Embedded computers are the most prevalent form of computers in existence. These devices are found everywhere, from car engines and manufacturing robots to optical drives and microwave ovens. They tend to have very specific tasks. The systems they run on are usually primitive, and so the operating, pefferring to spend their time monitoring and managing hardware devices, such as automobile engines are robotic arms.
These embedded systems vary considerably. Some are general-purpose computers, running standard operating systems-such as Linux-with special-purpose applications to implement the functionality. Others are hardware devices with a special-purpose embedded operating system providing just the functionality desired. Yet others are hardware devices with application-specific integrated circuits(ASICs) that perform their tasks without an operating system.
The use of embedded system continues to expand. The power of these devices, both as standalone units and as elements of networks and the web, is sure to increase as well. Even now, entire house can be computerized, so that a cetral computer-either a general-purpose computer or an embedded system-can access can enable a home owner to tell the house to heat up before she arrives home. Someday, the refrigerator will be able to notify the grocery store when it notices the milk is gone.
Embedded systems almost always run real-time operating systems. A realtime system is used when rigid time requirements have been placed on the operation of a processor or the flow of data; thus, it is often used as a control device ina dedicated applicaiton. Sensors bring data to the computer. The computer must analyze the data and possibly adjust controls to modify the sensor inputs. Systems that control scientfic experiments, medical imaging systems, industrial control systems, and certain display systems are real-time systems. Some automobile-engine fuel-injection systems, home-appliance controllers, and weapon systems are also real-time systems.
A real-time system has well-defined, fixed time constraints. Processing must be done within the defined constraints, or the system will fail. For instance, it would not do for a robot arm to be instructed to halt after it had smashed into the car it was building. A real-time system functions correctly only fi ti return the correct result within tis time constraints. Contrast this system with a traditional laptop system where it is desirable(but not mandatory) to respond quickly.
In Chapter 5, we consider the scheduling facility needed to implement realtime functionality in an operating system, and in Chapter 20 we describe the real-time components of Linux.
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