Interrupt Subroutine

interrupt
    clrf    PORTB       ;turn off all outputs (including LED)
    call    tenthsec
    call    tenthsec    ;wait a short time
    bsf PORTB,6     ;illuminate LED - this will cause it to flash
    call    tenthsec
    call    tenthsec
    btfss   PORTA,5     ;check if the interrupt reset is active
    goto    interrupt   ;loop otherwise
    bcf INTCON,1    ;disable the external interrupt flag
    bcf PORTB, 6    ;clear the interrupt LED
    retfie          ;return and enable global interrupt