Abstract A sequence of events is described, leading to the severe deterioration of an initially well conceived and cleanly implemented compiler. It is shown how an initial “optimization” implanted a latent bug in the compiler, how the bug was subsequently activated through innocent compiler modification, and how the compiler then deteriorated because of the incompetent correction of the bus manifestation. This exceedingly negative case history is presented in the hope of conveying to the reader a better feeling for the complex problems inherent to industrial software production and maintenance. The difficulty in proposing any constructive (and complete!) software engineering methodology is known and acknowledged; the study of an episode such as described in this paper might help put the difficulties, with which we are confronted, into better perspective.

Abstract As part of its effort to periodically investigate various new promising concepts and techniques, the Digital Equipment Corporation had sponsored a research project whose purpose it was to effect a limited exploratory implementation of a novel protective operating system framework, based on the kernel/domain architecture. The project was carried out in 1972, and its successful completion led to a number of observations and insights. This paper concentrates on the technological solutions which were developed for the translation of a theoretical model on to commercially available hardware (the DEC PDP‐11/45 minicomputer); specifically, the storage mapping technique and the intermodule call/return mechanism. We believe that we have made two contributions to the state of the art: firstly the identification of the doman's storage mapping properties which are expected to have an impact on the definition of storage class semantics for higher level programming languages to be developed for the domain environment, and secondly the development of a comprehensive intermodule communication mechanism combining all presently known forms of procedure activation within a single functional framework.

As part of its effort to periodically investigate various new promising concepts and techniques, the Digital Equipment Corporation has sponsored a research project whose purpose it was to effect a limited implementation of a protective operating system framework, based on the kernel/domain architecture which has increasingly been propounded in recent years.

Essential to any multi-process computer system is some mechanism to enable coexisting processes to communicate with one another. The basic inter-process communication (IPC) mechanism is the exchange of messages among independent processes in a commonly accessible data base and in accordance with some pre-arranged convention.By introducing several system wide conventions for initiating communication, and by utilizing the Traffic Controller it is possible to expand the basic IPC mechanism into a general purpose IPC facility. The Multics IPC facility is an extension of the central supervisor which assumes the burden of managing the shared data base and of respecting the IPC conventions, thus providing a simple and easy way for the programmer to use the interface.