Hello Philip. Wish you Happy New Year too ! :-)

> I have a question about bit manipulation.
> What I have is an input that reads '11010000' say. What I need to do is
> convert it to '00001101' say.
>
> One route seems to be to RRNCF four times. Would this do the job and not
> affect any other registers?
>
> Is there a better way to bit mask to create the desired result?

As i understand you want to move bits in following sequence:
bit_7 => bit_3
bit_6 => bit_2
bit_5 => bit_1
bit_4 => bit_0

This is may be done by executing SWAPF FILE,F

Another variant is to using RRF FILE,F or RLF FILE,F commands series:
Due to PIC16Cxx have no RRNCF instruction you should apply additional
command before (see example)

        RRF     FILE,W ;copy FILE.0 to Carry
        RRF     FILE,F
        RRF     FILE,F
; do it as many times as you need

There are also another trick was described by John Payson some times ago
(i admire him ability to find non-standart solution :-)

To swap 7<->0 6<->1 5<->2 4<->3 do the following sequence:

        MOVFW   FILE
        BTFSC   FILE,7
        XORLW   0b10000001
        BTFSC   FILE,0
        XORLW   0b10000001
        BTFSC   FILE,6
        XORLW   0b01000010
        BTFSC   FILE,1
        XORLW   0b01000010
        BTFSC   FILE,5
        XORLW   0b00100100
        BTFSC   FILE,2
        XORLW   0b00100100
        BTFSC   FILE,4
        XORLW   0b00011000
        BTFSC   FILE,3
        XORLW   0b00011000
        MOVWF   FILE


WBR Dmitry.