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Another form of a special-purpose operating system is the real-time system. A real-time 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 in a dedicated application. Sensor bring data to the computer. The computer must analyze the data and possilbly adjust controls to modify the senso inputs. Systems that control scientific experiments, medical imaging systems,industrial control systems, and certain disply 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 systems has well-defined, fixed time constaints. Processing must be done within the defined constraints, or the system will fail for must be done within the defined constraints, or the system will fail For intance, 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 constraints. Contrast this requirement to a time-sharing systems, where it is desirable( but not mandatory) to respond quicly, or to a batch system, which may have no time constraints at all.
Real time syatems come in two flavors: hard and soft. A hard real time system gurantees that critical tasks be completed on time. This goal requires that all delays in the syste be bouned, from the retrival of stored data to the time that it takes the operating system to finish any request made of it Such time constrints dictate the facilities that are avilable in hard real-time systems Secondary storage of any sort is usually limited or missing, with data instead being stored in short term memory or in read-only memory (ROM). ROM is located on nonvolatile storage devices that retain their contents even in the case of electric outage; most other types of memory are volatile. Most advanced operating system features are absent too, since they tend to seperate the user from the hardware, and that sepration results in uncertainly about the amounts of time an operatin will take. For instance, virtul memory is mixed. Since none of the existing genral-purpose operating systems support hard real time functionality, we don't concern ourselves with this type of system in this text
A less restrictive type of real-time systems is a soft real-time system, where a critical real-time task gets prority over other tasks, and retains that priority untill it completes. As in hard real-time systems, the operating-systems kernel delays need to bounded: A real time is an achievable goal that can be mixed with othr types of systems. Soft real-time systems, however, have more limited utility than hard real-time systems given their lack of deadline support,they are risky to use for industrial controls and robotics. They are useful however in servel areas, including multimedia, virtul reality, and advanced, scientific projects-such as undersea explation and planetary rovers. These systems neeed advanced operating systems features that can't be supported by hard it is finding its way into most current operating systems, including major version of UNIX.