This paper presents correctness proofs of Ada 95 preference control solution for the dining philosophers paradigm. Preference control is the ability to satisfy a request depending on the parameters passed in by the calling task and often also on the internal state of the server. In Ada 95, this schema is implemented with protected objects, entry families and requeue statements within the protected object. Our aim is to show that the preference control can be described in terms of states and transitions, similar to reactive automaton descriptions. This description, which can be done in terms of colored Petri nets, can lead jointly to validate the chosen implementation and to program it with protected objects and requeue statements of Ada 95. The paper is issued from an examination exercise for our students that have followed a course on Programming and Validation of Concurrent Applications and is presented in form of progressive steps for which the students were expected to give answer. The paper contains three programmed solutions and proofs of absence of deadlock and of starvation.

The purpose of this research is to develop and analyse addressing and protection structures which will help to run more secure programs and to limit the propagation of errors. The generalised notions of object, type of object and type extension are defined and a mechanism is presented which relies on descriptors and implements those notions into the addressing scheme. This mechanism allows the definition at execution time of new levels of protection and provide a tool for controlling the co-operation of mutually suspicious subsystems.

This paper describes the main design principles of the multiaccess system ESOPE. Emphasis is placed on basic ideas underlying the design rather than on implementation details. The main features of the system include the ability given to any user to schedule his own parallel processes, using system primitive operations, and the allocation/scheduling policy, which dynamically takes into account recent information about user behaviour.