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Introduction

A Real-Time Operating System (RTOS) is a computing environment that reacts to input within a specific time period. A real-time deadline can be so small that system reaction appears instantaneous. The term real-time computing has also been used, however, to describe "slow real-time" output that has a longer, but fixed, time limit.Learning the difference between real-time and standard operating systems is as easy as imagining yourself in a computer game. Each of the actions you take in the game is like a program running in that environment. A game that has a real-time operating system for its environment can feel like an extension of your body because you can count on a specific "lag time:" the time between your request for action and the computer's noticeable execution of your request. A standard operating system, however, may feel disjointed because the lag time is unreliable. To achieve time reliability, real-time programs and their operating system environment must prioritize deadline actualization before anything else. In the gaming example, this might result in dropped frames or lower visual quality when reaction time and visual effects conflict.

Real-time systems are computer systems that monitor, respond to, or control an external environment. This environment is connected to the computer system through sensors, actuators, and other input-output interfaces. It may consist of physical or biological objects of any form and structure. Often humans are part of the connected external world, but a wide range of other natural and artificial objects, as well as animals, are also possible.

The computer system must meet various timing and other constraints that are imposed on it by the real-time behavior of the external world to which it is interfaced. Hence comes the name real time. Another name for many of these systems is reactive systems, because their primary purpose is to respond to or react to signals from their environment. A real-time computer system may be a component of a larger system in which it is embedded; reasonably, such a computer component is called an embedded system.

Applications and examples of real-time systems are ubiquitous and proliferating, appearing as part of our commercial, government, military, medical, educational, and cultural infrastructures. Included are

vehicle systems for automobiles, subways, aircraft, railways, and ships
traffic control for highways, airspace, railway tracks, and shipping lanes
process control for power plants, chemical plants, and consumer products such as soft drinks and beer
medical systems for radiation therapy, patient monitoring, and defibrillation
military uses such as firing weapons, tracking, and command and control
manufacturing systems with robots
telephone, radio, and satellite communications
computer games
multimedia systems that provide text, graphic, audio, and video interfaces
household systems for monitoring and controlling appliances
building managers that control such entities as heat, lights, doors, and elevators