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For example, let us suppose that an interrupt routine for Port A starts at 0E12H, and that the interrupt address table will be stored at 0F80H. In order that the routine should be found correctly, the I register should contain 0FH, the value 80H should be sent to the control register of Port A, and finally, memory locations 0F80H and 0F81H should contain 12H and 0EH respectively (low byte first). At an interrupt, CPU interrupts are automatically disabled and must be re-enabled, if required, by the programmer.

Always end an interrupt service routine with the RETI instruction, as this is the only way to indicate to the PIO port, that the service routine is finished. This feature can cause some dismay to the unwary. Take the following example: everything is set up correctly, and the PORT interrupts correctly. However, unfortunately the interrupt routine crashes. No problem to our intrepid experimenter, he presses reset, debugs the interrupt routine and tries aqain, remembering to reset IM 2, I register and interrupt enable. Dismay! Nothing happens. No interrupt.

The problem is that the PIO still thinks that is is being serviced for its initial interrupt, and is internally inhibited from causing another. A useful routine to get out of this sort of problem is as follows:–

21 00 00         LD        HL, 0000H
ES               PUSH      HL
ED 5E            RETI

This will tell the PORT that its service routine is finished and then restart the monitor by executing from 0000H. It can be used at any time, if there is any doubt as to the status of a PIO.

Once the mode and interrupt control have been set, the Port interrupt may be enabled or disabled by sending 83H or 03H to the control register. This feature could form the basis of a generalized interrupt control program for a given system. However, it should be noted, that the correct way to disable a port interrupt, is to first of all disable CPU interrupts before the Port interrupt. This is because an interrupt by that Port, during the execution of the instruction to disable its interrupt, would cause a system crash.

Finally, when a Port has been disabled, an interrupt may be pending, so that when the Port is again enabled it will at once interrupt the CPU. This Pending interrupt may be cleared, if required, by sending an interrupt control word with bit 4 set. This is effective in all modes.

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