As the figure illustrates, we think of an external device as independent from the
processor except for digital signals that pass between them. In practice, of course, some
devices reside in the same enclosure with the processor, and both receive power from a
common source. However, we will ignore such details and concentrate on the control
signals.
Modern computers also arrange for the processor to control external devices. For
example, a processor can start a disk spinning, control the volume on an external speaker,
or turn off a printer. In the next chapter, we will learn how a modern computer
communicates control information to external devices.