GERRY COX <gcox@DEK.COM> wrote:
>I have  a  register pair,  ADC_H and ADC_L , which contain a 16 bit value
>representing an  ADC sample. I want to take successive samples and
>add them into a 32Bit accumulator. ACC_3, ACC_2, ACC_1, ACC_0.
>Speed is essential so code needs to be short. I am having trouble
>with the 16bit / 32 bit Add that I attempted to write.  It does funny things
>for example when I  add FFFF successively. I have written the rest of
>the loop  and the ADC control part  but just can't get the accumulate to
>work.
>
>Would someone please give me a clue on how to  do the add and how to handle
>the
>carrys. Or maybe send  a snippet of code that will help?
>

;*******************************************************************
;  Two byte add.   Standard PIC instruction set does not have an add
;  with carry instruction.
;  At exit, if C = 1, result is greater than 2 bytes.
;  Usage example:  add_word buff, new_data
;  Note: Zero flag NOT valid at exit
;*******************************************************************

add_word: macro aaa, bbb
   movf    (bbb+1), w
   addwf   (aaa+1), f
   movf    (bbb), w
   skpnc
   incfsz  (bbb), w
   addwf   (aaa), f
   endm



   Below is the companion subtract function.  Below that is a
   multibyte subtract.  You can use a similar technique to get
   a multibyte add.


;*******************************************************************
;  Two byte subtract.
;  At exit, if C = 1, result is positive.
;  Usage example:   sub_word new_count, old_count
;  Note: Zero flag NOT valid at exit
;*******************************************************************

sub_word: macro aaa, bbb
   movf    (bbb+1), w
   subwf   (aaa+1), f
   movf    (bbb), w
   skpc
   incfsz  (bbb), w
   subwf   (aaa), f
   endm


;*******************************************************************
;Multi byte subtract.    For example, subtract number from total:


   movf    n_1, w      ;subtract least significant byte
   subwf   t_1, f
                       ;2nd byte
   movf    n_2, w      ;subtract with borrow
   skpc
   incfsz  n_2, w
   subwf   t_2, f
                       ;3rd byte
   movf    n_3, w
   skpc
   incfsz  n_3, w
   subwf   t_3, f
                       ;4th byte
   movf    n_4, w
   skpc
   incfsz  n_4, w
   subwf   t_4, f



--
Bob Fehrenbach    Wauwatosa, WI     bfehrenb@execpc.com