8051 external interrupt INT0 issue?
Your code jumps to the ISR which ends in a RETI:

    org 0003H
    jmp isr

    ; ...

isr:
    ; any instruction you need
    reti


If you program in C, this will be handled for you automatically. For example, Keil-C51 places ISRs right at the start address if there is enough room, or jumps to it if not.