PIC Microcontroller Math Method

Increment / Decrement

Increment / Decrement 32 bit address and detect zero

; given address of 32 bit little-endian counter in W,
; increment or decrement the counter and return Z set if zero

;Inc32z 
;From Dmitry Kiryashov
inc32z:
        movwf   FSR
        clrz

        incfsz  INDF,F
        return

        incfsz  FSR,F
        incfsz  INDF,F
        return

        incfsz  FSR,F
        incfsz  INDF,F
        return

        incfsz  FSR,F
        incf    INDF,F
        return

;Dec32z
;From Rich Leggitt, Andrew Warren, and Dmitry Kiryashov
; 99.2% of the time, this takes 10 cycles w/call
dec32z:
        movwf   FSR
        decfsz  INDF,F
        goto    dec32nz

        incf    FSR,F
        movfw   INDF

        incf    FSR,F
        iorwf   INDF,W

        incf    FSR,F
        iorwf   INDF,W  ;get _Z finally
        return

dec32nz:
        clrz            ;set _Z=0
        incfsz  INDF,W
        return

        incfsz  FSR,F   ;doesn't corrupt _Z
        decfsz  INDF,F
        incfsz  INDF,W
        return

        incfsz  FSR,F   ;...
        decfsz  INDF,F
        incfsz  INDF,W
        return

        incfsz  FSR,F   ;...
        decfsz  INDF,F
        return
        return

;for the 18cxxx instruction set.
;From Scott Dattalo
; 10-16 cycles


dec32:
    movwf   FSR0L    ; the fsr's (three of them) on the 18cxxx
    clrf    FSR0H    ; are 16bits wide

    decf    POSTINC0,F,1 ;Indirect access that increments the fsr.

    bnz      dec32nz     ;this was bz

  ;The lsbyte is zero , so we don't need to propogate the borrow
  ;but we need to see if the higher bytes are zero.

    movf    POSTINC0,w,1 ;The first byte is zero
    iorwf   POSTINC0,w,1 ;check the next three
    iorwf   POSTINC0,w,1

    return

dec32nz
   ;decf on the 18cxxx parts affects C

    skpnc
     return

    decf   POSINC0,F,1
    skpc
     decf  POSTINC0,F,1

    skpc
     decf  POSTINC0,F,1

    clrz          ;was setz
    return