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Re: x86 real mode

From glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups comp.arch.embedded
Subject Re: x86 real mode
Date 2014-10-17 16:45 +0000
Organization Aioe.org NNTP Server
Message-ID <m1rh2d$31h$1@speranza.aioe.org> (permalink)
References <m1r8pj$che$1@speranza.aioe.org> <m1rafb$c7t$1@dont-email.me>

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David Brown <david.brown@hesbynett.no> wrote:
> On 17/10/14 16:23, Don Y wrote:

(snip)
>> What are the most conservative, *practical* expectations I can
>> make living in x86 real mode:  TEXT of 64K and DATA of (disjoint)
>> 64K?  BSS in it's own segment?  Or, shared with DATA?
 
> It should, I think be possible to have DS, CS and SS each pointing to a
> different 64 KB segment.  But .bss and .data would both be part of the
> data segment.  Access to other segments (including pointers to stack
> data, if SS is not the same as DS) is via "far pointers".

Some instructions, such as CALL, use a far pointer.

Others use a segment override prefix.

Most use DS: for accessing data, but you can put a CS:, ES:,
FS:, or GS: prefix on the instruction (or on the data operand),
and it will use the specified segment instead. 

If you have a far pointer, you load it into a segment register,
index register pair, and then use the appropriate prefix.

-- glen

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Thread

x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 07:23 -0700
  Re: x86 real mode Arlet Ottens <usenet+5@c-scape.nl> - 2014-10-17 16:31 +0200
    Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 07:45 -0700
      Re: x86 real mode Arlet Ottens <usenet+5@c-scape.nl> - 2014-10-17 16:53 +0200
        Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 08:28 -0700
          Re: x86 real mode Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-17 18:35 +0300
          Re: x86 real mode Arlet Ottens <usenet+5@c-scape.nl> - 2014-10-17 17:36 +0200
            Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 13:54 -0700
              Re: x86 real mode Grant Edwards <invalid@invalid.invalid> - 2014-10-17 21:02 +0000
                Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 14:54 -0700
              Re: x86 real mode upsidedown@downunder.com - 2014-10-18 10:09 +0300
          Re: x86 real mode Grant Edwards <invalid@invalid.invalid> - 2014-10-17 16:11 +0000
      Re: x86 real mode Stefan Reuther <stefan.news@arcor.de> - 2014-10-17 18:37 +0200
      Re: x86 real mode Les Cargill <lcargill99@comcast.com> - 2014-10-17 12:58 -0500
  Re: x86 real mode David Brown <david.brown@hesbynett.no> - 2014-10-17 16:52 +0200
    Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 16:45 +0000
  Re: x86 real mode Robert Wessel <robertwessel2@yahoo.com> - 2014-10-17 10:03 -0500
  Re: x86 real mode Grant Edwards <invalid@invalid.invalid> - 2014-10-17 16:07 +0000
  Re: x86 real mode upsidedown@downunder.com - 2014-10-17 19:57 +0300
  Re: x86 real mode Les Cargill <lcargill99@comcast.com> - 2014-10-17 12:57 -0500
  Re: x86 real mode Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-17 21:18 +0300
    Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 21:48 +0000
      Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 15:06 -0700
        Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 22:25 +0000
          Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 15:39 -0700
            Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-17 22:47 -0400
              Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 21:10 -0700
                Re: x86 real mode upsidedown@downunder.com - 2014-10-18 11:32 +0300
            Re: x86 real mode upsidedown@downunder.com - 2014-10-18 11:10 +0300
              Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-18 08:20 -0700
                Re: x86 real mode upsidedown@downunder.com - 2014-10-18 20:34 +0300
                Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-18 12:53 -0700
          Re: x86 real mode upsidedown@downunder.com - 2014-10-18 10:30 +0300
          Re: x86 real mode David Brown <david.brown@hesbynett.no> - 2014-10-18 20:22 +0200
      Re: x86 real mode upsidedown@downunder.com - 2014-10-18 10:22 +0300
        Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 09:06 +0000
          Re: x86 real mode Robert Wessel <robertwessel2@yahoo.com> - 2014-10-18 08:43 -0500
          Re: x86 real mode Gerd Brix <gbrix@gmx.eu> - 2014-10-18 16:39 +0200
          Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-18 17:52 -0700
            Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-20 01:35 -0400
              Re: x86 real mode upsidedown@downunder.com - 2014-10-20 10:14 +0300
                Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-20 14:12 -0400
              Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-20 08:15 -0700
    Re: x86 real mode Les Cargill <lcargill99@comcast.com> - 2014-10-17 18:39 -0500
      Re: x86 real mode Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-18 10:46 +0300
  Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-17 15:29 -0400
    Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-17 14:32 -0700
      Re: x86 real mode Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2014-10-18 00:09 +0200
        Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 22:31 +0000
          Re: x86 real mode upsidedown@downunder.com - 2014-10-18 12:04 +0300
            Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 23:22 +0000
            Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-18 20:59 -0700
      Re: x86 real mode upsidedown@downunder.com - 2014-10-18 11:53 +0300
        Re: x86 real mode Don Y <this@is.not.me.com> - 2014-10-18 09:34 -0700
    Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 22:00 +0000
      Re: x86 real mode Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2014-10-18 00:26 +0200
        Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-17 22:36 +0000
      Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-17 21:45 -0400
        Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 03:26 +0000
          Re: x86 real mode Robert Wessel <robertwessel2@yahoo.com> - 2014-10-17 23:26 -0500
            Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 05:42 +0000
          Re: x86 real mode Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-18 10:53 +0300
            Re: x86 real mode glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 09:12 +0000
        Re: x86 real mode Robert Wessel <robertwessel2@yahoo.com> - 2014-10-17 22:40 -0500
      Re: x86 real mode George Neuner <gneuner2@comcast.net> - 2014-10-17 22:07 -0400

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