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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Sun, 30 Jun 2024 16:18:55 +0000
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Thomas Koenig wrote:

> Michael S <already5chosen@yahoo.com> schrieb:
>> On Sun, 30 Jun 2024 10:44:34 -0000 (UTC)
>> Thomas Koenig <tkoenig@netcologne.de> wrote:
>>
>>> 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
>>> >
>>> > 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?
>>>
>>
>> Why would I want to use different address?
>
> Memory overlap and fragmentation after having started and stopped
> (or swapped out) too many processes.  Remember, these were
> physical-memory machines.

There were also the base-bounds machines.

>                           You could load a process to a certain
> place, but you had more running, and one of them was swapped out
> or terminated, it left block of available memory where the next
> process didn't necessarily fit.
>
> They would have fared better by assigning a base register (or two,
> one for data and one for code) invisible from problem state
> and handled by the OS.  Not sure why they didn't do so, but
> reading the literature seems to imply that they did not think it
> through.  Now, of course, we have the benefit of hindsight.

In that sense, virtual memory is simply an infinite amount of
base registers.