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Chapter 17

An Outline of the ICL 2900 Series System Architecture


J. L. Keedy1

Summary The system architecture of the ICL 2900 Series is outlined informally. Its central feature, the virtual machine concept, is described and related to virtual storage, segmentation and paging. The procedural approach is then discussed and its implementation by a stack mechanism is described. Further sections outline the protection mechanisms, and the instruction set and related features. Finally the virtual machine approach is related to global system activities.

The paper has been written such that it may he of interest to readers without a specialist knowledge of computer architecture.

Shortly after its announcement in October, 1974, the ICL 2900 Series2 was described in the popular computing press [Dorn, 1974] as little more than a copy of the B6700/7700 systems. It is easy to see how this happened, when one discovers that it is a stack oriented machine with a segmented virtual memory which makes extensive use of descriptors. In reality the implementation of these techniques is very different in the two computer families, and although a more serious attempt has been made to evaluate these differences [Doran, 1975] this is to some extent unsatisfactory since the author has, I believe, fallen into the same trap, albeit more subtly, of viewing the ICL 2900 through the eyes of someone thoroughly steeped in B6700 ideas. In fact, although the ICL 2900 has features in common with the B6700, radical differences exist, and some of the ICL 2900 features have more affinity to other systems, such as MULTICS [Organick, 1972]. Before the similarities and differences between such systems and the ICL 2900 Series can he fully appreciated, it is highly desirable that the ICL 2900 system architecture should first be understood in its own right. The real novelty of the architecture lies in the way in which its designers returned to first principles, and in the simplicity and elegance of the result. In this paper I shall therefore describe its architecture in a manner which attempts to reflect the thoughts of its designers, aiming at a level of description similar to Organick's description of the B6700 [Organick, 1973]. No attempt will be made to compare and contrast it with other systems, and it is hoped that the paper will provide an intelligible overview to readers without specialist knowledge of computer architecture.


1. The Virtual Machine

Faced with a problem to be solved using the computer, the user formulates a solution in a high level computer language such as COBOL or FORTRAN, and having satisfied himself of its correctness he will regard the resultant program as "complete." This is in one sense correct. His encoded algorithm will, if he has done his job well, be logically complete. However, even after it has been compiled, the user's program (or in more complex cases, his sequence of programs which comprise a job) must co-operate with other programmed subsystems (operating system, data management software, library routines, etc.) to solve the user's problem. The efficiency with which the problem is solved depends to a considerable extent on how the whole aggregate of necessary subsystems co-operates, and not merely on any one subsystem. It follows that it will be advantageous for a computer architecture to provide facilities for the efficient construction and execution of such aggregates, The 2900 Series explicitly recognises these aggregates, calling the environment in which each one operates a "virtual machine. "3 An aggregate itself is called a process image," its execution by a processor is a "process," and its state of execution as characterised by processor registers is its process state.

In the following sections we shall develop the idea of the virtual machine by considering its mainstore requirements, the dynamic relationship between its components, its protection requirements and its instruction set. But before we embark on this a few further remarks are necessary.

The fundamental concept, that each job runs in its own virtual machine containing all the code and data required to solve the application problem, allows the programmer to suppose that he is the sole user of the computer. But economic reality dictates that the real machine must be capable of solving several problems simultaneously, and this necessity for multiprogramming raises a set of problems which could threaten to destroy the advantages of the virtual machine approach. For example, how are the independent virtual machines co-ordinated, synchronised and scheduled? How, in view of high main storage costs, can separate process-images be permitted to have a private copy of common subsystems (e.g., the operating system)? How can virtual machines communicate with each other? Such questions will he borne in mind as we develop the concept of the virtual machine, and subsequently we shall consider them more directly, in an attempt

1Australian Computer Journal, vol. 9, no. 2, July 1977, pp. 53-62.

2References to the ICL 2900 Series in this paper are to the larger members of the new ICL range, which should not be confused with the ICL 2903 or the ICL 2904 computers.

3The term "virtual machine" has a wide variety of meanings in computer jargon. In this paper it is used consistently in the special ICL sense described here.


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