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SubjectRe: Virtual machines, etc.
There are IBM Principle of Operation manuals available at a cost, it's a
weighty piece of work.
The simplistic view is that multiple operating systems are time-sliced, with
each having it's own environment defined. When a timeslice expires, the running
system environment has to be stored away and the next system loaded into the
hardware, etc., etc.
Within each environment, there is also time-slice switching for associated
With the latest stuff, a single machine with multiple CPU's can be configured
as LPAR's (Logical Partitions), so that e.g you can configure an LPAR such that
the Operating system running under it uses specified CPU's and these CPU's can
be shared amongst other LPAR's as desired. You can can give an LPAR say, 30% of
one CPU, another 20% and a third 50%. Each LPAR also has to have assigned memory
and I/O channels which are dedicated resources, so you may define an LPAR to
have 1.3 CPU's (1 dedicated and 0.3 shared), 2 Gig of main storeage and 66
I add that this is a simple view and just a broad basic outline. The full
manual describes it all in very gory detail. It's been years since I've read the
POO seriously, then I felt I needed my lawyer with me.
The thing to remember about 370/390 is that it started out life as 360 and is
more complicated than it needs to be. With the call for more power, features and
flexibility, IBM had to graft on lots of stuff as to do a fundamental design
that certainly would be more efficient, would be prohibitive in terms of costs.
The net result is a big compromise which none-the-less gets the job done.
... Sid Boyce...Amdahl(Europe)...44-121 422 0375
Any opinions expressed above are mine and do not necessarily represent
the opinions or policies of Amdahl Corporation.

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