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Date:	Mon, 27 Aug 2001 16:41:28 +0200
To:	djgpp-workers AT delorie DOT com
From:	Pierre Muller <muller AT cerbere DOT u-strasbg DOT fr>
Subject:	Re: UMB access with %gs selector
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At 15:56 27/08/01 , vous avez écrit:
> >     Doesn't this mean that the higher part of a index is not used?
> > (i.e. if  $esi contain 0x10008,
> > will
> >     movl  %gs:(%esi),%eax
> > load into %eax
> > the content of linear address 0x10008 or 0x00008 ??)
> >
> > Can we really access to UMB area with a 16 bit selector ?
>
>Yes.
>
>_All_ selectors are 16-bit of which bits 3-15 are used to index into
>the GDT or LDT.

I think that you misunderstood my question.
   I know the segment register are 16 bit registers but I was talking about
something completely different:
   The descriptor of each selector allocated as a field that specifies if 
the OS should
consider it as a 16 bit or a 32 bit data area.

   But as the instruction explicitly have a 16 bit or 32 bit nature
(depending on some CR0 field IIRC) I don't know really if this
does influence the reading past the first megabyte or not.

   My question is more to know if the OS shouldn't forbid to
access anything above the 0xffff memory position relative to selector base 
if the
selector is marked as a 16 bit selector.

   If the answer to the first question is yes, then the second question is
of course, is there no error in the crt0.S code that sets up the rights
of this selector?



Pierre Muller
Institut Charles Sadron
6,rue Boussingault
F 67083 STRASBOURG CEDEX (France)
mailto:muller AT ics DOT u-strasbg DOT fr
Phone : (33)-3-88-41-40-07  Fax : (33)-3-88-41-40-99

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