Message-Id: <4.2.0.58.20010827163125.00bf6e90@ics.u-strasbg.fr> X-Sender: muller AT ics DOT u-strasbg DOT fr X-Mailer: QUALCOMM Windows Eudora Pro Version 4.2.0.58 Date: Mon, 27 Aug 2001 16:41:28 +0200 To: djgpp-workers AT delorie DOT com From: Pierre Muller Subject: Re: UMB access with %gs selector In-Reply-To: <200108271356.PAA14341@merope.lu.erisoft.se> References: <4 DOT 2 DOT 0 DOT 58 DOT 20010827140550 DOT 01b558e0 AT ics DOT u-strasbg DOT fr> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1"; format=flowed X-MIME-Autoconverted: from 8bit to quoted-printable by cerbere.u-strasbg.fr id QAA10236 Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by delorie.com id KAA03297 Reply-To: djgpp-workers AT delorie DOT com Errors-To: nobody AT delorie DOT com X-Mailing-List: djgpp-workers AT delorie DOT com X-Unsubscribes-To: listserv AT delorie DOT com Precedence: bulk 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