If I remember correctly, all of the "real"
members of the 3B family
(i.e. 3B2, 3B5, 3B15 and 3B20) shared a common "virtual"
instruction set called (I think) IS25 - it was the job of the assembler
to translate IS25 into the actual machine code for the specific
processor used in each machine.
IS25 was a little curious because it only defined
that were likely to be of use to the C compiler - thus there was a
"push" instruction so that the compiler could push function arguments
onto the stack, but no "pop" instruction because the C compiler
never generated it.
IS25: just so. I managed to retrieve the scanned PDF for
the manual for this virtual instruction set. It's an
internal memo, but I'll send it if anyone asks. It's
2.8MB of page images and is 108 pages long.
The memories of Lindsly and Lowenstein are also apposite.
AT&T donated quite a few machines (3B20 and 3B2) to universities,
and though they appreciated the thought, there were
various drawbacks--the gift didn't include maintenance, for