Date: Fri, 19 Jul 1996 16:31:29 +0200 (MET DST)
From: Mark Habersack <grendel AT ananke DOT amu DOT edu DOT pl>
To: Richard Philp <philpri AT ee2 DOT ee DOT cit DOT ac DOT nz>
cc: djgpp AT delorie DOT com
Subject: Re: Embedded code: LES
In-Reply-To: <philpri.40.837639716@ee2.ee.cit.ac.nz>
Message-ID: <Pine.NEB.3.93.960719161957.2019C-100000@ananke.amu.edu.pl>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

On Wed, 17 Jul 1996, Richard Philp wrote:

> 
> Hi all,
>     I've been needing to do some embedded code in some of my programs latly, 
> but theres a problem with one of the opcodes: LES . I've try lesl, but the 
> assembler doesn't like it, and when I make the program a library, with this 
> command in it, it keeps on crashing, and coming up with a sigsev error (Or 
> something similer. I know I've misspelt it.). Anyone know why? Or can 
> someone give me a reason for why the assembler doesn't like les? 
> Or is there an equlivent?

Well, by me the problem lies in what kind of pointers are you using (I assume
you are programming with djgpp - right?). DJGPP is a 32-bit FLAT model
environment and, as such, uses near 32-bit pointers (i.e. ones that have only
the offset part - selector is contained in one of the seg regs and usually
doesn't change). LES instruction requires you to use a selector::offset
address, both in 16- an 32-bit code. When you try to load it from 32-bit flat
mode pointer it stores the 16 upper bits in ES register (which should contain
SELECTOR which is an offset to LDT or GDT table - and NOT the real-mode
segment addres) and the lower 16 bits in UPPER part of the other LES operand
(pretty complicated description - sorry ;-)))) leaving the common 16-bit part 
of the register untouched (e.g. ax, bx etc) and possibly containing random
data. To load the ES register successfuly in 32-bits you'd need a 48-bit
pointer (not available directly from DJGPP, except for the farptr routines) of
the form SELECTOR:32_BIT_OFFSET. Then everything would be just fine.
If you want to access some data in assembler with DJGPP just load one of the
indexing 32-bit registers with the pointer value supplied - in most cases you
may safely assume that the corresponding segment registers contains the
correct selector (unless YOU have modified it earlier).
Hope that helps

   Mark 

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