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In computer science, real-time computing (RTC), or reactive computing describes hardware and software systems subject to a "real-time constraint", for example operational deadlines from event to system response. Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines".〔Ben-Ari, M., "Principles of Concurrent and Distributed Programming", Prentice Hall, 1990. ISBN 0-13-711821-X. Ch16, Page 164〕 Real-time responses are often understood to be in the order of milliseconds, and sometimes microseconds. A system not specified as operating in real time cannot usually ''guarantee'' a response within any timeframe, although ''actual'' or ''expected'' response times may be given. A real-time system has been described as one which "controls an environment by receiving data, processing them, and returning the results sufficiently quickly to affect the environment at that time." The term "real-time" is also used in simulation to mean that the simulation's clock runs at the same speed as a real clock, and in process control and enterprise systems to mean "without significant delay". Real-time software may use one or more of the following: synchronous programming languages, real-time operating systems, and real-time networks, each of which provide essential frameworks on which to build a real-time software application. Systems used for many mission critical applications must be real-time, such as for control of fly-by-wire aircraft, or anti-lock brakes on a vehicle, which must produce maximum deceleration but intermittently stop braking to prevent skidding. Real-time processing ''fails'' if not completed within a specified deadline relative to an event; deadlines must always be met, regardless of system load. == History == The term ''real-time'' derives from its use in early simulation, in which a real-world process is simulated at a rate that matched that of the real process (now called real-time simulation to avoid ambiguity). Analog computers, most often, were capable of simulating at a much faster pace than real-time, a situation that could be just as dangerous as a slow simulation if it were not also recognized and accounted for. Minicomputers, particularly in the 1970s onwards, when built into dedicated embedded systems such as CAT scanners, increased the need for low-latency priority-driven responses to important interactions with incoming data and so operating systems such as Data General's RDOS (Real-Time Disk Operatings System) and RTOS with background and foreground scheduling as well as Digital Equipment Corporation's RT-11 date from this era. Background-foreground scheduling allowed low priority tasks CPU time when no foreground task needed to execute, and gave absolute priority within the foreground to threads/tasks with the highest priority. Real-time operating systems would also be used for time-sharing multiuser duties, for example Data General Business Basic could run in the foreground or background of RDOS (and would introduce additional elements to the scheduling algorithm to make it more appropriate for people interacting via dumb terminals. Once when the MOS Technology 6502 (used in the Commodore 64 and Apple II), and later when the Motorola 68000 (used in the Macintosh, Atari ST, and Commodore Amiga) were popular, anybody could use their home computer as a real-time system. The possibility to deactivate other interrupts allowed for hard-coded loops with defined timing, and the low interrupt latency allowed the implementation of a real-time operating system, giving the user interface and the disk drives lower priority than the real-time thread. Compared to these the Programmable Interrupt Controller of the Intel CPUs (8086..80586) generates a very large latency and the Windows operating system is neither a real-time operating system nor does it allow a program to take over the CPU completely and use its own scheduler, without using native machine language and thus surpassing all interrupting Windows code. However, several coding libraries exist which offer real time capabilities in a high level language on a variety of operating systems, for example Java Real Time. The Motorola 68000 and subsequent family members (68010, 68020 etc.) also became popular with manufacturers of industrial control systems thanks to this facility. This application area is one in which real-time control offers genuine advantages in terms of process performance and safety. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Real-time computing」の詳細全文を読む スポンサード リンク
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