What's 'real' about real-time systems?

A real-time system is a concurrent computing system whose computations and actions must satisfy time constraints. Guaranteeing that such a system will meet its constraints can be viewed either in terms of an extended model of program correctness or as a problem of scheduling, e.g. establishing a feasible schedule. However, a crucial distinction between real-time and other concurrent programs is not merely that of time, but that the former must often execute on systems with limited resources. The authors give a short account of a limited resource semantics of program execution. The model can serve both as the basis for a formal specification system for real-time programs and to characterize real-time scheduling problems.<>