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Consider a software system in which the inputs represent digital data from hardware such as imaging devices or other software system's and the outputs are digital data that control external hardware such as displays. The time between the presentation of a set of inputs and the appearance of all the associated outputs is called the response time. A real-time system is one that must satisfy explicit bounded response time constraints to avoid failure. Equivalently, a real-time system is one whose logical correctness is based both on the correctness of the outputs and their timeliness. Notice that response times of, for example, microseconds are not needed to characterize a real-time system - it simply must have response times that are constrained and thus predictable. In fact, the misconception that real-time systems must be "fast" is because in most instances, the deadlines are on the order of microseconds. But the timeliness constraints or deadlines are generally a reflection of the underlying physical process being controlled. For example, in image processing involving screen update for viewing continuous motion, the deadlines are on the order of 30 microseconds. In practical situations, the main difference between real-time and non-real-time systems is an emphasis on response time prediction and its reduction.
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