On Thu, 8 Mar 2001, myke predko wrote:

> Hi James,
>
> The only thing that I wasn't sure about was the lines:
>
>  ;; subtract 1s from copy of counter
>  movlw d'1000000'&0xff
>  subwf t0,f
>  btfss status,c
>  decf t1,f
>
> This is an eight bit subtract of a 24 bit value, but you are not bringing
> the carry to the most significant byte ("t2").  I haven't done any kind of
> analysis on what this can do to you, but I'm sure every once in a while it
> will bite you.
>
> I would suggest something like:
>
>  ;; subtract 1s from copy of counter
>  movlw d'1000000'&0xff
>  subwf t0,f
>  btfsc status,c
>   goto $ + 5  ;; If No Borrow, Subtract from the Middle Byte
>  movlw d'1'
>  subwf t1,f
>  btfss status,c
>   decf t2,f ;;  "t1" was Zero upon Entry into Subtraction Routine, Decrement
> t2

If you wanna do this why not do it like:

  clrf  FSR   ;) Intentional obfuscation
  movlw d'1000000'&0xff
  subwf t0,f
  RLF   FSR,w
  subwf t1,f
  RLF   FSR,w
  subwf t2,f

If FSR is always 0, then the clear only needs to be done once.

another variation:

  movlw d'1000000'&0xff
  subwf t0,f
  movlw 1
  skpnc
   subwf t1,f
  skpnc
   subwf t2,f

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