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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: architecture, The Design of Design
Date: Wed, 08 May 2024 03:37:31 -0700
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anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> John Levine <johnl@taugh.com> writes:
>>
>>> Well, yes, but another 360 innovation was the whole idea of computer
>>> architecture, as well as the term.  It was the first time that the
>>> programmer's view of the computer was described independently of any
>>> implementation.
>>
>> I don't buy it.  An architecture is just a description of system
>> behavior, and surely there were descriptions of system behavior
>> before System/360.  Even in the 1950s companies must have changed
>> implementations of a given model while still conforming to its
>> earlier description.
>
> Sure, the 7094 was a compatible successor of the 704,

I think a more accurate description is to say that the IBM 709 was
an upgraded version of the IBM 704, the IBM 7090 was a compatible
replacement for the 709, and the IBM 7094 was an upgraded version
of the 7090.  In both cases the upgrades included changes.  The
7094, for example, still had a three-bit field to select an index
register, but there were 7 index registers, not 3, only one of
which could be selected rather than OR-ing together all the index
registers whose bits were on.  To be fair I should add that the
7094 could be run in a compatible mode where only 3 index registers
were used, with OR-ing like in the earlier models, but there were
other changes (or maybe only additions) as well.  The 7094 may have
been upward compatible relative to the 7090, but it wasn't plug
compatible, and TTBOMU wasn't even upward compatible relative to
the 704.

> but the idea of
> implementation independence turns out to be much more profound than
> most people (probably including its inventors at the time) realized.

The point I was trying to make upthread (and whose significance seems
to have been missed by some people) is that the important lessons had
already been learned and understood -- by some key people at IBM,
although certainly not all -- before the System/360 effort started.
It isn't an accident that IBM decided to make an upward- and downward-
compatible family of computer models.  That the System/360 effort
ended up producing a system description that is independent of any
particular model, and the benefits that accrue as a result, is simply
a consequence of that earlier and deeper understanding.