;***ISR Vectors***
    org 0x0000
    ljmp    Start
    
    org 0x0003
    ljmp    ExtInt0

    org 0x000B
    ljmp    Tmr0Int

;***Code Starts***
Start:
    .
    ;***System Init.***
    .
    mov IE, 13H
    .
;***Your background process***
Main:
    jnb KBDataReady, Main
    .
    ;***Process data in buffer***
    ;***Send to memory mapped UART's***
    .
    clr KBDataReady
    sjmp Main

;***Ext. Int0 ISR***
ExtInt0:
    jb  KBDataReady, ExitExt0
    push ....
    .
    clr TR0     ;Reload timer0 timeout value
    mov TL0, #LOW TIMER0_VALUE
    mov TH0, #HIGH TIMER0_VALUE
    .
    .
    ;Do your bit/byte process here
    ;Check Rx buffer ptr boundry
    ;If ptr OK, save data to buffer
    .
    .
    setb TR0
    pop ...
ExitExt0:
    reti
    
Tmr0Int:
    clr TR0
    setb KBDataReady
    reti