Re: architecture, The Design of Design

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.