[rescue] More IBM emulation fun

Bill Bradford mrbill at mrbill.net
Tue Feb 10 01:07:26 CST 2009

On Mon, Feb 09, 2009 at 11:08:00PM -0700, Scott M wrote:
> At this URL, this doc:  Introduction_To_VM370_Course_Jul75.pdf
> has this chapter heading:  "VIRTUAL MACHINE DESCRIPTION". 
> Virtualization in 1975?  Really? 

Yes.  It's not a new idea by a long shot.

>From http://en.wikipedia.org/wiki/Full_virtualization

"The prototypical illustration of full virtualization is in the control
 program of IBM's CP/CMS operating system btrated with IBM's CP-40 research
 system in 1967, then distributed via open source in CP/CMS in 1967-1972,
 and re-implemented in IBM's VM family from 1972 to the present."

> I guess I should not have been surprised, since the "VM" in VM370 
> stands for "Virtual Machine".  
> http://en.wikipedia.org/wiki/VM370
> However with so little memory available, it seems strange that 
> anyone would want to put more than one OS on a machine of that era. 

>From http://en.wikipedia.org/wiki/System/370

"The original System/370 line underwent several architectural improvements
during its roughly 20-year lifetime. The first and most significant change
was the introduction of virtual memory, which was first made generally
available in 1972 via IBM's "System/370 Advanced Function" announcement."


"Virtual memory is a computer system technique which gives an application
program the impression that it has contiguous working memory (an address
space), while in fact it may be physically fragmented and may even overflow
on to disk storage."

> Page 8 of this doc shows a "256K DOS BATCH" virtual machine running 
> on a S/370 Model 145 along with five "320K CMS" virtual machines. 
> But DOS didn't exist in 1975!   -Turns out this isn't MS-DOS, but 
> IBM DOS/360 or some variant of it: 
> http://en.wikipedia.org/wiki/DOS/360

Right.  DOS as in "Disk Operating System", and that term is not restricted
to the PC-DOS or MS-DOS software from the 80s and 90s that most of us are
familiar with.

> Maybe DOS/360 and CMS were single-user and virtualization was a way 
> to make the machine multi-user?  

That's one reason to run VM - give multiple users each their own
installation of CMS.  Another reason was to make maximum use of the
hardware and keep it busy as much as possible.

You can actually run VM/370 (aka VM/ESA, aka z/OS nowdays) within itself,
multiple levels deep, if you want.

This is a really good read:


Be sure and check out the linked PDFs in the "References" section.


Bill Bradford 
Houston, Texas

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