This brief sketch illustrates how the Processor communicates with memory and devices.

Devices also need to get the attention and "service" from the processor.

This can be accomplished by hardware interrupts.

A device can signal the processor by asserting the INTR signal (or its equivalent) in the control bus.

This INTR line is connected to the control unit of the processor and is checked by the processor in step 4 of the processor instruction cycle.