>I have a need for a 16 bit counter which is decremented in my interrupt
>service routine which is called every 10ms. It should be easy, I know, but
>I cannot see the simple solution. I have written a test routine as follows:-

Try this one:

decloop:        MOVWF   lo_byte         ; Get the low byte
                BTFSC   STATUS,Z        ; Check to see if its zero
                DECF    hi_byte         ; Its zero, decrement high byte.
                DECF    lo_byte         ; Decrement the low byte
                BTFSS   hi_byte,7       ; Check for underflow
                GOTO    decloop         ; NOT done, keep on going.
finished:       ...                     ; do finish processing here.

Some things to note:
        1) The MOVWF instruction sets the zero bit. No need to do anything
           else.
        2) The code works by testing the value in the low byte on entry,
           if its zero, then the high byte should be decremented as well,
           otherwise it skips the decrement high byte instruction.

        3) The end of the loop is detected by an underflow of the counter,
           this is cheap to do (one instruction) but limits the counter to
           15 bit values. If you really need 16 bits, change the sequence
                BTFSS   hi_byte,7       ; check for underflow
                GOTO    decloop         ; NOT done, keep on going
           to be
                MOVF    lo_byte,W       ; move low byte into W
                IORWF   hi_byte,W       ; Or in the high byte
                BTFSS   STATUS,Z        ; Check the zero flag
                GOTO    decloop         ; continue (non-zero)

It adds two instructions but that gives you a full 16 bits of resolution.

Hope this helps,
--Chuck