Path: news.eternal-september.org!eternal-september.org!feeder3.eternal-september.org!i2pn.org!i2pn2.org!.POSTED!not-for-mail
From: quadibloc <quadibloc@gmail.com>
Newsgroups: comp.arch
Subject: Re: Why I've Dropped In
Date: Fri, 13 Jun 2025 11:52:13 +0000
Organization: novaBBS
Message-ID: <697be23c211740763457a5f3b09df55d@www.novabbs.com>
References: <0c857b8347f07f3a0ca61c403d0a8711@www.novabbs.com> <dd6e28b90190e249289add75780b204a@www.novabbs.com> <ec821d1d64555055271e3b72f241d39b@www.novabbs.com> <8addb3f96901904511fc9350c43917ef@www.novabbs.com> <102b5qh$1q55a$2@dont-email.me> <48c03284118d9d68d6ecf3c11b64a76b@www.novabbs.com> <102cd09$23hcu$1@dont-email.me> <42cac84be6fb35b9e468cc174919bc4a@www.novabbs.com> <102eske$2nqdn$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Info: i2pn2.org;
	logging-data="328842"; mail-complaints-to="usenet@i2pn2.org";
	posting-account="GSAUMsvIs05PgSAevbIzdWiOy1BcuThtiv166p5NnMk";
User-Agent: Rocksolid Light
X-Rslight-Site: $2y$10$iHmUHhtH192V3wkk8sqWQuuhRkFZTvjjj67hgIddkBBGqPyay1yRa
X-Spam-Checker-Version: SpamAssassin 4.0.0
X-Rslight-Posting-User: 7260c650ae4d5ba82d3b6b1eab0ac1b8653ff052

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memory superseded.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