We said that a bus can be used to connect a processor to an I/O device. In fact,
most buses are shared, which means that a single bus is used to connect the processor
to a set of I/O devices. Similarly, if a computer contains multiple processors, all the
processors can connect to a shared bus.
To permit sharing, an architect must define an access protocol to be used on the
bus. The access protocol specifies how an attached device can determine whether the
bus is available or is in use, and how a set of attached devices take turns using the bus.