Real-time systems
Real-time systems play a considerable role in our society, and they cover a spectrum from the very simple to the very complex. Examples of current real-time systems include the control of domestic appliances like washing machines and televisions, the control of automobile engines, telecommunication switching systems, military command and control systems, industrial process control, flight control systems, and space shuttle and aircraft avionics.All of these involve gathering data from the environment, processing of gathered data, and providing timely response. A concept of time is the distinguishing issue between real-time and non-real-time systems. When a usual design goal for non-real-time systems is to maximize system’s throughput, the goal for real-time system design is to guarantee, that all tasks are processed within a given time. The taxonomy of time introduces special aspects for real-time system research.
Real-time operating systems are an integral part of real-time systems. Future systems will be much larger, more widely distributed, and will be expected to perform a constantly changing set of duties in dynamic environments. This also sets more requirements for future real-time operating systems.
This seminar has the humble aim to convey the main ideas on Real Time System and Real Time Operating System design and implementation.
INTRODUCTION
Timeliness is the single most important aspect of a real -time system. These systems respond to a series of external inputs, which arrive in an unpredictable fashion. The real-time systems process these inputs, take appropriate decis ions and also generate output necessary to control the peripherals connected to them. As defined by Donald Gillies “A real-time system is one in which the correctness of the computations not only depends upon the logical correctness of the computation but also upon the time in which the result is produced. If the timing constraints are not met, system failure is said to have occurred.”
It is essential that the timing constraints of the system are guaranteed to be met. Guaranteeing timing behaviour requires that the system be predictable.
The design of a real -time system must specify the timing requirements of the system and ensure that the system performance is both correct and timely. There are three types of time constraints:
Ø Hard: A late response is incor rect and implies a system failure. An example of such a system is of medical equipment monitoring vital functions of a human body, where a late response would be considered as a failure.
Ø Soft: Timeliness requirements are defined by using an average respons e time. If a single computation is late, it is not usually significant, although repeated late computation can result in system failures. An example of such a system includes airlines reservation systems.
Ø Firm: This is a combination of both hard and soft t imeliness requirements. The computation has a shorter soft requirement and a longer hard requirement. For example, a patient ventilator must mechanically ventilate the patient a certain amount in a given time period. A few seconds’ delay in the initiation of breath is allowed, but not more than that.
One need to distinguish between on -line systems such as an airline reservation system, which operates in real-time but with much less severe timeliness constraints than, say, a missile control system or a telephone switch. An interactive system with better response time is not a real-time system. These types of systems are often referred to as soft real time systems. In a soft real -time system (such as the airline reservation system) late data is still good dat a. However, for hard real -time systems, late data is bad data. In this paper we concentrate on the hard and firm real-time systems only.
Most real -time systems interface with and control hardware directly. The software for such systems is mostly custom -developed. Real -time Applications can be either embedded applications or non -embedded (desktop) applications. Real -time systems often do not have standard peripherals associated with a desktop computer, namely the keyboard, mouse or conventional display monitors. In most instances, real-time systems have a customized version of these devices.
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