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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: The Design of Design
Date: Fri, 26 Apr 2024 07:39:13 -0000 (UTC)
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John Levine <johnl@taugh.com> schrieb:

> S/360 invented eight bit byte addressed memory with larger power of 2
> data sizes, which I think all by itself is enough to explain why it
> survived. All the others, which were word or maybe decimal digit
> addressed, died. Its addresses could handle 16MB which without too
> many contortions was expanded to 2GB, a lot more than any other design
> of the era. We all know that the thing that kills architectures is
> running out of address space.

Brooks wrote that the design was supposed to have been 32-bit
clean from the start, but that the people who implemented the BALR
instruction (which puts some bits of the PSW into the high-value
byte) didn't follow that guideline.  He blamed himself for not making
that sufficiently clear to all the design team.

He also commented on the carefully-designed gaps in the opcode space;
extensibility was designed in from the beginning.  @John S: Another
important point about S/360 you might want to follow, as Mitch
keeps telling you...

> I thought the PDP-10 was swell, but even if DEC had been able to
> design and ship the Jupiter follow-on to the KL-10, its expanded
> addressing was a kludge. It only provided addressing 8M words or about
> 32M bytes with no way to go past that.

Reading

http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp10/KC10_Jupiter/ExtendedAddressing_Jul83.pdf

I concur that it was a kludge, but at least they seem to have
allowed for further extension by reserving a 1-1- bit pattern,
as an illegal indirect word.

However, one questions.  Designs like the PDP-10 or the UNIVAC
(from what I read on Wikipedia) had "registers" at certain
memory locations.  On the PDP-10, it even appears to have been
possible to run code in the first memory locations/registers.

It seems that the /360 was the first machine which put many
registers into a (conceptually) separate space, leaving them open
to implementing them either in memory or as faster logic.

Is that the case, or did anybody beat them to it?