Embedded processors can be broken into two broad categories:
ordinary microprocessors (μP) and microcontrollers (μC), which have many more
peripherals on chip, reducing cost and size. Contrasting to the personal
computer and server markets, a fairly large number of basic CPU architectures
are used; there are Von Neumann as well as various degrees of Harvard
architectures, RISC as well as non-RISC and VLIW; word lengths vary from 4-bit
to 64-bits and beyond (mainly in DSP processors) although the most typical
remain 8/16-bit. Most architectures come in a large number of different
variants and shapes, many of which are also manufactured by several different
companies.
An embedded system is a computer system designed to do one
or a few dedicated and/or specific functions often with real-time computing
constraints. It is embedded as part of a complete device often including
hardware and mechanical parts. By contrast, a general-purpose computer, such as
a personal computer (PC), is designed to be flexible and to meet a wide range
of end-user needs. Embedded systems control many devices in common use today.
Embedded systems contain processing cores that are typically
either microcontrollers or digital signal processors (DSP). The key
characteristic, however, is being dedicated to handle a particular task. They
may require very powerful processors and extensive communication, for example
air traffic control systems may usefully be viewed as embedded, even though
they involve mainframe computers and dedicated regional and national networks
between airports and radar sites (each radar probably includes one or more
embedded systems of its own).
Since the embedded system is dedicated to specific tasks,
design engineers can optimize it to reduce the size and cost of the product and
increase the reliability and performance. Some embedded systems are
mass-produced, benefiting from economies of scale.
Physically embedded
systems range from portable devices such as digital watches and MP3 players, to
large stationary installations like traffic lights, factory controllers, or the
systems controlling nuclear power plants. Complexity varies from low, with a
single microcontroller chip, to very high with multiple units, peripherals and
networks mounted inside a large chassis or enclosure.
In general, "embedded system" is not a strictly
definable term, as most systems have some element of extensibility or
programmability. For example, handheld computers share some elements with
embedded systems such as the operating systems and microprocessors that power
them, but they allow different applications to be loaded and peripherals to be
connected. Moreover, even systems that do not expose programmability as a
primary feature generally need to support software updates. On a continuum from
"general purpose" to "embedded", large application systems
will have subcomponents at most points even if the system as a whole is
"designed to perform one or a few dedicated functions", and is thus
appropriate to call "embedded".
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