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22 Part 1 ½ Fundamentals Section 1 ½ Abstraction and Notation

is usual in specifying a controller in an overall description of a system. (The next level of detail goes to the structure of the actual operations and instructions and belongs to the ISP level, not the PMS level.)

As noted earlier, there is significant advantage to defining a PMS diagram at a more detailed level. Thus the notation provides for recursive definition, as in the case of the paper-tape transducer in Fig. 1. There is both a 300 char/s tape reader and a 100 char/s tape punch.

For the Pc in Fig. 1, the manufacturer's name is capitalized and preceded by a single vertical prime or quotation mark: 'PDP-8. By convention, generic names and abbreviations always appear lowercase and proper names always begin uppercase. We have given a few parameters: the data-types, the technology, etc.

We have used several different ways of saying the same thing in Fig. 1 in order to show the range of descriptive notations. Thus the 64 Teletypes are shown by describing a single connection through a switch and putting the number of links in the switch above the connecting line.

Consider, finally, the Pc in Fig. 1. We have given a few parameters: the data-types, the processor state, the word length, etc. These few parameters are easily inferred from the Mp. The basic operation time in a processor is a small multiple of the read time of its Mp. Thus it is predictable that the Pc stores and reads information in 2 x 1.5 m s (one for instruction fetch, one for data fetch). Again, where this is not the case (as in the CDC 6600) it is necessary to say so. Similarly, the word size in the Pc is the same as the word size of the Mp: 12 data hits. More generally, the Pc must have instructions that take care of evoking all the components of the PMS structure. These instructions do not see the switches and controls as distinct entities; rather, they speak directly to the operation of the M's and T's connected via these switches and controls.

Other summary parameters could have been given for the Pc. None of them would come close to specifying its behavior uniquely, although to those knowledgeable in computers still more can be inferred from the parameters given. For instance, knowing both the data-types available in a Pc and the number of instructions, one can come very close to predicting exactly what the instructions are. Nevertheless, the way to describe a Pc in frill detail is not to add larger and larger numbers of summary parameters. It is more direct and more revealing to develop a description at the level of instructions, which is the ISP description.

Because of the evolution of technology, PMS components continue to decrease in physical size. Consider the PDP-5, the immediate predecessor of the PDP-8. When introduced in 1963, the PDP-5 processor logic required 100 boards and occupied 2,100 in2 of hoard space. By 1971, the PDP-8/E required only three hoards and 240 in2. In 1976 the Intersil 6100 implemented the PDP-8 ISP on a single silicon chip approximately 1/4 in on a side. If the PDP-8 ISP were reimplemented today, the processor would occupy a small fraction of the chip area. The next decade will be one of design at the PMS level with many PMS components being integrated into individual semiconductor chips.

Let us end this introduction to the PMS descriptive system by returning to a critical item in its design philosophy. A descriptive scheme for systems as complex and detailed as digital computers must have the ability to range from extremely complete to highly simplified descriptions. It must permit highly compressed descriptions as well as extensive ones and must permit the selective suppression or amplification of whatever aspects of the computer system are of interest to the user. PMS attempts to fulfill these criteria by providing simple conventions for detailed description with additional conventions that permit abbreviation and abstractions, almost without limit. The result is a notation that may seem somewhat fluid, especially on first contact in such a brief introduction as this. But once assimilated, PMS seems to allow some of the flexibility of natural language within enough notational controls to enhance communication considerably.

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