Bits 13 and 3 of the  interrupt registers  are reserved for external
interrupts.

Bit 13, EXTER, becomes a 1 when the system line called INT6* becomes a
logic 0. Bit 13 generates a  level 6 interrupt .

Bit 3, PORTS, becomes a 1 when the system line called INT2* becomes a
logic 0. Bit 3 causes a  level 2 interrupt .

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