Re: architecture, The Design of Design

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

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.

Well, yes, but another 360 innovation was the whole idea of computer
architecture, as well as the term. It was the first time that the
programmer's view of the computer was described independently of any
implementation.

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.

I wonder if that was the way that the 704 OR'ed the index registers.
There were three of them, numbered 1, 2, and 4, so if your index field
was 5, it OR'ed registers 1 and 4. It subtracted the index (or the
OR'ed combination of indexes) from the base address, so it would have
taken some really tricky programming to make use of that. But someone
must have since they documented it and it continued to work on the
709, 7090, and 7094 until they provided 7 index registers and a mode
bit to switch between the old OR and the new 7 registers.

I have never found anything that says whether it was deliberate or an
accident of the 704's implementation, and I have looked pretty hard.

--
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