Implementing atomic actions in Ada 95

Atomic actions are an important dynamic structuring technique that aid the construction of fault-tolerant concurrent systems. Although they were developed some years ago, none of the well-known commercially-available programming languages directly support their use. This paper summarizes software fault tolerance techniques for concurrent systems, evaluates the Ada 95 programming language from the perspective of its support for software fault tolerance, and shows how Ada 95 can be used to implement software fault tolerance techniques. In particular, it shows how packages, protected objects, requeue, exceptions, asynchronous transfer of control, tagged types, and controlled types can be used as building blocks from which to construct atomic actions with forward and backward error recovery, which are resilient to deserter tasks and task abortion

Reasoning About the Reliability of Multi-version, Diverse Real-Time Systems

This paper is concerned with the development of reliable real-time systems for use in high integrity applications. It advocates the use of diverse replicated channels, but does not require the dependencies between the channels to be evaluated. Rather it develops and extends the approach of Little wood and Rush by (for general systems) by investigating a two channel system in which one channel, A, is produced to a high level of reliability (i.e. has a very low failure rate), while the other, B, employs various forms of static analysis to sustain an argument that it is perfect (i.e. it will never miss a deadline). The first channel is fully functional, the second contains a more restricted computational model and contains only the critical computations. Potential dependencies between the channels (and their verification) are evaluated in terms of aleatory and epistemic uncertainty. At the aleatory level the events ''A fails" and ''B is imperfect" are independent. Moreover, unlike the general case, independence at the epistemic level is also proposed for common forms of implementation and analysis for real-time systems and their temporal requirements (deadlines). As a result, a systematic approach is advocated that can be applied in a real engineering context to produce highly reliable real-time systems, and to support numerical claims about the level of reliability achieved

On twisted forms and relative algebraic K-theory

This paper introduces a new approach to the study of certain aspects of Galois module theory by combining ideas arising from the study of the Galois structure of torsors of finite group schemes with techniques coming from relative algebraic $K$-theory.Comment: Final version. To appear in the Proceedings of the London Mathematical Societ

An optimal synchronous bandwidth allocation scheme for guaranteeing synchronous message deadlines with the timed-token MAC protocol

This paper investigates the inherent timing properties of the timed-token medium access control (MAC) protocol necessary to guarantee synchronous message deadlines in a timed token ring network such as, fiber distributed data interface (FDDI), where the timed-token MAC protocol is employed. As a result, an exact upper bound, tighter than previously published, on the elapse time between any number of successive token arrivals at a particular node has been derived. Based on the exact protocol timing property, an optimal synchronous bandwidth allocation (SBA) scheme named enhanced MCA (EMCA) for guaranteeing synchronous messages with deadlines equal to periods in length is proposed. Thm scheme is an enhancement on the previously publiibed MCA scheme

Physical processes in planetary rings

A summary of research performed in 1990 is presented. The subject areas covered include perturbed narrow rings and the dynamics of circumplanetary dust. Progress made in the area of perturbed narrow rings includes: (1) the possible discovery of an undocumented moonlet in the environs of Saturn's F ring; and (2) the investigation of the consequences of a close satellite perturbing a narrow ring using numerical simulation. Progress made in the area of circumplanetary dust includes: (1) studies of the motion of circumplanetary dust under the action of radiation pressure and various electromagnetic processes; and (2) the initiation of a systematic explanation of the curious consequences of some of the perturbations that act on small particles

Propulsion sizing program

Computer program was written to evaluate and define optimum design parameters of low pressure auxiliary propulsion system. APS will provide attitude and translational control of space shuttle vehicles. Program evaluates subsystem design parameters such as engine chamber pressure, mixture ratio, expansion ratio, and subsystem component weight and size

Investigations of planetary ring phenomena

Faint planetary rings, their dynamical behavior and physical properties, were the main focus of the research efforts. The motion of weakly-charged dust through the gravitational and magnetic fields of Jupiter were examined. Several topics concerning features of Saturn's rings were addressed. The origin and fate of the Uranian ring dust is presently being studied