[Coco] Mod10 Suggestions

[Dave Philipsen]

On 2/16/2017 10:05 PM, Mark McDougall wrote:
>
> Can't help myself! :(
> Here's my take on it.
> Assembles but ***NOT TESTED***!
>
> On optimisation to the algorithm; check the result for >10 on each 
> loop iteration - means you don't have to do an extra loop at then end!
>
>
>         .list   (meb)     ; macro expansion binary
>         .area   idaseg (ABS)
>
>         .org    0x1200
>
> ccd:    .ds     16        ; credit card number
> result: .ds     1
> status: .ds     1
>
> start:  clrb
>         stb     result    ; zero sum
>         stb     status    ; flag=bad
>         ldx     #result   ; ptr CCN
>         ldb     #16       ; 16 digits
>
> loop:   lda     ,x+       ; get digit (0-9)

Here's a problem.  It's not the credit card digit itself that is being 
tested for odd/even.  It's the position of the digit.  So you need to do 
a  'bitb #1' instead of a 'bita #1'.

>         bita    #1        ; odd/even?
>         bne     accum     ; odd, skip
>         lsla              ; x2
>         cmpa    #10       ; 2 digits?
>         blo     accum     ; no, skip
>         suba    #9        ; add digits
> accum:  adda    result    ; add to result
>         cmpa    #10       ; less than 10?
>         blo     store     ; yes, skip
>         suba    #10       ; discard tens
> store:  sta     result    ; store result
>
>         decb              ; done all digits?
>         bne     loop      ; no, loop
>
>         tsta              ; result=zero?
>         bne     done      ; no, flagged bad already
>         deca              ; $FF
>         sta     status    ; update status
>
> done:   rts
>
>         .end    start
>
> Regards,
>