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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Sun, 30 Jun 2024 16:05:59 -0000 (UTC)
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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.  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. 

Precisely! This is what the, roughly contemporary, Univac 1108 did.  It
worked will for several decades.  Eventually the 1108's successors went
to a paging scheme (but with ~ 16 KB pages), to avoid the fragmentation
issues from using variable sized chunks (called banks in 1100
terminology) of memory.



> Not sure why they didn't do so, but
> reading the literature seems to imply that they did not think it
> through. 


Yes.  John Levine said they thought that changing the, user visible,
base registers would be doable  :-(

The invisible base registers may require an extra adder in the CPU when
computing addresses, but this is much less overhead than paging
requires.


> Now, of course, we have the benefit of hindsight.


Yup!



-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)