Date: Thu, 19 Sep 1996 01:25:17 -0700 (PDT)
From: Samuel Vincent <svincent AT cs DOT sonoma DOT edu>
To: leathm AT gbrmpa DOT gov DOT au
cc: Mihai Moise <moisemih AT ift DOT ulaval DOT ca>, djgpp AT delorie DOT com
Subject: Re: Quick inline asm question...
In-Reply-To: <199609190031.KAA02429@gbrmpa.gov.au>
Message-ID: <Pine.GSO.3.94.960919011959.4107A-100000@zippy>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

On Thu, 19 Sep 1996, Leath Muller wrote:

> > > 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.

Well if all you wanted was to multiply by two...
I believe shifting the register over by one bit would accomplish this
in a lesser amount of time.

As far as adding one 16 bit register to a 32 bit register...
You have the best way there..
Zero a spare 32 bit register (spare register on an x86 cpu you say?
yeah right ;)  Move the 16 bit register to the 16 bit register occupying
the lower 16 bits of the 32-bit register you just cleared.  (In other
words clear ebx, then move something into bx)  Then ebx becomes a
a 32-bit register holding the same value your original 16 bit register
did..  Allowing you to use it with all the other nifty 32-bit registers.

Now I know of zero-extend op-codes..  but I'm unsure as to what these
really are..  (I don't remember)..  They may be addition, multiply,
whatever..

-Sam