>As I said, I've used RETLW many times and never struck this
>problem, so I must have been lucky. Although having said
>that, if I had I'd have worked it out then instead of now.

Well you must have been real lucky if you ignored all the recommendations
given in the app note on doing these tables. Let us just examine your code
for a moment :)

>But this adds W to PCL and returns immediately to 00xx, not
>to 01E8
>
>01E2         movwf   devno
>                  call    bitnum
>01E8         movwf   temp
>01EA         bra    cp_num
>
>0204  bitnum
>                  addwf   pcl
>                  retlw    0x01
>                  retlw    0x02
>                  etc

Your code is crossing a 256 byte page boundary - not a problem in itself
provided the correct labels are used when calculating the offset into the
table, rather than using
    movlw    high $    ; potential to get 0x01 instead of correct 0x02 if
crossing page boundary
                       ; needs to be "movlw    high bitnum+1
    movwf    pclath    ; ready for computed jump

and then as others have pointed out there is no code to load PCLATH at all.
Based on this (assuming PCLATH is 0x00 as appears to be the case from what
you post) your code is going to go to 0x0005 + W instead of 0x0205 + W

>As the code is still evolving I'll be ditching the RETLW in
>favour of TBLPTR

Why, adding two lines of code in accordance with the app note on doing
things this way will bring it all correct. To make sure that I get the
correct high byte of the address to put into PCLATH I always use a label
after the "addwf pcl" instruction to allow for crossing the page boundaries.

The other trick of course is to make sure your W is always even, as someone
else pointed out. This is real easy seeing you are saving the W reg anyway.
My reckoning is that the code will then be as follows :-

         movwf   devno
         movlw    high bitnum_table
         movwf    pclath
         movf    devno,w
    ;    addwf    devno,w    ; only needed if we do not know w is always
even
    ;    btfsc    status,c    ; ditto
    ;    incf    pclath,f    ;     - inc pclath if doubled W is >255
         call    bitnum
         movwf   temp
         bra    cp_num

bitnum
         addwf   pcl
bitnum_table
         retlw    0x01
         retlw    0x02
;         etc

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