Ole-Johan Dahl

SIMULA: an ALGOL-based simulation language

This paper is an introduction to SIMULA, a programming language designed to provide a systems analyst with unified concepts which facilitate the concise description of discrete event systems. A system description also serves as a source language simulation program. SIMULA is an extension of ALGOL 60 in which the most important new concept is that of quasi-parallel processing.

The development of the SIMULA languages

The organizers of this conference have told us that we should write at least 25 pages of manuscript, but that we may produce as many pages more as we wanted. Perhaps they did not envisage the possible consequences, but we have taken their words at face value. This paper has implied a vast amount of work and archeological activities. We are grateful to SIGPLAN for defining a task to which resources had to be allocated by our institutions and which forced us to write down an account of our work from 1961 to 1971. While we are writing this preface, those years are very much alive to us. We realize that we lived through this period in a state of semimadness, a combination of very hard work, frustration, and euphoria.

Coroutine sequencing in a block structured environment

A certain primitive sequencing operation is defined, applicable as an extension to Algol-like languages. The operation serves the double purpose of “intermediate exit” from or “reentry” into procedure-like block instances. A second primitive, which corresponds to a fully symmetric coroutine linkage, is derived from the former. An abstract notation is introduced for the concepts of textual and dynamic enclosure of block instances. The effects of the sequencing primitives are explored in terms of this notation, and certain results are proved. Finally applications are given within the language framework of Simula 67.

Analysis of an algorithm for priority queue administration

A special kind of priority queue structure, priority trees (p-trees), and an algorithm for building such trees are investigated. The main part of the paper is devoted to a mathematical analysis of the algorithm showing that the expected number of key comparisons to insert a “random” element into a “random”p-tree withn nodes isO((logn)2). Also some practical experiments, comparing different types of priority queue strategies, are presented.

The roots of object orientation: the Simula language

The development of the programming languages Simula 1 and Simula 67 is briefly described. An attempt is made also to explain the cultural impact of the languages, in particular the object-oriented aspects.

Generator induction in order sorted algebras

Linguistic and semantic consequences of combining the ideas of order sorted algebras (as in OBJ) and generator induction (as in Larch) are investigated. It is found that one can gain the advantages of both, in addition to increased flexibility in defining signatures and generator bases. Our treatment also gives rise to typing control stronger in a certain sense than that of OBJ, as well as the detection of inherently inconsistent signatures.

An Approach to Correctness Proofs of Semicoroutines

The paper discusses local correctness criteria and local correctness proofs of semicoroutines subject to certain simplifying assumptions. A nontrivial worked example is given.

A presentation of the specification and verification project "ABEL"

This is an overview of the ABEL-project which is led The first section describes the main goals of the project. The second section describes research activities associated with the project, emphasizing areas that are assumed to be relevant to the VERkshop. The last section presents the ABEL85 language through a non-trivial example.

IX – SIMULA Session

Publisher Summary The development of the SIMULA I and SIMULA 67 languages was carried out at the Norwegian Computing Center (NCC). However, the early background for the development is the work at the Norwegian Defence Research Establishment (NDRE) in the 1950s. A BULL punched-card calculator had, in 1951–1953, been extended at the NDRE into a card-programmed electromechanical computer. In 1954, it was decided that NDRE should acquire a Ferranti MERCURY computer, then at the design and construction stage, and in the following years the NDRE milieu developed basic software which was fairly advanced for its time. In the late 1950s, the NDRE milieu started work in language design, in which Garwick and OJD were particularly active. This chapter discusses the four main stages that the SIMULA I language went through. At the NCC, the use of SIMULA I for simulation programming started immediately and spread rapidly. In 1965, three SIMULA I courses were given at NCC. The use of SIMULA I up to December 1965 is reported in Nygaard (1965b). A later version of this report covers the period up to June 1968.

Litt om symbolbruken i ALGOL

Some critical remarks are made in connection with the use of symbols in ALGOL. The underlined words and the for-statements are especially considered.