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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: architecture, The Design of Design
Date: Wed, 8 May 2024 14:18:04 +0300
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On Wed, 8 May 2024 10:03:51 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:

> Michael S <already5chosen@yahoo.com> schrieb:
> 
> > My impression is that until S/360 there was no such thing as
> > different by 100% SW compatible models.  
> 
> I think the important thing was that S/360 was designed and built,
> right from the start, as a _series_ of compatible computers, which
> were upward- and downward-compatible.  They had the challenge
> of designing an architecture where the instructions for the
> high-end supercomputers still needed to work (although slowly)
> on the low-end bread and butter machines, and what was efficient
> on the low-end bread and butter machines should not constrain the
> high-end supercomputers.
> 

Of course, there is a theory and there is a practice.
In practice, downward compatibility lasted ~half a year, until Model 20.
Upward compatibility did not fare much better and was broken
approximately one year after initial release, in Model 67.
That is, if I didn't get upward and downward backward.

According to my understanding, since ~1970, IBM completely gave up on
all sorts of compatibility except backward compatibility. In more
recent decades it was further reduced to application-level backward
compatibility.

> Most other computer series were built one at a time, with successors
> usually extending the previous ones (which IBM also did with the /370,
> series).  The VAX may have been another such line - DEC did not
> release several models all at once, but they did release the cheaper
> and slower 11/750 after they had released the 11/780.

I'd think that by 1977 (VAX) backward compatibility was widespread in
the industry.