From:	Vik Heyndrickx <Vik DOT Heyndrickx AT rug DOT ac DOT be>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Operations on conventional memory
Date:	Wed, 07 Jan 1998 11:01:40 +0100
Organization:	University of Ghent, Belgium
Lines:	37
Message-ID:	<34B35284.38E1@rug.ac.be>
References:	<1B6D42C6A39 AT fs2 DOT mt DOT umist DOT ac DOT uk>
NNTP-Posting-Host:	eduserv1.rug.ac.be
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Anthony.Appleyard wrote:
>   and its relatives similarly. Which PC instructions can I prefix `byte 0x65'
> to to make them operate on conventional memory? If e.g. I wanted a quick way
> to compile `x &= y', where x is in conventional memory, would this work?:-

You can prefix any instruction with ".byte 0x65", even though, some
instructions do not benefit from it. In general you can (should) only
use it before instructions that do explicit memory addressing (excluding
the implicit push/pop argument) with %ds or %ss as default segment
register.
".byte 0x65" is a way in gnu assembler to let the CPU know it should use
the %gs register to reference the offset against. A full table (by
heart)

 .byte 0x26  ES
 .byte 0x2e  CS
 .byte 0x36  SS
 .byte 0x3e  DS
 .byte 0x64  FS
 .byte 0x65  GS

Know what you do when using this table! You can't use any segment
register in general just for any purpose. Wrong use can and will crash
your compu.
 
> extern __inline__ void farpokeandb(unsigned short selector,
>          unsigned long offset, unsigned char value) {
>   __asm__ __volatile__ ("movw %w0,%%gs\n" " .byte 0x65 \n" " andb %b1,(%k2)"
>       : : "g" (selector), "qi" (value), "r" (offset)); }

Looks fine to me (after a quick look).
BTW "x&=y" will work faster (and allows for optimization).

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