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'Divide routine error'
1997\03\27@105614 by Bob Fehrenbach

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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

  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
  goto    check_sign
  movf    n_3, w
  subwf   t_3, w
  goto    check_sign
  movf    n_2, w
  subwf   t_2, w
  goto    check_sign
  movf    n_1, w
  subwf   t_1, w
  goto    check_count

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

  bsf     STATUS, C

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


Bob Fehrenbach    Wauwatosa, WI

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