> This routine is compact, but unreliable in case of RAM disruption:
>         clrc                    ;Shift in a 0.
>         rlf     scanbit,f       ;Do the shift.
>         skpnc                   ;Skip if 1 didn't come out.
>         rlf     scanbit,f       ;If 1 came out, put it to bit 0.
>
> If the RAM location 'scanbit' contains a value other than one of the form
> having a single 1 and 7 zeros, it won't work.

The point about self-stabilizing vs non-self-stabilizing code is an important
one (if a glitch that may occur once every few days disrupts operation for
a few seconds that's often not as bad as one that disrupts operation indef-
initely).

>                                                Now consider this routine:
>
>         clrc                    ;Shift in a 0
>         rlf     scanbit,f       ;Rotate it in.
>         movf    scanbit,f       ;Test if new value is 0
>         skpnz                   ;It's not, shifted value OK.
>         bsf     scanbit,0       ;Reset scan to first bit.

If it's okay (or desirable) to have the result in W as well as F, it can be
done safely in 4 cycles:

        movlw   1
        btfss   scanbit,7
         rlf    scanbit,w
        movwf   scanbit