Re: The Design of Design

John Levine <johnl@taugh.com> writes:

According to MitchAlsup1 <mitchalsup@aol.com>:
>

For a modern ISA, the architect should specify that various bits
of the general format "must be zero"* when those bits are not used
in the instruction.

>
That was another innovation of the 360.  It specifically said that
unused bits (of which there were a few) and unused unstructions (of
which there were a lot) were reserved.  The unused bits had to be
zero, and the instructions all trapped.

I would describe this not so much as an innovation but just as
applying a lesson learned from earlier experience.  Some earlier IBM
model (don't remember which one) had the property that instructions
were somewhat like microcode, and some undocumented combinations of
bits would do useful things.  Needless to say these undocumented
behaviors were discovered by programmers in the wild, and when IBM
went to produce a newer model they found that they had to duplicate
these "accidental features" or old programs wouldn't work on the
newer, supposedly compatible, model.  To stop that from happening
again, the people doing System/360 insisted that there be no
"accidental" behaviors, especially since an "accidental" behavior
would likely be different on different models of the same line.

I remember hearing or reading about this somewhere, but I don't
remember where.  It may have been when I took Fred Brooks's
architecture class when I was a grad student.