Red face mode on.

Sometime ago in response to someone's request for a divide routine
I posted a 32 by 32 bit routine.  Yesterday I received a private
message from Lieven Leroy pointing out that in the four byte subtract,
the carry was incorrectly propagated.  (Great catch)  Here is the
revised version.


;*******************************************************************
;  Division:  Divide 32 bits by 32 bits,
;             32 bit quotient and remainder.
;  Standard shift and subtract algorithm.
;
;   q_4,3,2,1 / n_4,3,2,1 -> q_4,3,2,1   rem in t_4,3,2,1
;
;  Uses count
;
;  Execution time: Extremes seem to be 620 to 1350 clock cycles
;  Program memory: 52
;  Author: BF
;  Bug reported by Lieven Leroy regarding carry propagation in four
;  byte subtract - fixed.
;*******************************************************************

div32b32: macro
   local   loop, check_sign, check_count
   clrf    t_1
   clrf    t_2
   clrf    t_3
   clrf    t_4

   movlw   32
   movwf   count

loop:
   rlf     q_1, f
   rlf     q_2, f
   rlf     q_3, f
   rlf     q_4, f
   rlf     t_1, f
   rlf     t_2, f
   rlf     t_3, f
   rlf     t_4, f

   movf    n_4, w
   subwf   t_4, w
   skpz
   goto    check_sign
   movf    n_3, w
   subwf   t_3, w
   skpz
   goto    check_sign
   movf    n_2, w
   subwf   t_2, w
   skpz
   goto    check_sign
   movf    n_1, w
   subwf   t_1, w
check_sign:
   skpc
   goto    check_count

   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

   bsf     STATUS, C

check_count:
   decfsz  count, f
   goto    loop
   rlf     q_1, f
   rlf     q_2, f
   rlf     q_3, f
   rlf     q_4, f

   endm

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