Real-Time Systems

Real-Time Systems

Another form of a special-purpose operating system is the real-time system. A real-time system is used when there are rigid time requirements on the oper­ation of a processor on the flow of data, and thus is often used as a control device in a dedicated application. Sensors bring data to the computer. The computer must analyze the data and possibly adjust controls to modify the sensor inputs. Systems that control scientific experiments, medical imaging systems, industrial control systems, and some display systems are real-time systems. Also included are some automobile-engine fuel-injection systems, home-appliance controllers, and weapon systems. A real-time operating 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 is considered to function correctly only if it returns the correct result within any time constraints. Contrast this requirement to a time-sharing system, where it is desirable (but not mandatory) to respond quickly, or to a batch system, where there may be no time constraints at all.

There are two flavors of real-time systems. A hard real-time system guarantees that critical tasks complete on time. This goal requires that all delays in the system be bounded, from the retrieval of stored data to the time that it takes the operating system to finish any request made of it. Such time constraints dictate the facilities that are available 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 separate the user further from the hardware, and that separation results in uncertainty about the amount of time an operation will take. For instance, virtual memory is almost never found on real-time systems. Therefore, hard real-time systems conflict with the operation of time-sharing systems, and the two cannot be mixed. Since none of the existing general-purpose operating systems support hard real-time functionality, we do not concern ourselves with this type of system in this text. A less restrictive type of real-time system is a soft real-time system, where a critical real-time task gets priority over other tasks, and retains that priority until it completes. As in hard real-time systems, kernel delays need to be bounded: A real-time task cannot be kept waiting indefinitely for the kernel to run it. Soft real time is an achievable goal that is amenable to mixing with other types of systems. Soft real-time systems, however, have more limited utility than do hard real-time systems. Given their lack of deadline support, they are risky to use for industrial control and robotics. There are several areas in which they are useful, however, including multimedia, virtual reality, and advanced scientific projects such as undersea exploration and planetary rovers. These systems need advanced operating-system features that cannot be supported by hard real-time systems. Because of the expanded uses for soft real-time functionality, it is finding its way into most current operating systems, including major versions of UNIX.