Re: Why I've Dropped In

On 6/12/2025 8:09 PM, quadibloc wrote:

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.

No.  Other systems in the S/360 time frame (i.e. before virtual memory) used a system "base register", that was hidden from the user (but was in its context), that was set by the OS when the program was loaded, or if it was swapped out, when it was swapped in again.  It was reloaded whenever the program gained control of the CPU.  Besides the advantage of not requiring a user register for that purpose, it allowed a program to be swapped in to a different memory address than it was swapped out from, a feature the S/360 didn't enjoy.
snip

Of course, then why did the 68020 support it, I could ask.

Someone more familiar with the 68020 would have to answer that.
But in any case, the answer to Thomas's original question is that there is no use case for it now, and the cost in instruction bits is too large to consider using them.
--
  - Stephen Fuld
(e-mail address disguised to prevent spam)