Nearly all devices have a master interrupt enable control flag that can be set and cleared by under programmed I/O by the controlling processor. Its output is just ANDed with the local interrupt source.
The programmed I/O uses the write enable (wen) signal to guard the transfer of data from the main data bus into the control register. A ren signal is used for reading.
The principal of programming is
With only a single interrupt wire to the processor, all interrupt sources share it and the processor must poll around on each interrupt to find the device that needs attention.
Enchancement: a vectored interrupt makes the processor branch to a device-specific location.
Interrupts can also be associated with priorities, so that interrupts of a higher level than currently being run preempt.