Berner was also chairing a session at the second international conference on information processing hosted by IFIP. In the following couple of years Dahl and Nygaard spent a lot of time teaching Simula. In C. Hoare introduced the concept of record class construct, which Dahl and Nygaard extended with the concept of prefixing and other features to meet their requirements for a generalized process concept. This paper became the first formal definition of Simula In June a conference was held to standardize the language and initiate a number of implementations.
Dahl proposed to unify the Type and the Class concept. This lead to serious discussions, and the proposal got rejected by the board. These implementations were ported to a vide range of platforms. The TOPS implemented the concept of public, protected, and private member variables and methods, that later got integrated into Simula Simula 87 is the latest standard and is ported to a vide range of platforms.
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There are mainly tree implementations:. In February they received the A.
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When the contract between NTNF and Univac was signed on 24 October , the software systems to accompany the computer were not yet ready. However, Univac assured NTNF through a penal clause in the contract that these systems would be available in time for delivery.
Introduction to SIMULA 67
The original delivery date was set for March , but for various reasons was later postponed. When the computer eventually arrived, in August , it became apparent that the software systems provided did not meet the standards promised by Univac. This situation did not improve considerably, and by March NCC had accumulated a claim on Univac amounting to approximately 1. However, progress was being made, and in June NCC finally considered the software situation satisfactory.
By then NCC's standing claim on Univac had become substantial, and important people within the Univac organization found themselves in a most awkward position. So, in order to maintain friendly terms with Univac, and to avoid unnecessary 'head-rolling' within that company, NCC agreed to accept an upgrade of the hardware configuration instead of a cash payment. In short, this meant that NCC got a very powerful computer configuration on exceedingly favorable terms. After a few weeks of assembling and testing, the computer was finally operative, approximately four months after schedule.
The vital financial question was at last settled, and the technical premises fairly well clarified. However, despite these promising conditions it would still take close to a year before the development of SIMULA really took off. The reason for this seemingly unexpected delay is partly technical, partly political, and clearly shows how science and research are largely dependent upon external premises.
Ever since Univac's offer was first known to NTNF in June , they had emphasized the fact that an engagement involving such a heavy investment would necessarily imply that the bulk of NCC's available resources had to be directed strictly towards the business side of the institute's activities.
In this respect it can be asserted that the UNIVAC came to represent a double- edged sword, at least as far as basic research activities were concerned. It is somewhat difficult to establish exactly how this situation might have affected the development of SIMULA, since this project was financed by Univac. It is evident, however, that the engagement restricted research latitude in general, and gave way for a professional profile which, at least to a certain extent, resembled the situation before For Kristen Nygaard, who in December was appointed NCC's director of research, this outlook must have been most disquieting, and he obviously found it difficult to accept that research should be pushed into 'a small corner.
According to the board's resolution of 11 December, , these new departments were meant to engage in practical commissions as well as applied research on specific target areas within NCC's mandate. Since software development had not previously been an integrated part of NCC's activity, and moreover required highly specific professional expertise, it must have been somewhat difficult to ascertain under which department it actually belonged.
Conflicts of interests must also have been important with regard to another of the board's reasons for reorganizing the institute. This responsibility rested first and foremost with NCC's director, Bjoern OErjansen, but it also applied to the rest of the staff. As pointed out, Kristen Nygaard did not quite share this opinion, and accordingly he did what he could to prevent basic research from being curtailed by scarce resources.
Nygaard's activity in this regard created a most difficult administrative situation for NCC's director, and for various reasons untenable social conditions within NCC developed. The conflict reached a climax in March when three employees were fired for, among other things, conspiracy against OErjansen and Nygaard. Since the politics surrounding these events were rather complex, I will not engage in further discussions here.
Olaussen in December For Kristen Nygaard the heavy strain caused by the working conditions in wake of the crisis lead to a sick leave during the autumn of It might therefore have been a matter of necessity that the board subsequently decided to establish a Department for Special Projects and put Nygaard in charge as director of research. The new department should focus on important problems within NCC's mandate, of such character that they could not be adequately treated by any of the other departments. As examples of this kind of activity the board puts forward: software development, research in new areas of computer application, special projects related to the development of NCC's professional profile, public relations and consulting services for important customers.
As we can see, it can easily be asserted that this was a 'playground' especially designed to suit Kristen Nygaard. In this way they could keep him occupied, and prevent him from interfering with administrative matters, and simultaneously provide a suitable forum for software development. Let us now return to the technical matters. If we compare these initial ideas with the actual outcome of their scientific endeavor, the SIMULA I compiler, we find that there is a rather distinct difference between these two positions.
In the following I will point out a few reasons for this change of goals. In other words, this meant that a SIMULA program had to operate strictly within the framework of ALGOL 60, and as we shall see, this proved to be a serious obstacle when simulation aspects were involved. However, at this early stage they were mainly preoccupied with the idea that customers in a simulation model could be depicted as ALGOL blocks and characterized by using local variables. At that time, this idea looked rather promising since ALGOL's recursive block mechanism did cater for multiple occurrences of user-defined data structures.
INTRODUCTION TO SIMULA
In short, the problem facing Dahl and Nygaard at this stage was that ALGOL 60's procedure calls and storage allocation mechanisms operated strictly according to a stack principle, whereas objects customers in a simulation model rather tended to behave according to the queue principle. In light of this, they subsequently realized that they would not achieve their design objectives unless they found a way to get around ALGOL's rigorous stack regime. During the summer and autumn of , while Kristen Nygaard was preoccupied with problems in the political arena, Ole-Johan Dahl commenced work on a new storage allocation scheme based on a two-dimensional free area list.
This change of strategy opened a whole new set of perspectives on SIMULA, and accordingly, they were compelled to start over again. This time they derived the basic concepts through a variety of thorough case studies, ranging from job shops via airport departure systems to epidemiological studies. With regard to the original network concept, they eventually discovered that this could just as well be regarded the other way around, i.
This in turn led to the realization that an in-between, or dual point of view could profitably be adopted. From this perspective the customers were regarded as active in moving from station to station, but passive in their interaction with the service parts of the various stations. See Figure 8 As a result of this vital construct, the joint activity within the system itself now became the one general principle applying to wide classes of systems.
In light of this new understanding they found that the simple network concept seemed too narrow and thus inappropriate, and for these reasons it was subsequently abandoned. In short, a process can be understood as a generalized ALGOL procedure with quasi-parallel properties.
The system was now understood as consisting of a series of interacting processes operating in quasi- parallel as ALGOL stacks within the main program execution. The implementation effort was solely conducted by Ole-Johan Dahl.