The need for more powerful and faster systems gave birth to
multiprocessing and multi-tasking systems. But to achieve this, cost
and reliability were not to be sacrificed. To reduce cost it is vital to
share resources, but to do so requires reliable means of arbitration.
In a multiprocessing system, a single bus may be shared between
various processors or intelligent peripherals. The resources shared
by processors (Figure 1) are generally termed as global resources
and those shared between the local processor and the peripherals
(Figure 2) are typically known as local resources. Whether local or
global, there always exists a protocol that will connect and
disconnect various devices to and from the shared resources.
Various bus architectures in existence today have different ways of
doing this.
No matter what the protocol of a specific bus, there is always a
method which dictates how arbitration shall be performed between
two or more devices. Some systems employ synchronous arbitration
and some use an asynchronous approach. The third option is not to
use arbitration at all, but instead to employ time-multiplexing. This is
used mainly in data communications by dividing the common media
into various time slots. Each processor (station) is assigned a
predetermined time for using the media. If the station does not need
to use the media during its assigned time-slot, it may pass control to
the next station. This obviously results in an inefficient use of the
bus bandwidth.