Hi Peter,

If you combine the increment and decrement code with the range check,
it might be more efficient:

; Increment lower 2 bits of File, 0->1->2->0
        movlw 1
        btfsc File, 1
         movlw -2
        addwf File, f

; Decrement lower 2 bits of File, 0->2->1->0
        movf File, w
        andlw 0x03
        movlw -1
        skpnz
         movlw 2
        addwf File, f



 Nikolai

> Subject: Re: [PIC]: Modulo 3 code
> From: Peter L. Peres (plp@ACTCOM.CO.IL)
> Date: Sat Jun 15 2002 - 12:07:42 PDT

> On Sat, 15 Jun 2002, Scott Dattalo wrote:

>>On Sat, 15 Jun 2002, Peter L. Peres wrote:
>>
>>> Hi,
>>>
>>> I need to make a modulo 3 counter in a file. I've come up with this code:
>>>
>>>   movlw 0x03
>>>   andwf File,w  ;
>>>
>>>   xorlw 0x03    ; 11 -> 00
>>>   btfss STATUS,Z
>>>   xorlw 0x03    ; undo xorlw above
>>>
>>>   movwf File
>>>
>>> I actually need the movwf at the end to be like this because I have to add
>>> in some flags. Surely there is a MUCH shorter way to do this ...
>>>
>>> Peter
>>>
>>> PS: The code has to be constant run time

>>    how 'bout
>>
>>    rrf    File,w
>>    andwf  File,w
>>    andlw  1
>>    addwf  File,f
>>
>>Scott

> I did not specify clearly, my mistake. File contains flags excepting the
> lowest two bits. It can count up or down. The down counting is already
> handled elsewhere (in a similar way), so is the up counting.

> This is because the counter switches between base 3 and base 4 at runtime.
> The only thing is I need to fix is the state 11 which does not exist in
> base 3.  Thus my code. Sorry about being unclear.

> This code is clearer I hope:

>   movf  File,w
>   andlw 0x03
>   xorlw 0x03
>   movlw 0xFC
>   btfsc STATUS,Z
>   andwf File,f

> On the second look it is not that long ... thanks anyway.

> Peter

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