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This book tells how to design digital systems. Whenever a book proposes ~o teach you how to design systems of type X, you are entitled to ask two questions and receive specific answers:

Only then will you know whether you should be interested in designing such systems, whether you want systems that perform the stated tasks, and whether the technology proposed is reasonable. There are many other reasons why you might wish to read or study a book on the design of type-X systems: The beauty of the designs; the rigor of the intellectual discipline; the view afforded of a complex intellectual structure; the pleasure of mastering yet one more art. All these, though real, should be secondary. Design is the art of obtaining useful devices, and technologies are fashioned to make design practical without excessive creativity and intelligence. First things first.

Herewith we attempt to answer the two basic questions.


Digital systems process information. They do calculations on information input in specified physical forms, delivering answers represented in some other (or the same) physical form. They sense the current state of physical devices, such as instruments or manufacturing tools, and control them to achieve given objectives. They acquire information at a given time and in a given place, and make that information available at some later time and in some place. They monitor ongoing physical processes at discrete times so that they perform as desired, recording errors when they occur and deactivating a process if it moves outside of stated bounds of safety and reliability.

If these statements seem to you too general -- that we should answer our question by stating-specific capabilities -- then it is you who must revise your notions, not us who must become more definite. For a peculiarity of digital systems is that they are capable of performing any specified task of processing information and, further, that they can perform any collection of such tasks, doing each when commanded. Not only are 'they capable of doing this, in some technical sense of possibility, but they are the main practical device for doing it in an ever widening domain of application.

There are many technologies for processing information and for sensing and controlling physical devices. For instance an autopilot on an airplane senses broadcasted external signals from which it can determine the plane's position and then manipulate the plane's controls to guide it along a predetermined path. Currently these devices are entirely analog, i.e. continuous, working with the intensities and phase relationships of received electromagnetic signals, making all the computations of how to respond in terms of voltages in fixed circuits, and effecting the. controls by setting other voltages appropriately. These analog systems have been developed to a high state of performance. and reliability. However, add a little complexity to the task, especially if it involves remembering past behavior or arbitrary future plans (e.g., a flight plan), and make digital technology a little cheaper, and digital systems may become practical competitors for the task.

Digital systems include digital computers, which are the machines normally


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