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From: jgd@cix.co.uk (John Dallman)
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Sun, 30 Jun 2024 18:38 +0100 (BST)
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In article <v5s173$jl70$1@dont-email.me>, SFuld@alumni.cmu.edu.invalid
(Stephen Fuld) wrote:

> John Dallman wrote:

> > Virtual memory was pretty new technology at the time, and 
> > required a disk or drum. The central idea of /360 was having 
> > the same ISA across a wide range of machines, and virtual 
> > memory wasn't affordable at the low end at the time, AFAICS. 
> 
> But IIRC even low end S/360s required a disk, at least to IPL(boot)
> from. 

Are you sure? Per Wikipedia, the lowest-end real S/360, the Model 30,
could run with only card equipment, running BPS, or with only tape drives,
under TOS. 

<https://en.wikipedia.org/wiki/IBM_System/360_Model_30#System_software>

BOS was a really minimal OS for an 8KB RAM machine with one disc drive,
and DOS was less minimal. 

The Model 30 was apparently one of the most popular machines in the early
days of S/360. Being able to build such small machines was a strong
commercial consideration for the company, and thus the architecture. 

> But Virtual memory is more expensive than the hidden base
> registers, as you need page tables. probably a TLB, etc.

Yup, and it pushes up the minimum amount of RAM you need. 

> > So the problem wasn't with MFT memory management APIs or their
> > implementation, but that a /360 site had to find a set of 
> > partition sizes that allowed for all the combinations of 
> > programs that they needed to run simultaneously. This was 
> > inevitably wasteful of memory, because each partition had 
> > to allow for the largest program that could be required to 
> > run in it. 
> > 
> > Is that correct?
> 
> That is correct for OS/MFT, but not for OS/MVT.

Right. And MVT had dynamic memory allocation, but since it used direct
physical addresses, it suffered progressive fragmentation, and would
presumably have to be IPL'ed at intervals? 

John