Virtual machines

[Rob Warnock]

Andrew Klossner <andrew at orca.UUCP> writes:
+---------------
| CMS is a single-user IBM 370 operating system.  When it runs
| stand-alone on a 370, it supports a single user.  Several instances of
| it can be running under CP, each in a distinct virtual machine...
|
| There is some user interface in CP, but not much ... it's mostly of the
| "simulate a front panel" variety, like the command to boot an operating
| system in your virtual machine.
+---------------

I find it interesting to compare TOPS-10, which was originally (in the
PDP-10/30 configuration) a single user monitor. When it became a timesharing
system, the user view of a single-user system persisted. Your "job" was a
virtual machine, albeit with system calls in the "hardware" (much like the
later versions of CP with the extended "diagnose" call).

You managed your "core image" via front-panel-like commands, such as "examine"
and "deposit". It was perfectly legal to sit and type octal machine code into
(your job's) core and run it. You could "get" an image, run it, halt it, patch
it, and "save" it again. When a program exited, the core image was still there.

In fact, certain large programs depended on this -- after compiling one
of them you would run it one time so it could build its symbol tables (or
whatever), then it changed its starting address and exited so you could save
it with the startup code already done (a trick "ps" could use, no doubt!).

Rob Warnock
Systems Architecture Consultant

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