Thnx very much..
I have understand a lot...

But I have another question.. When I use timer 0, is it the must for me to use type:

ORG 0BH ; as it is the interupt vector of
; the timer 0
AJUMP TIMER_0_INT

I would like to ask where will the code of TIMER_0_INT be? will it be stayed at 0BH? if yes, i.e. the code of Timer 0 interupt subroutine will be limited between the address of 0BH and 1BH?