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The goal of this chapter is to give the reader insight into the operation of computers. Again RTM's are used -- here for fabricating computers. That we have used RTM's before in a slightly different context does not seem to limit our ability to present new concepts with them. In fact, since we use a fixed set of components, now well-known to the reader, it is feasible to carry out the description and analysis at a much deeper level than when we have to worry about implementation details. In describing a computer we can show the details of the structure and behavior by allowing the reader to look at the complete internal structure of the computer.


Several computers are presented for various reasons. Holding with the policy that repetition reinforces learning, we. also include at least two problems to reinforce concepts. Three C's are given: the Crtm-1, the first minicomputer constructed from RTM's; the PDP-8/RTM, based on a common minicomputer, the PDP-8; and the PDP-16/M, a subminicomputer which is microprogrammed.

The Crtm-1 is given both for historical and pedagogical reasons. Besides having been the first, it is nearly the simplest computer that can be constructed from RTM's, requiring only about 200 control wires. Because it is so simple, we believe that anyone who is unfamiliar with general purpose stored program computers can understand them using Crtm-1 as a model. Computers of about its size take about 8 to 12 hours to define, document, wire, and get operational. Therefore, we hope every reader who has not had the joy of designing and implementing a language or a computer will take the time to design and build one with RTM's, or at least carry out the paper exercise. He should then spend at least as long writing programs for it, and possibly re-iterate the design.

The PDP-8/RTM is given to provide an exam pie of an existing minicomputer, which was designed independently (actually, long ago in 1965) of the work in RTM's. Educational examples are almost always oversimplified. This is especially true of computers, since they have many seemingly irrelevant details. But these details are in fact an essential part of the design -- of what makes the computer useful in the real world. Consequently, it is important to design and analyze an actual computer.

The PDP-16/M is presented because in applying the PDP-16 (i.e., RTM's) to a problem, a 16/M configuration might be the best solution. Also,, since the 16/M is like a microprogram computer with no higher level instruction-set, it gives another view of microprogramming.

In terms of gradual transition from hardwired RTM systems to fully developed computers, the order should be 16/M, CRTM-1, PDP-8/RTM. We place the 16/M at the end in this chapter for reasons of pedagogy.

Several additional small computers are described briefly as problems: a general register structure machine, a desk calculator, and a machine using the DMar.


In describing Crtm-1, a notation for computer instruction-set description, ISP, is introduced. ISP should be relatively familiar by now, since it is the programming-language-like notation used in the RTM flowchart and data part diagrams. By using the notation, a one-dimensional text string (such as in conventional programming) expresses the structure and behavior of a digital


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