subject=Re:[PIC:] coding challenge: divide by 7
source= http://www.piclist.com/piclist/2004/03/12/163317a.txt?

After some thinking:

---
        ;Reoptimized version of division by 7, by Koen van Leeuwen
        ;Classic algorithm.
        ;Number to be divided in W
        ;Result: integer result in outcome, remainder in W
        ;Trashes: W, STATUS, outcome
        ;Instructions/cycles: 25


        ;According to the datasheets,
        ;sublw is w = k - w
        ;addlw is w = w + k
        ;It is kinda odd.
        ;I also read that the add instructions do not add carry. Also odd.
        ;But if it is correct, I guess the following code should work:
        ;WARNING: untested code ahead!

        clrc
        clrf    outcome         ;start with clean reg
        addlw   d'-224'         ;-(7<<5)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
                                ;(=whether outcome is negative or not)
                                ;and clear carry since leftmost bit of
                                ;outcome is always zero
        btfss   outcome,0       ;If carry was 0, subtraction underflowed.
        addlw   d'224'          ;so fix it
                                ;NOTE: this generates a carry

        addlw   d'-112'         ;-(7<<4)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
        btfss   outcome,0       ;If carry was 0, subtraction
underflowed.
        addlw   d'112'          ;so fix it

        addlw   d'-56'          ;-(7<<3)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
        btfss   outcome,0       ;If carry was 0, subtraction underflowed.
        addlw   d'56'           ;so fix it

        addlw   d'-28'          ;-(7<<2)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
        btfss   outcome,0       ;If carry was 0, subtraction underflowed.
        addlw   d'28'           ;so fix it

        addlw   d'-14'          ;-(7<<1)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
        btfss   outcome,0       ;If carry was 0, subtraction underflowed.
        addlw   d'14'           ;so fix it

        addlw   d'-7'           ;-(7<<0)check whether its too big
        rlf     outcome,f       ;shift in carry(/borrow)
                                ;(=whether outcome is negative or not)
                                ;and clear carry since leftmost bit of
                                ;outcome is always zero

        return
        end

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