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:

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 hadn't thought about it that way.
It does make sense that on a timesharing system, virtual memory meant
that different users would not have to share the same memory space, so
programs wouldn't have to be relocatable.
But if you drop base registers for that reason, suddenly you are forced
to always use virtual memory. So this ISA stops being suitable for
smaller-sized systems without this big-system feature. That would be the
argument I would be tempted to use here.
It could be fallacious, since _really_ small systems get along just fine
without base registers - minicomputers in the old days, the TI 9900, and
so on. So base-index addressing may just serve a nonexistent
intermediate category.
Of course, then why did the 68020 support it, I could ask.
John Savard