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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Sun, 30 Jun 2024 11:05:12 -0000 (UTC)
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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.  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.