Re: backward architecture, The Design of Design

Michael S <already5chosen@yahoo.com> writes:

[...]

Can you, please, define the meaning of upward and downward
compatibility?  I had never seen this terms before this thread,
so it is possible that I don't understand the meaning.

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.

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;  I don't have
enough information to be precise.)  The point is that most models
were compatible with both higher-numbered ("upwards compatible")
and lower-numbered ("downwards compatible") models.