In this paper, we consider the preemptive scheduling of hard-real-time sporadic task systems on one processor. We first give necessary and sufficient conditions for a sporadic task system to be feasible (i.e., schedulable). The conditions cannot, in general, be tested efficiently (unless P = NP). They do, however, lead to a feasibility test that runs in efficient pseudo-polynomial time for a very large percentage of sporadic task systems.

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In this 25th year anniversary paper for the IEEE Real Time Systems Symposium, we review the key results in real-time scheduling theory and the historical events that led to the establishment of the current realtime computing infrastructure. We conclude this paper by looking at the challenges ahead of us.

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A~stract-Two important characteristics of time-critical systems are: the requirement to satisfy stringent timing constraints, and the need to guard against an, imperfect execution environment. In this paper, we formalize the safety analysis of timing properties in real-time syste~s. Our analysis is based on a formal logic: RTL (Real-Time Logic) which is especially suitable for reasoning about the timing behavior of systems. Given the for~al specification of a~ystem a~d a safety assertion to be analyzed, our goal is to relate the safety assertion to the systems specification. There are three distinct eases: 1) the safety assertion is, a theorem derivable from the systems specification, 2) the safety assertion is,~nsatisfia~le with respect to the systems specification, or 3) the negation of the safety assertion is satisfiable under certain conditions. A systematic method for performing safety analysis will be presented.'

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