🚀 go-pugleaf

Anne & Lynn Wheeler wrote:
> 
> "Stephen Fuld" <s.f...@worldnet.att.net> writes:
> >
> > Since there have been lots of answers to number 1 and to you last paragraph,
> > but none to number 2, I'll take a shot at it.  I am far from an expert here,
> > but AFAIK you nailed one of the reasons but there are others.
> >
> > Some people run VM so thy can run lots of CMS users ...
> >
> > Some people run VM so they can take advantage of modern hardware ...
> >
> > Many independent software vendors run it to allow them to support several
> > versions of several different operating systems ...
> 
...

An important additional case was that people inside IBM ran VM so
they could more easily bring up the next version of the
"strategic" operating system - e.g., MVS (now zOS). Possibly more
than anything else, this ensured that VM would always be around.

An amusing related story: A friend of mine, who used to work on
VM in the mid-70s, particularly the Virtual Memory part (duelling
acronyms here), was having trouble getting system time on new
machines to bring up the latest version. The MVS (or whatever it
was then) folks were hogging all available machines. So one
night, he did a midnight anti-repair and sabotaged the system
console. MVS wouldn't come up without it. VM didn't care; it
could use anything able to do text I/O. Result: he got to put a
version of VM on the system, as a "favor" so the MVS folks could
continue work, using VM which provided a virtual console -- and
he could use that hardware too, to bring up the next VM release.

Related -- Once upon a time that same friend told me that you
actually have to be able to do three levels of virtualization of
memory to be general -- the top-level VM system just doing it is
not enough; you have to be able to nest a VM system virtualizing
what the top-level system is also virtualizing, and a third level
below that. I never did understand why the third level was
necessary, but this guy was very, very good and knew that code
intimately, so he was probably right.

(I knew that if I wanted long enough Lynn & Anne would fill in
lots of the other answers to the original question. :-))

Greg Pfister