From: korpela AT albert DOT ssl DOT berkeley DOT edu (Eric J. Korpela)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Quick inline asm question...
Date: 19 Sep 1996 17:54:34 GMT
Organization: Cal Berkeley-- Space Sciences Lab
Lines: 62
Message-ID: <51s1cq$epe@agate.berkeley.edu>
References: <51mrb2$o5f AT news DOT cais DOT com>
NNTP-Posting-Host: albert.ssl.berkeley.edu
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <51mrb2$o5f AT news DOT cais DOT com>,
David Charlton <charlton AT the-hermes DOT net> wrote:
>	Okay, I have a quick question, if it's really easy or stupid 
>please don't flame me... anyway, suppose I wanted to add one of the 
>16-bit registers to a 32-bit register (this is for a basic graphics 
>routine) -- how would I do that? I've tried "addw %%ax, %%eax", and "addl 
>%%ax, %%eax", but it doesn't work. I have a feeling it should be simple, 
>but if it's simple it isn't obvious (to me) ;)... so what do I do?
>	Thanks in advance...

You have a couple of choices depending upon whether your 16 bit quantity
is signed or unsigned.  Here we are adding the 16 bit quantity in ax to
the 32 bit quantity in ebx.

for unsigned addition....

   xor    %ecx,%ecx
   movw   %ax,%cx
   addl   %ecx,%ebx

--or--

   movzwl %ax,%ecx
   addl   %ecx,%ebx

--or--

   shll   $16,%eax
   shrl   $16,%eax
   addl   %eax,%ebx

--or--

   andl   $0x0000FFFF,%eax
   addl   %eax,%ebx

+many more

For signed addition...

   cwtl
   addl   %eax,%ebx

--or--

   movswl %ax,%ecx
   addl   %ecx,%ebx

--or--

   shll   $16,%eax
   sarl   $16,%eax
   addl   %eax,%ebx

+ many more

Eric
-- 
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