Hi Dody,
you can work it out like this...

<code>
ORG XXXXh ; timer IVT.
......
......
ljmp your_routine
reti

org yyyyh ; main program etc goes here...
......
.....
.....
....
your_routine:
; your routine code goes here and finally
; return it back
ret
</code>
; rest of the code..
...
...
end