|
|
Definition of Real-Time Systems
There are many ways to define a real-time system. Perhaps one of the most comprehensive definitions is: "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 at which the result is produced. If the timing constraints of the system are not met, system failure is said to have occurred." (The source for this definition is the real time FAQ in the Internet's comp.realtime news group.)
As an example, consider the bottling equipment in a beverage plant. It's not sufficient if the equipment attempts to cap bottles at regular time intervals. If it's off by just a fraction of a second, then the bottle may not be capped properly and the beer will be spoiled. That is, the time at which the machine caps the bottle is a critical part of the process.
Practically speaking, this means that real-time applications fall into two primary groups—those that can respond in "hard" real-time, and other "soft" real-time applications with less severe requirements. Simply stated, a hard real-time operating system must, without fail, provide a response to some kind of event within a specified time window. This response must be predictable and independent of other activities undertaken by the operating system. Providing this response implies that system calls will have a specified, measured latency period. Hard real-time systems often employ specific hardware devices with special device drivers.
Under this definition, Microsoft® Windows NT™ Workstation is not a hard real-time operating system. Rather, it is a general-purpose operating system that has the capability to provide very fast response times, but is not as deterministic as a hard real-time system.
E-mail: 
info@telesys.ru
