----------
>Dudes:
>
>As many of you know, I periodically post programming "challenges"
>here... Usually, they're of the "Tell me how this code fragment
>works" form.
>
>This time, it's a little different...


>The following PIC 16C5x code tests for divisibility by 3, using an
>algorithm credited to Lewis Carroll, the dope-smoking mathematician
>and author of "Alice in Wonderland".  Carroll's algorithm, of course,
>was designed for base 10, so it tested for divisibility by 11.
>
>;
>; Test for divisibility by 3.
>; Enter with the number to be tested in "TEST".
>; Exits with W = 0 if the number's not divisible by 3,
>;            W = 1 if the number IS divisible by 3.
>;

The following is what I call the "one potato, two potato" method.


;    *********************************************


           list         p=16c54

TEST        equ         9
STATUS      equ         3
CBIT        equ         0
ZBIT        equ         2
WREG        equ         0
TRUE        equ         1
FALSE       equ         0


start       call        divby3

            nop
            nop

;           start of routine

divby3      movf        TEST,WREG       ;into w reg
            btfsc       STATUS,ZBIT     ;test for inital zero
            retlw       FALSE           ;yes, can't devide zero

;           main loop       aa10 is if no need to clear carry

loop        bcf         STATUS,CBIT     ;clear carry

aa10        movf        TEST,WREG       ;see if done
            btfsc       STATUS,ZBIT     ;is w zerO
            retlw       TRUE            ;yes, so good and done

            rrf         TEST            ;look for next bit

            btfss       STATUS,CBIT     ;not the bit?
            goto        aa10            ;yes, didn't find first bit


;           got a bit,  look for another good bit

aa20        bcf         STATUS,CBIT     ;clear carry

aa22        rrf         TEST            ;look for good bit

            btfsc       STATUS,CBIT     ;found bit?
            goto        loop            ;yes


;           no good bit, look for bad bit

aa30        movf        TEST,WREG       ;see if done
            btfsc       STATUS,ZBIT     ;is w zerO
            retlw       FALSE           ;yes, so bad and done


            rrf         TEST            ;look for bad bit

            btfss       STATUS,CBIT     ;not a bad bit?
            goto        aa20            ;yes, so go back for good bit

;           got a bad bit, now next bit is bad, then good

            bcf        STATUS,CBIT      ;clear carry
            goto       aa30             ;look for offsetting bad bit

            end

;   *****************************************************************


   ---------
   Bill Cornutt
   billcorn@infoserv.com
   Located in Ione California USA.
   A small town in Northern California.
   Sitting against the foothills of the Mother Lode.
   ----------------------------------------------------