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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