Real Time Systems
Real-Time systems are becoming pervasive. Typical examples of real-time systems include Air Traffic Control Systems, Networked Multimedia Systems, Command Control Systems etc. In a Real-Time System the correctness of the system behavior depends not only on the logical results of the computations, but also on the physical instant at which these results are produced. Real-Time systems are classified from a number of viewpoints i.e. on factors outside the computer system and factors inside the computer system. Special emphasis is placed on hard and soft real-time systems. A missed deadline in hard real-time systems is catastrophic and in soft real-time systems it can lead to a significant loss. Hence predictability of the system behavior is the most important concern in these systems. Predictability is often achieved by either static or dynamic scheduling of real-time tasks to meet their deadlines. Static scheduling makes scheduling decisions at compile time and is off-line. Dynamic scheduling is online and uses schedulabilty test to determine whether a set of tasks can meet their deadlines. The present paper talks about static and dynamic scheduling algorithms and operating systems support for these mechanisms.Real-Time systems span several domains of computer science. They are defense and space systems, networked multimedia systems, embedded automative electronics etc. In a real-time system the correctness of the system behavior depends not only the logical results of the computations, but also on the physical instant at which these results are produced. A real-time system changes its state as a function of physical time, e.g., a chemical reaction continues to change its state even after its controlling computer system has stopped. Based on this a real-time system can be decomposed into a set of subsystems i.e., the controlled object, the real-time computer system and the human operator. A real-time computer system must react to stimuli from the controlled object (or the operator) within time intervals dictated by its environment. The instant at which a result is produced is called a deadline. If the result has utility even after the deadline has passed, the deadline is classified as soft, otherwise it is firm. If a catastrophe could result if a firm deadline is missed, the deadline is hard. Commands and Control systems, Air traffic control systems are examples for hard real-time systems. On-line transaction systems, airline reservation systems are soft real-time systems.