computer science
Real-time systems
The design of real-time systems is becoming
increasingly important. Computers have been incorporated
into cars, aircraft, manufacturing assembly lines, and
other applications to control processes as they
occur--known as "in real time." It is not
practical in such instances to provide input to the computer,
allow it to compute for some indefinite length of time,
and then examine the output. The computer output
must be available in a timely fashion, and the processor
(or processors) must be carefully chosen and the tasks
specially scheduled so that deadlines are met.
Frequently, real-time tasks repeat at fixed time
intervals; for example, every so many seconds, sensor
data are gathered and analyzed and a control signal
generated. In such cases, scheduling theory is utilized
by the systems designer in determining how the tasks
should be scheduled on a given processor. A good example
of a system that requires real-time action is the
antilock braking system (ABS) on most newer vehicles;
because it is critical that the ABS instantly react to
brake-pedal pressure and begin a program of pumping the
brakes, such an application is said to have a hard
deadline. Some other real-time systems are said to have
soft deadlines, in that, although it is deemed important
to meet them, no disaster will happen if the system's
response is slightly delayed; an example is ocean
shipping and tracking systems. The concept of "best
effort" arises in real-time system design, not only
because soft deadlines may sometimes be slipped, but
because hard deadlines may sometimes be met by computing
a less than optimal result. For example, most details on
an air traffic controller's screen are approximations--e.g.,
altitude, which need not be displayed to the nearest
inch--that do not interfere with air safety.
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