Re: backward architecture, The Design of Design

According to Tim Rentsch  <tr.17687@z991.linuxsc.com>:

Michael S <already5chosen@yahoo.com> writes:
>
[...]
>

Can you, please, define the meaning of upward and downward
compatibility?

It's still in the S/360 Principles of Operation on page 5.

The System/360 model 20 is described in TDOD as being "upward
compatible", which means that programs that run on a model 20
could be run on higher-numbered models, but usually not vice
versa.

Not really. It only had 8 16-bit registers, numbered 8 to 15, with a
mutant form of addressing. If the high bit of the base register was 1,
it worked normally, if 0 the low three bits were prefixed to the
displacement which allowed absolute addressing. In assembler programs
we pretended that base registers 1, 2, 3, contained 4K, 8K, 12K.

The application instructions were a subset of the 360's, but the I/O
was completely different and much simpler, as were the interrupts. You
could write application code that would work on both a /20 and a real
360 if you were careful, but you needed different I/O libraries.

Most models of System/360 had the property that code that runs on
model M would also run on model N > M and on model K < M, for other
models in the set.  (The model 20, and arguably the model 30, were
exceptions, and probably some other models as well;

As I said, the /20, /44, and /67 were special, but all of the other
models had the same instruction set and I/O structure. The /30 was
quite slow but it was a real 360 and ran DOS or OS just like any other
model, just slower.
--
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