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Chapter 8

Structural Levels of the PDP-81

C. Gordon Bell / Allen Newell / Daniel P. Siewiorek

A map of the PDP-8 design hierarchy, based on the Structural Levels View of Chap. 2, is given in Fig. 1, starting from the PMS structure, to the ISP, and down through logic design to circuit electronics. These description levels are subdivided to provide more organizational details such as registers, data operators, and functional units at the register transfer level.

The relationship of the various description levels constitutes a tree structure, where the organizationally complex computer is the top node and each descending description level represents increasing detail (or smaller component size) until the final circuit element level is reached. For simplicity, only a few of the many possible paths through the structural description tree are illustrated. For example, the path showing mechanical parts is missing. The descriptive path shown proceeds from the PDP-8 computer to the processor and from there to the arithmetic unit, or more specifically, to the Accumulator (AC) register of the arithmetic unit. Next, the Logic implementing the register transfer operations and functions for the jth bit of the Accumulator is given, followed by the flip-flops and gates needed for this particular implementation. Finally, on the last segment of the path, there are the electronic circuits and components from which flip-flops and gates are constructed.

Abstract Representations

Figure 1 also lists some of the methods used to represent the physical computer abstractly at the different description levels. As mentioned previously, only a small part of the PDP-8 description tree is represented here. The many documents which constitute the complete representation of even this small computer include logic diagrams, wiring lists, circuit schematics, printed circuit board photo etching masks, production description diagrams, production parts lists, testing specifications, programs for testing and diagnosing faults, and manuals for modification, production, maintenance, and use. As the discussion continues down the abstract description tree, the reader will observe that the tree

1Originally printed in C. G. Bell, J. C. Mudge, and J. E. McNamara, Computer Engineering: A DEC View of Hardware System Design, Digital Press, 1978, pp. 209-228.


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