Bob,

 Isn't it as simple as checking the MSB.  That's the sign bit.  
 If it's a 1,it's negative. Then just add this to the other register.
 You now have the result of x-y.  You are subtracting by addition.
 You don't have to subtract by subtracting.  

                                                Jim



> Sorry guys I fat fingered my previous message.  Here's the rest of it.
> 
> Guys, I have a value in W that's signed.  Adding this value to a set of
> two variables is easy enough:
> addwf     blahL,f
> clrf         wreg
> addwfc   blahH,f
> 
> W contains a signed value.  If the value of W is a negative, doing the
> following doesn't work (to subtract it instead of adding it.)
> 
>     addwf    blahL,f
>      clrf        wreg
>      bnc       subtract
>      addwfc blahH,f
>subtract
>      subwfb, blahH,F
> 
> I see why this is incorrect, but I'm not sure how you test for a 2's
> complement value in order to properly skip the subtraction if the value
> in W is not setting the carry or borrow flag when adding the lower
> byte.  Can someone please steer me down the correct path?
> 
> Regards,
> Bob
> 
> 
> 
> 
> 
> On 7/14/06, Bob J. <rocketbob@gmail.com> wrote:
>>
>> Guys, I have a value in W that's signed.  Adding this value to a set
>> of two variables is easy enough:
>>
>> addwf     blah,f
>> clrf         wreg
>> addwfc
>>
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