Re: Why I've Dropped In

On Thu, 12 Jun 2025 15:44:14 +0000, Stephen Fuld wrote:

On 6/12/2025 8:00 AM, quadibloc wrote:

On Wed, 11 Jun 2025 17:05:13 +0000, Thomas Koenig wrote:
>

What is the use case for having base and index register and a
16-bit displacement?

>
The IBM System/360 had a base and index register and a 12-bit
displacement.

>
Yes, but as I have argued before, this was a mistake, and in any event
base registers became obsolete when virtual memory became available
(though, of course, IBM kept it for backwards compatibility).

I have been thinking about this, and I don't think that base registers
only existed to allow program relocation in a crude form that virtual
memory superseded. They also existed simply to avoid having to have a
displacement field large enough to address all of memory in every
instruction. In fact, I think this was the primary reason, and using
them to relocate code and data was a nice idea that came after.

Most microprocessors have a base register and a 16-bit displacement.
>
So this lets my architecture be a superset of both of them.

>
But your different ISA format, etc. means that it is not a true
superset.  That is, any S/360 program would have to be recompiled to run
on your architecture.  So it is only for some sort of "conceptual", but
not actual compatibility that is only for assembler language programmers
(and compiler writers).

There is the old saw about programmers who can write FORTRAN programs in
any language. The intent here isn't compatibility, but simply that
assembler programmers aren't forced to adopt a new mindset.
John Savard