "numerous other processors", and provide Microsoft's "full line of system software products", including BASIC and other languages. The first port was for the Z8001 16-bit processor: the first customer ship was January 1981 for Central Data Corporation of Illinois,: 4 followed in March 1981 by Paradyne Corporation's Z8001 product.: 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]: 3 
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 [note 2] Tandy more than doubled the Xenix installed base when it made TRS-Xenix the default operating system for its TRS-80 Model 16 68000-based computer in early 1983, and was the largest Unix vendor in 1984. 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.: 17  There was also a port for IBM System 9000.
SCO had initially worked on its 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. Microsoft and SCO then further engaged Human Computing Resources Corporation (HCR) in Canada, and a software products group within Logica plc in the United Kingdom, as part of making further improvements to Xenix and porting Xenix to other platforms. In doing so, Microsoft gave HCR and Logica the rights to do Xenix ports and to license Xenix binary distributions in those territories.
In 1984, a port to the 68000-based Apple Lisa 2 was jointly developed by SCO and Microsoft and it was the first shrink-wrapped binary product sold by SCO. The Multiplan spreadsheet was released for it.
In its 1983 OEM directory, Microsoft said the difficulty in porting to the various 8086 and Z8000-based machines 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.": Introduction
A generally available port to the unmapped Intel 8086/8088 architecture was done by The Santa Cruz Operation around 1983. 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.
The later 286 version of Xenix used the integrated MMU present on this chip, by running in 286 protected mode. 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, version 3.0 was upgraded to a Unix System III code base,: 9  a 1984 Intel manual for Xenix 286 noted that the Xenix kernel had about 10,000 lines at this time.: 1–7 It was followed by a System V R2 codebase in Xenix 5.0 (a.k.a. Xenix System V).
"Microsoft hopes that Xenix will become the preferred choice for software production and exchange", the company stated in 1981. Microsoft referred to its own MS-DOS as its "single-user, single-tasking operating system", and advised customers that wanted multiuser or multitasking support to buy Xenix. It planned to over time improve MS-DOS so it would be almost indistinguishable from single-user Xenix, or XEDOS, which would also run on the 68000, Z8000, and LSI-11; they would be upwardly compatible with Xenix, which Byte in 1983 described as "the multi-user MS-DOS of the future". Microsoft's Chris Larson described MS-DOS 2.0's Xenix compatibility as "the second most important feature". His company advertised DOS and Xenix together, describing MS-DOS 2.0 (its "single-user OS") as sharing features and system calls with Xenix ("the multi-user, multi-tasking, Unix-derived operating system"), and promising easy porting between them.
After the breakup of the Bell System in 1982, AT&T started selling System V. Microsoft, believing that it could not compete with Unix's developer, decided to abandon Xenix. The decision was not immediately transparent, which led to the term vaporware. It agreed with IBM to develop OS/2, 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 slightly less than 20% of SCO (this amount prevented both companies from having to disclose the exact amount in the event of an SCO IPO). And SCO would acquire both of the other companies that had Xenix rights, Logica's software products group in 1986 and HCR in 1990. When Microsoft eventually lost interest[clarification needed] in OS/2 as well, the company based its further high-end strategy on Windows NT.
In 1987, SCO ported Xenix to the 386 processor, a 32-bit chip, after securing knowledge from Microsoft insiders that Microsoft was no longer developing Xenix. 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.
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 1988. All internal Microsoft email transport was done on Xenix-based 68000 systems until 1995–1996, when the company moved to its own Exchange Server product.
SCO released its SCO Unix as a higher-end product, based on System V R3 and offering a number of technical advances over Xenix; Xenix remained in the product line. In the meantime, AT&T and Sun Microsystems completed the merge of Xenix, BSD, SunOS and System V R3 into System V R4. The last version of SCO Xenix/386 itself was System V R2.3.4, released in 1991.
Aside from its AT&T Unix base, Xenix incorporated elements from BSD, notably the vi text editor and its supporting libraries (termcap and curses). Its kernel featured some original extensions by Microsoft, notably file locking and semaphores,: 1.12 while to the userland Microsoft added a "visual shell" for menu-driven operation instead of the traditional UNIX shell. 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.
OEMs often added further modifications to the Xenix system.
Trusted Xenix was a variant initially developed by IBM, under the name Secure XENIX; later versions, under the Trusted Xenix name, were developed by Trusted Information Systems. It 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 be made available only to those 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 DoD'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 as late as 1995.
Microsoft licensed Seventh Edition Unix from AT&T in 1978 to produce the Xenix operating system initially for the PDP-11.
In the late 1970s Microsoft licensed Unix source code from AT&T, which at the time was not licensing the name Unix.
Xenix 286 is Intel's value-added version of the Xenix operating system released by Microsoft Corporation.
"The Santa Cruz Operation (SCO) was formed in 1979 by Larry and Doug Michels as a technical management consulting business. [...] SCO then changed its focus from consulting to the custom porting of Unix system software and applications. The first version of Unix which SCO developed and sold was called Dynix, a name subsequently used by Sequent. The operating system was based on Seventh Edition Unix and ran on the PDP-11. [...] In 1982, a joint development and technology exchange agreement was reached between SCO and Microsoft bringing together engineers from SCO and Microsoft to further enhance the Xenix operating system which was increasing in popularity. Microsoft and SCO worked together with Logica in the UK and HCR in Canada, producing enhancements to Xenix and porting Xenix to other platforms.
In 1984 a port of Xenix was made to the Apple Lisa by SCO and Microsoft, and was subsequently sold successfully by SCO as their first binary product, showing the success of the shrink-wrapped market. A port was also made to the Tandy model 16B.
In 1983 the PC [XT] emerged. SCO started porting to the 8088 but concentrated on the 8086, producing a release of SCO Xenix in 1984 which ran in 640 Kbytes with a 10 Mbyte hard disk. The release could support three or more users simultaneously, had multiscreen (virtual console) facilities, Micnet local area networking and enhancements added from 4.2BSD.
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.