Unix History

[Henry Spencer]

> This raises the burning question:  whatever happened to
> Systems I and II, and, especially, IV? ...

Adam Reed and Matt Perez have already answered this moderately well, but
I'll throw in a few more tidbits.  System I was, approximately, PWB, which
was released.  System II was an improved PWB, incorporating some useful
things like a souped-up shell; it was never released because just then
AT&T was not sure it wanted to continue distributing useful new goodies
to potential competitors.  After the decision to continue with distribution
was made, System III was released, well after it was in use internally.
System IV never made it out because it was already in use when SysIII came
out, and the decision to bring external releases into sync with internal
ones overtook it.  System V was then released.  Since "System V" has now
become a magic marketing buzzword, the top-level numbering is absolutely
frozen for external purposes, and all future releases will be V.something
rather than VI, VII, etc.

Fortuitously, this happens to avoid 4, 6, and 7, which correspond to other
well-known flavors of Unix.

If you want to get a look at what System IV was like, check out the PDP11
distribution of System V.  PDP11 SysV is really SysIV.  Although AT&T won't
admit it in so many words, they effectively abandoned work on PDP11 Unix a
long time ago.  For example, although some of the SysV performance work
wouldn't fit on the 11, *some* of it would.  None of it was applied to
PDP11 SysV.  The PDP11 SysV shared-memory stuff is also different from and
incompatible with the regular SysV version.
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