From: leathm AT solwarra (Leath Muller)
Message-Id: <199609190031.KAA02429@gbrmpa.gov.au>
Subject: Re: Quick inline asm question...
To: moisemih AT ift DOT ulaval DOT ca (Mihai Moise)
Date: Thu, 19 Sep 1996 10:31:17 +1000 (EST)
Cc: djgpp AT delorie DOT com
Reply-To: leathm AT gbrmpa DOT gov DOT au
In-Reply-To: <32401FF0.41C67EA6@ift.ulaval.ca> from "Mihai Moise" at Sep 18, 96 12:14:40 pm
Content-Type: text

> > Well, in as much as I understand 16 vs 32-bit assembly, the 'e*x'
> > registers
> > are simply 32-bit extended versions of the '*x' registers.  So, %%ax is
> > in
> > actuality the lower 16 bits of %%eax.  What you are trying to do is add
> > the register to itself, which I doubt will work.
 
> Actually, adding a register to itself just might work. But intel
> provides no opcodes to mix 16-bit and 32-bit registers in an
> instruction. Besides, even if such opcodes existed, the resulting
> algorithm would not be portable.

You can add a register to itself...its a great way to do a quick multiply.
(x 2). As for adding a 16 bit register to a 32bit one, what you could do is
make sure the top 16 bits of a 32 bit register are zeroed, and perform
an addl. using another register ie:

	xorl	%%ebx, %%ebx		// 1 cycle
	movw	%%ax, %%bx			// 2 cycles
	addl	%%ebx, %%eax		// 1 cycle

Leathal.