Here is my stab at fixing the 16bit counter problem

test   movlw  04
       movwf  hibyte     ;some test data

loop   movf   lobyte,w
       iorwf  hibyte,w
       btfsc  status,z
       goto   finish
       decf   lobyte                              *  changed line
       btfss  _v         ; #define ALUSTA,3 _v    *  added line
       goto   loop
       decfsz hibyte
       goto   loop
finish goto   finish
       end

Your basic flaw is that you were decrementing the high byte when the low byte
went to all zeros. You need to decrement the high byte when the low byte goes
to all ones!  I think this will work? Something tells me there is a
better (more efficient?) way to do this though?

Hope this helped

 Roy