; any code here; even main(), wow! (=
; define JUMP to subroutine of timer 1
CSEG AT 0x1B
LJMP ISR_T1
; put any other segments here
isr_t1_cseg SEGMENT CODE
            RSEG isr_t1_cseg
            USING 1 ; register bank if needs
ISR_T1:
; the code of your ISR created