Difference between different unix versions

[Guy Harris]

>As far as I know, there are 3 flavors of unix: BSD which is public
>domain, System V which is moderately priced and Xenix which is
>astronomically expensive.

Incorrect.  There are several flavors, almost *all* of which are derived
from AT&T code and thus not public domain.  The three flavors you list
are all derived from AT&T code, and thus none are public domain.

>Just what is the difference between the 3 flavors.

There are several differences.  BSD is derived from an earlier VAX
version called 32V, which is pretty much a PDP-11 version called V7
ported to the VAX.  It has picked up some features from later UNIX
versions.  4BSD supports demand paging on machines that have it; it
prefers a "large" machine (with 32-bit "int"s, for example, and with
support for demand paging), although it has allegedly been ported, or is
in the process of being ported, to some set of 80286-based machines. 

In 4.2BSD and later releases, it comes standard with support for TCP/IP
networking.

System V is derived indirectly from V7 as well.  It is also derived from
an earlier PDP-11 version called PWB/UNIX.  It has picked up some
features from 4BSD.  Later versions support demand paging, but those
later versions prefer a "large" machine as well.  As distributed by
AT&T, it does not come standard with TCP/IP support; the latest versions
have a framework called STREAMS into which said support can be dropped,
and various people have basically ported various 4.xBSD versions of
TCP/IP into that framework.

Xenix is, I think, derived from V7 in its earlier incarnations, from V7
and System III (which is basically a S5 predecessor) in later
incarnations, and, I think, V7 and S5 in its most recent incarnations. 
I don't think TCP/IP comes standard with Xenix from Microsoft; I don't
know which Xenix versions support it.

>And if all of them can run any unix program, what makes one buy Xenix,
>when they can get BSD?  

1) Not all of them can run any UNIX program.  No feature set of those
   versions is entirely a subset of the feature set of some other
   version.

2) You can't get BSD for every machine in existence; the same applies to
   vanilla System V as distributed by AT&T and to Xenix.