Input/output (I/O) devices transfer information without altering it between the external world and one or more internal components. I/O devices can be secondary memories, for example disks and tapes, or devices used to communicate directly with users, such as video displays, keyboards, and mice.
The communication channels that tie the system together can either be simple links that connect two devices or more complex switches that interconnect several components and allow any two of them to communicate at a given point in time. When a switch is configured to allow two devices to exchange information, all other devices that rely on the switch are blocked, i.e. they must wait until the switch can be reconfigured.
A common convention used in drawing simple ``stick figures'' of
computer systems is the PMS notation [32]. In a
PMS diagram each
major component is represented by a single letter, e.g. P
for processor, M for memory,
or S for switch. A subscript on a letter
distinguished different types of components,
e.g. 
for primary memory and 
for cache memory. Lines connecting two components represent
links, and lines connecting more than two components represent a
switch. Although they are primitive and might appear at first
glance to be too simple, PMS diagrams convey quite a bit of
information and have several advantages, not the least of which
is they are independent of any particular manufacturer's
notations.
