Hello Mike.

> What's the best way to guarantee a valid output with only one execution
> of the update routine?  Maybe changing the whole approach to use a
> counter that counts from 0 to 7, then generating ths scan mask from it
> (Most likely any real application will require both a count and a mask
> anyway).  The counter can be guaranteed valid just by ANDing it with 7.


How about following example:

;       get a lowest "1" bit in a byte

        movfw   scanbit
        sublw   0
        andwf   scanbit,f
        skpnz                   ;if _Z=1 than _C=1 too (see sublw)
        bsf     scanbit,_N      ;if _Z=1 than init scanbit with 1<<_N
                                ;where _N is desirable position

;       calc. next scan_value

        rlf     scanbit,w
        rlf     scanbit,f
;

WBR Dmitry.