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| From | torbenm@diku.dk (Torben Ægidius Mogensen) |
|---|---|
| Newsgroups | comp.arch |
| Subject | Re: What are the various alternate uses for the PC's LSB ? |
| References | <iq732c$ku0$1@speranza.aioe.org> |
| Date | 2011-05-09 16:31 +0200 |
| Message-ID | <7z4o54q72z.fsf@ask.diku.dk> (permalink) |
| Organization | SunSITE.dk - Supporting Open source |
whygee <yg@yg.yg> writes: > Hello, > > I hope that the old guard here can provide some wisdom and knowledge :-) > > In the context of a fixed-size instruction and byte-oriented computer architecture, > such as ARMv7, MIPS, SPARC and so on, the Least Significant Bit of the instruction > pointer is wasted and cleared. > > ARM Thumb uses it to define the current instruction set mode, > which makes a lot of sense for them. > > So can people here tell me what other uses of this LSB have > been tried in the past ? Early ARM processors used the last two bits of the PC to store the processor mode. This was long before Thumb, so the modes were user, supervisor, interrupt and fast interrupt. The same ARMs also used the top four bits of the PC to store arithemtic flags. This had the advantage that you could store both PC and flags using a single word, but it limited the PC to a 256 MB range. Later ARM designs moved both flags and processor status to a separate (unnumbered) register and later still Thumb used the lsb to indicate 16/32-bit mode. If you want to use the lsb for something, I suggest it is for something that is useful to change and/or preserve across jumps or calls, as you can change/save it along with the PC. Torben
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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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