Asis Goswami

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.<<ETX>>

ISL: An Interval Logic for the Specification of Real-time Programs

ISL is a linear-time temporal logic for specifying properties of programs in execution intervals which are sequences of states. The end points of intervals are specified using instances of state predicates (or assertions) or time values. Abstract intervals, delimited by formulae over states in a computation, are used as the first step in constructing a timed specification. This is then transformed to incorporate timing, first by logical formulae and then using concrete time domains. Refinements are introduced to define time domains and timing properties and include refinement to programming constructs. We outline a way to specify resource limitations along with the functional and timing properties of programs. The specification method is illustrated with some examples.

Ordered ports—a language concept for high-level distributed programming

A new language concept for high-level distributed programming is proposed. Programs are organised as a collection of concurrently executing processes. Some of these processes, referred to as liaison processes, have a monitor-like structure and contain ports which may be invoked by other processes for the purposes of synchronisation and communication. Synchronisation is achieved by conditional activation of ports and also through port control constructs which may directly specify the execution ordering of ports. These constructs implement a path-expression-like mechanism for synchronisation and are also equipped with options to provide conditional, non-deterministic and priority ordering of ports. The usefulness and expressive power of the proposed concepts are illustrated through solutions of several representative programming problems. Some implementation issues are also considered.

Semancitcs of Real-time Distributed Programs

A semantic model for distributed real-time programs is proposed. The semantics is state-based and compositional. It preserves the basic properties of process autonomy and considers their nondeterministic execution in a dense time domain. The internal actions and communications of a command are treated in a uniform way to obtain a simple semantic domain. An ordering on this domain for information approximation is developed. The absence of global objects in the semantics of a command makes it possible for modular changes to adapt the model for different communication mechanisms and different execution environments. To illustrate this, we show how process executions can be modelled in an environment with limited processors. The proposed semantics models termination, failure, divergence, deadlock, and starvation, and supports an arbitrary degree of parallelism.

A functional style of programming with CSP-like communication mechanisms

This paper introduces CSP-like communication mechanisms into Backus’ Functional Programming (FP) systems extended by nondeterministic constructs. Several new functionals are used to describe nondeterminism and communication in programs. The functionals union and restriction are introduced into FP systems to develop a simple algebra of programs with nondeterminism. The behaviour of other functionals proposed in this paper are characterized by the properties of union and restriction. The axiomatic semantics of communication constructs are presented. Examples show that it is possible to reason about a communicating program by first transforming it into a non-communicating program by using the axioms of communication, and then reasoning about the resulting non-communicating version of the program. It is also shown that communicating programs can be developed from non-communicating programs given as specifications by using a transformational approach.

Semantics for Specifying Real-Time Systems

Real-time programs are the least well understood of concurrent programs: apart from being subject to all the usual problems associated with concurrency and communication they must interact with a variety of agents’ at points in the execution that have a specified ordering relation with time. So, for example, the familiar partial order over the execution of the statements of a concurrent program must be made more elaborate to accommodate time-ordering, and this requires a semantics that can account for programs whose execution may be constrained by limitations on the availability of resources.