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The well-known periodic task model of Liu and Layland [10] assumes a worst-case execution time bound for every task and may be too pessimistic if the worst-case execution time of a task is much longer than the average. In this paper, we give a multiframe real-time task model which allows the execution time of a task to vary from one instance to another by specifying the execution time of a task in terms of a sequence of numbers. We investigate the schedulability problem for this model for the preemptive fixed priority scheduling policy. We show that a significant improvement in the utilization bound can be established in our model. [ABSTRACT FROM AUTHOR]

We explain why pre-run-time scheduling is essential if one wishes to guarantee that timing constraints will be satisfied in a large complex hard-real-time system. We examine some of the major concerns in pre-run-time scheduling and consider what formulations of mathematical scheduling problems can be used to address those concerns. This paper provides a guide to the available algorithms. [ABSTRACT FROM AUTHOR]

This editorial introduces the January 1993 issue of the periodical "IEEE Transactions on Software Engineering" which focuses on software for critical systems. The issue contains a selection of significant papers from the SIGSOFT '91 conference. Papers by Ricky Butler and George Finelly, John Rushby and Friedrich von Henke, and others address different aspects of the topic. [ABSTRACT FROM AUTHOR]

An approach to specification of requirements and verification of design for real-time systems is presented. A system is defined by a conventional mathematical model for a dynamic system where application specific states denote functions of real time. Specifications are formulas in duration calculus, a real-time interval logic, where predicates define durations of states. Requirements define safety and functionality constraints on the system or a component. A top-level design is given by a control law: a predicate that defines an automaton controlling the transition between phases of operation. Each phase maintains certain relations among the system states; this is analogous to the control functions known from conventional control theory. The top-level design is decomposed into an architecture for a distributed system with specifications for sensor, actuator, and program components. Programs control the distributed computation through synchronous events. Sensors and actuators relate events with system states. Verification is a deduction showing that a design implies requirements. [ABSTRACT FROM AUTHOR]