Real-Time Constraints

(Invited Article, May 2000)
Pao-Ann Hsiung
Institute of Information Science
Academia Sinica, Taipei, Taiwan.
( Encyclopedia of Computer Science and Technology,
Volume 45 - Supplement 30, (ISBN: 0-8247-2298-1)
Allen Kent and James G. Williams, Editors,
Marcel Dekker, Inc., New York, USA, May 2001.)


Restrictions on the timings of events are called real-time constraints. A system with real-time constraints is called a real-time system. Not merely the performance, but also the feasibility of such systems depends on the satisfaction of real-time constraints. Hard constraints must be satisfied for system correctness, while the violation of soft constraints only degrades a system. Specification of real-time constraints either requires extensions of traditional programming languages through annotations and logic expressions, or the use of temporal logics or some formal declarative language with temporal constructs. Stringent restrictions on timings complicate system design and verification. The time model can be dense or discrete, thus giving different methods for real-time system synthesis and verification. This article surveys the specification, design, and verification of real-time constraints and systems. Different models are presented for handling real-time constraints, such as object-oriented model, formal model, and dynamic model. Design techniques for real-time hardware and for real-time software applications are introduced and discussed. Verification techniques related to real-time constraints, such as clock shielding, are also presented.