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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: backward architecture, The Design of Design
Date: Wed, 29 May 2024 21:43:15 -0700
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"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.

> 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.
That is very different from deciding IBM wasn't going to sell
the old hardware.  Brooks points out that the decision to
drop all further development of IBM's six existing product
lines was made by CEO Thomas Watson (Jr).