Re: What are the various alternate uses for the PC's LSB ?

From	whygee <yg@yg.yg>
Newsgroups	comp.arch
Subject	Re: What are the various alternate uses for the PC's LSB ?
Date	2011-05-09 09:02 +0200
Organization	Aioe.org NNTP Server
Message-ID	<iq86a2$vue$1@speranza.aioe.org> (permalink)
References	<iq732c$ku0$1@speranza.aioe.org> <015bd6ff-60b2-4e63-94e8-9795c5ee478b@k17g2000vbn.googlegroups.com>

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Hello,

robertwessel2@yahoo.com wrote:
> zSeries (the current incarnation of S/360) does something similar
> with  the low bit and the high bit of a 32 bit bottom half of a
> register.  All instructions are 2, 4 or 6 bytes long, and are aligned
> on halfword (2 byte) boundaries.  Various combinations of those two
> bits select between 24, 31 and 64 bit addressing.  Prior to XA mode
> (circa ~1980), addressing was 24 bit.  XA extended that to 31 bits,
> and used the high bit of the 32 bit registers to store (in some cases)
> a mode indication, and then a special branch-with-mode-setting
> instruction could switch from 24 to 31 bit mode or vice versa, which
> made calling between 24 and 31 code reasonably straight-forward.  With
> the introduction of 64 bit mode (circa 2000), the low bit of the
> addresses were used to select between 64 bit mode and the 24/31 bit
> modes.

Wow...

I try to avoid that kind of "mode bit craze" for many reasons,
and I design the YASEP in such a way that if the architecture
is not powerful enough, one is compelled to use another one, better
suited to the task (a big brother of the YASEP could appear one day).

It is possible today because software is comparatively less
expensive than on IBM systems : it's easier to recompile or
reassemble for a new target, while IBM (and customers) has (had)
so much precious hand-written "legacy" code... In a GNU world,
that makes less sense.

In the early iterations of the YASEP architecture, I played
with the idea of using the MSB of the addresses but that made
many things more complex rather than simplify coding and implementation.

Any more examples of architectures playing with the LSB and MSB ?

yg
-- 
http://ygdes.com / http://yasep.org

Thread

What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-08 23:01 +0200
Re: What are the various alternate uses for the PC's LSB ? kym@kymhorsell.com - 2011-05-09 04:07 +0000
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-09 08:45 +0200
Re: What are the various alternate uses for the PC's LSB ? "robertwessel2@yahoo.com" <robertwessel2@yahoo.com> - 2011-05-08 23:58 -0700
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-09 09:02 +0200
Re: What are the various alternate uses for the PC's LSB ? Quadibloc <jsavard@ecn.ab.ca> - 2011-05-09 09:43 -0700
Re: What are the various alternate uses for the PC's LSB ? MitchAlsup <MitchAlsup@aol.com> - 2011-05-09 09:38 -0700
Re: What are the various alternate uses for the PC's LSB ? "robertwessel2@yahoo.com" <robertwessel2@yahoo.com> - 2011-05-09 11:14 -0700
Re: What are the various alternate uses for the PC's LSB ? Anne & Lynn Wheeler <lynn@garlic.com> - 2011-05-09 14:51 -0400
Re: What are the various alternate uses for the PC's LSB ? torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-05-09 16:31 +0200
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-09 18:16 +0200
Re: What are the various alternate uses for the PC's LSB ? mac <acolvin@efunct.com> - 2011-05-09 17:27 +0000
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-09 19:38 +0200
Re: What are the various alternate uses for the PC's LSB ? rpw3@rpw3.org (Rob Warnock) - 2011-05-09 20:23 -0500
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-10 03:04 +0200
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-11 14:23 -0700
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-12 02:45 +0200
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-12 20:41 -0700
Re: What are the various alternate uses for the PC's LSB ? George Neuner <gneuner2@comcast.net> - 2011-05-14 23:24 -0400
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-15 09:18 -0700
Re: What are the various alternate uses for the PC's LSB ? Michael S <already5chosen@yahoo.com> - 2011-05-15 10:00 -0700
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-15 12:06 -0700
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-15 12:12 -0700
Re: What are the various alternate uses for the PC's LSB ? Michael S <already5chosen@yahoo.com> - 2011-05-15 16:16 -0700
Re: What are the various alternate uses for the PC's LSB ? nmm1@cam.ac.uk - 2011-05-15 20:58 +0100
Re: What are the various alternate uses for the PC's LSB ? Michael S <already5chosen@yahoo.com> - 2011-05-15 15:59 -0700
Re: What are the various alternate uses for the PC's LSB ? nmm1@cam.ac.uk - 2011-05-16 08:58 +0100
Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-16 07:43 -0700
Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-16 22:46 +0200
Re: What are the various alternate uses for the PC's LSB ? timcaffrey@aol.com (Tim McCaffrey) - 2011-05-19 16:13 +0000
Re: What are the various alternate uses for the PC's LSB ? George Neuner <gneuner2@comcast.net> - 2011-05-15 20:33 -0400
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-11 05:53 +0200
Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-11 07:10 +0200
Re: What are the various alternate uses for the PC's LSB ? Quadibloc <jsavard@ecn.ab.ca> - 2011-07-04 09:39 -0700
Re: What are the various alternate uses for the PC's LSB ? Quadibloc <jsavard@ecn.ab.ca> - 2011-07-04 10:23 -0700

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