Re: architecture, The Design of Design

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

My impression is that until S/360 there was no such thing as different
by 100% SW compatible models.

>
I think a counterexample is the LGP-30 (1956) and its successor
the LGP-21 (1963).

They were pretty close but it says on the intertubes that the -30 put
memory words 9 apart on the drum and the -21 put them 18 apart, which
I presume means that you would need to arrange your data differently
to get good performance.

Another example may be the IBM 709 and IBM 7090, both done in the 1950s.

They were pretty similar but the 7090 had a more complex channel and
new instructions to manage it. There was a trap mode that caught the
old I/O instructions they used to run 704 or 709 coe on a 7090 but of
course not vice versa.

Compare that to S/360 where every model had the same channel
interface, the same I/O instructions, and the same I/O interrupts.

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