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Section 3

Processors defined by a microprogram

Processors defined by a microprogram have only recently come into existence, although Wilkes suggested the idea in 1951. The discussion in Chap. 3 (page 71) suggests reasons why this controversial idea has taken so long to be adopted.

Microprogramming and the design of the control circuits in an electronic computer

Chapter 28 is an extension of an earlier paper by Wilkes. It includes an example of a microprogrammed processor (page 337). In the earlier paper, The Best Way to Design an Automatic Computing Machine [Wilkes, 1951a], the essential ideas of microprogramming were first outlined.

The observation that an instruction set, or ISP, should be looked at as a program to be interpreted is the basis of micro- programming. The idea of an ISP is our acknowledgment that we, too, view a processor as a program.

There is little to say about this chapter; it is historical, yet timely and well written. Microprogramming, like other of Wilkes' ideas, is present in many of our computers.

The design of a general-purpose microprogram- controlled computer with elementary structure

The SD-2 computer (Chap. 29) is described by Kampe in a casual but highly communicative fashion. Most engineers tend to be somewhat formal and stuffy when describing the machines they have designed. This formal ruse can be used to make the design seem difficult but well founded-certainly not arbitrary. Kampe truthfully admits to making decisions in a somewhat arbitrary fashion.

The SD-2 microprogram structure, unlike that of the IBM System 360 models, has a P.microprogram which is similar to the external Pc which it defines. As such, the main question about this design is whether it is cheaper to have a single, hard-wired Pc rather than a computer within a computer. The Packard Bell 440 [Boutwell and Hoskinson, 1963] is an example of a better-known Pc whose internal P resembles the SD-2.

The authors of this book feel that, when the internal and external P's are so similar, it may be better to have a single P which suits both needs. To gain speed and still define powerful functions, Mp could be made up of both the conventional Mp and a small, fast Mp.

The Hewlett-Packard HP 9100A computing calculator

The HP 9100A (Chap. 20) is discussed in Part 3, Sec. 4, page 235.

Microprogrammed implementation of EULER on the IBM System 360/Model 30

This microprogrammed processor in Chap. 32 is also discussed as a language processor in Part 4, Sec. 4, page 348.


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