From: alaric AT abwillms DOT demon DOT co DOT uk (Alaric B. Williams)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Quick inline asm question...
Date: Fri, 20 Sep 1996 18:18:24 GMT
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To: djgpp AT delorie DOT com
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leathm AT solwarra (Leath Muller) wrote:
>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

If your 16 bit number might be negative, padding with zeros won't
work; you need to use an instruction called movsx in intel syntax,
"move with sign extension". 

Erm... the AS info entries say:

---------------8<--------------

   Almost all opcodes have the same names in AT&T and Intel format.
There are a few exceptions.  The sign extend and zero extend
instructions need two sizes to specify them.  They need a size to
sign/zero extend *from* and a size to zero extend *to*.  This is
accomplished by using two opcode suffixes in AT&T syntax.  Base names
for sign extend and zero extend are `movs...' and `movz...' in AT&T
syntax (`movsx' and `movzx' in Intel syntax).  The opcode suffixes are
tacked on to this base name, the *from* suffix before the *to* suffix.
Thus, `movsbl %al, %edx' is AT&T syntax for "move sign extend *from*
%al *to* %edx."  Possible suffixes, thus, are `bl' (from byte to
long),
`bw' (from byte to word), and `wl' (from word to long).

--------------8<--------------

So I guess to add %bx to %eax we could:

movswl %bx,%ebx
addl %ebx,%eax

Assuming movswl likes both operands being the same register (in a
different guise)!

If the 16 bit number starts in %ax, consider one of these babies (with
different scissors!):


-------------B<---------------

   The Intel-syntax conversion instructions

   * `cbw' -- sign-extend byte in `%al' to word in `%ax',

   * `cwde' -- sign-extend word in `%ax' to long in `%eax',

   * `cwd' -- sign-extend word in `%ax' to long in `%dx:%ax',

   * `cdq' -- sign-extend dword in `%eax' to quad in `%edx:%eax',

are called `cbtw', `cwtl', `cwtd', and `cltd' in AT&T naming.  `as'
accepts either naming for these instructions.


-------------B<---------------

Ie, to add %ax to %edx:

cwtl
addl %eax,%edx

I presume cwtl is faster that movswl %ax,%eax!

>Leathal.

ABW (who loves the 386 instruction set!)
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