Date: Tue, 27 Jun 1995 10:59:02 -0700
To: loiselse AT ift DOT ulaval DOT ca (Sebastien Loisel)
From: kbaca AT skygames DOT com (Kevin Baca)
Subject: Re: Inline assembler in GCC for PC
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu

>void SETMODE(short mode)
>        {
>        asm("mov ax,%0 \n int 10h"
>           : 
>           : "f" (mode)
>           );
>        }

First, GCC asm uses AT&T op codes, not Intel.  This means that source and dest
operands are reversed.  Try this:

void SETMODE(short mode)
        {
        asm("mov %0,ax \n int 10h"
           : 
           : "f" (mode)
           : "eax"
           );
        }

Also, notice the last colon up there.  This informs the compiler that you
are using
the register and it can do any saving that needs to be done.  An even better way
to do the above would be like this:

void SETMODE(short mode)
        {
        asm("int 10h"
           : 
           : "a" (mode)
           );
        }

That "a" up there tells the compiler to place the variable mode in eax.
Sometimes,
the variable will already be in eax and the extra mov you have in your code
will be
avoided.  Plus, in this case you don't need the extra colon at the end
because the
compiler already knows you're using eax.

As for info, I get most of mine from the GCC info files and the asm info
files.  The
GCC info files explain inline assembly (just search for "inline") and the
asm info
files explain everything else.  Also, look in the farptr.h header file for
examples
of inline assembler AND inline functions (another useful, though non-protable,
GNU extension).  If you use linux, look in the string.h header file in the
kernel source
tree for some excellent examples of GNU inline assembly.

One more thing (and this isn't explained in either doc) - the "d" that is
appended
to opcodes like "movsd" and "stosd" is changed to an "l" (ell) in GNU asm.