Thanks, Andy, for the DETAILED explanation of adders, subtraction, and
complements.  I have a somewhat related question.

What does the carry flag represent after a PIC subtraction?  I've
heard from one place that it should be thought of as a "non-negative
result" flag, but that doesn't always work.  The PSIM simulator gave
the following reslts for these values:

-4 - -2 = -2  No Carry
-4 -  2 = -6  Carry
-2 -  4 = -6  Carry
-2 - -4 =  2  Carry
 4 - -2 =  6  No Carry
 2 - -4 =  6  No Carry

My 16C84 code looked like this:

   movlw   -4
   movwf   temp         ; temp = -4
   movlw   -2           ; w = -2
   subwf   temp, 0      ; w = temp - w  (-4 - -2)
                        ; result is -2 (FE) and NO CARRY (positive?)

I was bit by this mis-understanding this weekend and my resolution
was to AND the result with 0x80 and test for zero to determine a
non-negative result.

Thanks, Byon
______________________________________
Byon Garrabrant  N6BG  byon@netcom.com