According to https://en.wikipedia.org/wiki/Xenix by the beginning of the nineties, SCO was selling 32-bit 386 versions of both Xenix and Unix. According to https://news.ycombinator.com/item?id=12235900 their Unix cost $1000, and I have an unsourced figure of $500 for Xenix. From the above Wikipedia article

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.

What exactly were the technical advantages that made their Unix worth more than Xenix?

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    One starting point: Xenix was a port of UNIX system III, where SCO UNIX was a port of UNIX System V. Feb 25, 2021 at 4:25
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    But I gather that the question is asking about the differences between Xenix and SCO UNIX at the time of the latter’s release, not the end of the Xenix line. Feb 25, 2021 at 6:08
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    @Schezuk Something ain't right...the PC/XT was an 8088 machine. There were definitely some 8086 machines that ran Xenix - e.g., Altos 586 - I had one, but I didn't know better at the time - what I knew was the CP/M & DOS world so I got it with MP/M-86 - which was actually quite nice, though I had to hack it a bit to get past some stupid limitations. Feb 25, 2021 at 7:11
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    There was also a 286 version of Xenix but it was horribly slow. The 386 version was much snappier even when run on the same hardware. My first work PC when I started working in 1990 was a 386/33 and had Xenix 286 installed. After testing 2 or 3 things I reformatted the disk to install DOS 3.3. I was recruted for C embedded programming and was not in the DB division which used Informix on SCO-Xenix/Unix. Feb 25, 2021 at 8:25
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    The differences between SCO-Xenix and SCO-Unix felt like comparing 2 Linux distributions that are 4 years apart, i.e. a lot of small details have changed but the global feel is the same. Feb 25, 2021 at 8:27

3 Answers 3


Using SCO UNIX describes the history of XENIX and SCO UNIX and provides a brief summary of the technical differences.

As Raffzahn explains, SCO UNIX is the successor to XENIX. XENIX is a licensed version of UNIX; it was called XENIX because initially, AT&T didn’t allow its licensees to use the UNIX trademark. This was relaxed in 1989, which allowed SCO to use the trademark.

However there’s more than a name change to SCO UNIX compared to XENIX, enough to warrant a different pricing structure (at least, enough for marketing to justify a different pricing structure). SCO UNIX was intended to be a “more compatible” release, based on AT&T System V Release 3.2:

  • unlike XENIX, SCO UNIX could run any 386 System V binary;
  • SCO UNIX was POSIX conformant with a few minor exceptions;
  • SCO UNIX was X/Open conformant;
  • SCO UNIX was a DoD “trusted” system (capable of running at C2 security level).

On the technical side, SCO UNIX introduced the Acer Fast File System, and added a number of device drivers.

None of this would have been impossible to add to XENIX of course, and SCO could have released an updated version of XENIX with all the above. Renaming the product to “SCO UNIX” helped support the UNIX compatibility story, and allowed SCO to continue selling multiple products in parallel without introducing too much confusion:

  • XENIX 286 for 286 PCs;
  • XENIX 386 for users of 386 PCs requiring a small and fast operating system;
  • SCO UNIX for users interested in greater compatibility and security, at the cost of reduced performance on the same hardware.
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    I wouldn't consider it really a big enough change - after all, Xenix went thru several changes S7 -> SIII -> SV before with simply changing it's version number, so going from R2.3 to R3.2 while adding a few feature seams no big issue. Or is it? In fact, as I understand it, they were still for most parts the same source tree. But yeah, marketing defies all logic - always :)
    – Raffzahn
    Feb 25, 2021 at 11:31
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    Right, I agree it’s not so much about the changes themselves as marketing, with the availability of the name... Feb 25, 2021 at 11:35
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    Put another way, even if the reasoning behind the segmentation isn’t consistent compared to previous Xenix releases, there are technical differences between Xenix and SCO UNIX, which is what the question is about. Feb 25, 2021 at 12:08
  • True every new release contains new material. I just think it's important to point out that these are less than it might sound and the sales only overlapped in part like usual with release cycles, as the question seems under the assumptions there were many or fundamental ones.
    – Raffzahn
    Feb 25, 2021 at 12:25
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    The licensing arrangement with AT&T may have incurred additional costs per down-stream license sold under the Unix name, or SCO may have been amortizing a large up-front expense in that regard. The bean counters may have worried that SCO-Unix would disrupt their current Xenix revenue streams. Really, all of the above may have contributed to the additional cost.
    – jwdonahue
    Feb 25, 2021 at 20:42

What exactly were the technical advantages that made their Unix worth more than Xenix?

For most parts: The Name. Otherwise it's simply the next release of SCO's unixoide OS.

They were only sold in parallel for a short time (ca 1989/90). While the latest Xenix version was based on System V R2.3, SCO Unix started out as System V R3.2. But using the same driver interface as Xenix.

Xenix had a long history of development since ca. 1980, but when SCO acquired the rights to the Unix name, it was clear that an 'all new' product was to be released :))

Originally Xenix was developed by Microsoft. MS licenzed Unix V7 already in the late 1970s. They did, at that time, firmly believe that Unix would be the future OS for all of their products (they were a language company) - and most of the market. They did license the source code, but not the name, so they came up with Xenix as their brand name for Unix.

MS did not sell Xenix to end customers, they only OEMed it to other companies. Much the same way they sold MS-DOS only to OEM before DOS 3.2. Notable licencees were Intel, IBM, Tandy, Siemens (Sinix) or, well, SCO.

Xenix developed from being a System 7 over System III to System V related. Unlike often assumed, Unix does not need a fully fledged MMU, it only simplifies life. In fact, SCO's twist on Xenix was the low end market. Their Xenix 3.0 (System III alike at that time) was intended for IBM-PC (MS-Xenix did of course use 286 protected mode). Not a rocket, more a soap box racer, still, worked quite fine and covered a market other unixoid systems didn't.

It wasn't until the mid 1980's (85?) that SCO Xenix was as well available for 286 class machines - now System V alike. At a later point 386 got native support as well.

In the late 80s (1987?) MS sold all rights, code and customers of Xenix to SCO for a considerable stake in SCO. Now SCO did all development on their own.

Again later (1989?) SCO bought the right to the Unix name. After that the next major Xenix release was of course renamed SCO Unix. SCO Xenix was, under that name, still maintained for 3 years for existing customers - like other releases before - but otherwise the name was history.

Other MS licencees, like Siemens, continued their development independent way into the 90s and 2000s.


One other difference: Xenix 2.3.2 did not have a block buffer cache. Every version of SCO Unixs I've used did.

Consequence: On an IBM PS/2 model 80 (?, 20 MHz 386, Micro Channel) the max throughput to the hard drive was 35 KB/s. On an AST Research 486 with a DPT SmartCache SCSI controller (high end for 1992!) we maxed out at 45 KB/s.

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