Re: ancient OS history, ARM is sort of channeling the IBM 360

On Sun, 30 Jun 2024 10:44:34 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:
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John Dallman <jgd@cix.co.uk> schrieb:

In article <87ed8e7os5.fsf@localhost>, lynn@garlic.com (Lynn
Wheeler) wrote:
>

back to IBM decision to add virtual memory to every 370 ... aka MVT
storage management was so bad that regions had to be specified four
times larger than used

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What was the problem with the memory management? My experience of
systems without virtual memory doesn't include any that shared the
machine among several applications, so I have trouble guessing.

>
Imagine a process which resides at a certain address.  It contains
code, data, and pointers to data.  Now you swap it out and want
to reload it.  You can use the same base address, then everything
is fine.  Or you can use a different one, where do the pointers
point, especially registers which contain addresses?
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Why would I want to use different address?

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The /360 tried to solve this via base pointers, which all addresses
were supposed calculated relative to to.  Hence the RX and RS
instraction all had a base register + 12 bit offset for their
addressing modes - swapping out the base registers (if you knew
which ones they were, was this info in the executable?)  should have
worked.  But the SS instructions for decimal arithmetic did not have
base pointers, so that solution did not work in the general casse.
>
Going to virtual memory from the start would have saved the
base pointer issue, and would have allowed 16-bit displacements,
also saving registers in the case where 12-bit displacements were
not enough.