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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: backward architecture, The Design of Design
Date: Thu, 30 May 2024 14:12:01 -0700
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"Stephen Fuld" <SFuld@alumni.cmu.edu.invalid> writes:

> Tim Rentsch wrote:
>
>> "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid> writes:
>>
>>> Tim Rentsch wrote:
>>>
>>>> "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid> writes:
>>>>
>>>>> The key innovation that IBM made with the S/360 was to announce
>>>>> systems with a wide range of performance *at the same time*,
>>>>> i.e. different Y values and the same X value.
>>>>
>>>> I would argue that this property is only one of three factors
>>>> that made System/360 successful, and perhaps the least important
>>>> of the three.  The other two factors are, one, addressing both
>>>> business computing and scientific computing rather than having
>>>> separate models for the two markets, and two, replacing and
>>>> discontinuing all of IBM's other lines of computers.  I think
>>>> it's hard to overstate the importance of the last item.
>>>
>>> I didn't mean to imply that the performance range was the only
>>> factor in S/360's success.  Just that with S/360, IBM was the first
>>> to use that strategy, and it was a factor in its success.
>>
>> We agree that having multiple price/performance models helped
>> System/360 succeed.  Where I think we don't agree is how big
>> a factor it was,or how innovative it was.  Supporting multiple
>> models that differ only in price/performance is an obvious
>> idea, even in the early 1960s.
>
> I don't have an opinion on how big a factor it was, but if you think it
> was innovative, can you name any other computer manufacturer who did
> it, i.e. announced at the same time multiple models with difference
> performance?

I think it was an obvious idea at the time, even before IBM started
work on System/360.  What made System/360 different was not the idea
of having a common architecture but the large range of performance for
the various models.  Besides being an impressive feat technically, it
clearly showed IBM's commitment to the architecture, not just in the
present but for many years into the future, and I believe that
commitment being demonstrated (ignoring for the moment the
discontinuing of other product lines) was the larger part of the
success of System/360.

>>> As to the other two factors you mentioned, I don't necessarily
>>> disagree, but I do want to note that discontinuing older lines of
>>> computers was factiltated by the ability of various S/360 models to
>>> emulate various older computers.  So a site that had, say a 1401,
>>> could upgrade to a S/360 mod 30, which could run in 1401 emulation
>>> mode, so sites could keep their old programs running until they
>>> were replaced by newer nativve S/360 applications.  Similarly for
>>> 7080 emulation on s60/65s.  There were probably others that I don't
>>> know about.
>>
>> Read the chapter on System/360 in The Design of Design and you
>> may change your mind.  It isn't surprising that IBM provided
>> a path for people who wanted to keep running their old software.
>
> Again, did any other manufactorer at the time provide, in their new
> models, emulation of their older models with radically different
> archotectures?

Emulation was not a new idea, and there were historical precedents,
for example the 7094 being able to run 7090 code even though how
indexing was done on the two machines was completely different
(admittedly this example is on a smaller scale than emulating a
completely different architecture).  Also it isn't like IBM knew
going in that emulation would be the way that they would address the
issue of bringing old customers forward.  The System/360 effort
started in 1961 and was announced in April 1964.  Quoting from TDOD:
"At a crucial point in January 1964, William Harms, Gerald Ottoway,
and William Wright devised almost overnight a microprogrammed
emulation of the 1401 on the Model 30.  This mightily addressed the
biggest single customer conversion problem."  IBM knew they needed
to provide a way forward but didn't know at the start how they would
do that.  It was a happy byproduct of the decision to use microcode
in the smaller 360 models that emulation was possible, however IBM
didn't realize or plan that until fairly late in the game.