Re: backward architecture, The Design of Design

According to Michael S  <already5chosen@yahoo.com>:

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.

The 360/22, /25, /30, /40, /50, /65, /75, and /85 were all compatible
implementations of S/360. You could write a program that ran on any of
them, and it would also run on larger and smaller models.

The /20, /44, and /67 were each for special markets. The /20 was
basically for people who still wanted a 1401 (admittedly a pretty big
market), the /44 for realtime, and the /67 for a handful of
time-sharing customers. The /67 was close enough to a /65 that you
could use it as one, often /67 timesharing during the day, and /65
batch overnight. The /91 and /95 were also compatible except that the
/91 left out decimal arithmetic, which OS/360 would trap and slowly
emulate if need be.

According to my understanding, since ~1970, IBM completely gave up on
all sorts of compatibility except backward compatibility.

No. When hey updated the architecture and then shipped multiple
implementations of each one. So when they went to S/370, there was the
370/115, /125, /135, /138, /145, /148, /158, and /168 which were
upward and downward compatible as were the 303x and 434x series. The
/155 and /165 were originally missing the paging hardware but later
could be field upgraded.

The point here is that you could write a program for any model, and
you could expect it to work unmodified on both larger and smaller
models.  Later one there was S/390 and zSeries, again each with models
that were both upward and downward compatible.

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

More like 1957. The IBM 705 was mostly backward compatible with the
702, and the 709 with the 704. But only in one direction -- if you
wanted your 709 program to work on a 704, you had to be careful not to
use any of the new 709 stuff, and since the I/O was completely
different, you needed suitable operating systems or at least I/O
libraries.
--
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly