Due to continual progress in the large-scale integration of semiconductor circuits, parallel computing principles can already be met in low-cost sys tems: numerous examples exist in image processing, for which special hard ware is implementable with quite modest resources even by nonprofessional designers. Principles of content addressing, if thoroughly understood, can thereby be applied effectively using standard components. On the other hand, mass storage based on associative principles still exists only in the long term plans of computer technologists. This situation is somewhat confused by the fact that certain expectations are held for the development of new storage media such as optical memories and "spin glasses" (metal alloys with low-density magnetic impurities). Their technologies, however, may not ripen until after "fifth generation" computers have been built. It seems that software methods for content addressing, especially those based on hash coding principles, are still holding their position firmly, and a few innovations have been developed recently. As they need no special hardware, one might expect that they will spread to a wide circle of users. This monograph is based on an extensive literature survey, most of which was published in the First Edition. I have added Chap. ?, which contains a review of more recent work. This updated book now has references to over 1200 original publications. In the editing of the new material, I received valuable help from Anneli HeimbUrger, M. Sc. , and Mrs. Leila Koivisto.

Designers and users of computer systems have long been aware of the fact that inclusion of some kind of content-addressable or "associative" functions in the storage and retrieval mechanisms would allow a more effective and straightforward organization of data than with the usual addressed memories, with the result that the computing power would be significantly increased. However, although the basic principles of content-addressing have been known for over twenty years, the hardware content-addressable memories (CAMs) have found their way only to special roles such as small buffer memories and con trol units. This situation now seems to be changing: Because of the develop ment of new technologies such as very-large-scale integration of semiconduc tor circuits, charge-coupled devices, magnetic-bubble memories, and certain devices based on quantum-mechanical effects, an increasing amount of active searching functions can be transferred to memory units. The prices of the more complex memory components which earlier were too high to allow the application of these principles to mass memories will be reduced to a fraction of the to tal system costs, and this will certainly have a significant impact on the new computer architectures. In order to advance the new memory principles and technologies, more in formation ought to be made accessible to a common user.

Biology of Memory investigates the biological basis of memory and covers topics ranging from short- and long-term post-perceptual memory to memory storage processes, memory microstructures, chemical transfer, and neuronal plasticity. The activity of neuronal networks in the thalamus of the monkey is also examined, along with integrative functions of the thalamocortical visual system of the cat. Comprised of 20 chapters, this book begins with an overview of the mechanisms that inhibit or interfere with short- and long-term memory, followed by a discussion on different retrieval mechanisms for short- and long-term memory. The reader is then introduced to the role of short- and long-term memory in the formation, retention, and utilization of associations, together with the link between memory and the medial temporal regions of the brain. Subsequent chapters focus on anatomical and chemical changes in the brain during primary learning; cellular models of learning and cellular mechanisms of plasticity in Aplysia; trace phenomena in single neurons of hippocampus and mammiliary bodies; and plasticity in single units in the mammalian brain. The book concludes with a description of Occam (Omnium-Gatherum Core Content Addressable Memory), a computer program for a content addressable memory in the central nervous system. This monograph will be useful to biologists, behavioral psychologists, neuropsychologists, neurophysiologists, biophysicists, and biochemists as well as computer scientists and mathematicians.

Two significant things have happened since the writing of the first edition in 1983. One of them is recent arousal of strong interest in general aspects of "neural computing", or "neural networks", as the previous neural models are nowadays called. The incentive, of course, has been to develop new com puters. Especially it may have been felt that the so-called fifth-generation computers, based on conventional logic programming, do not yet contain in formation processing principles of the same type as those encountered in the brain. All new ideas for the "neural computers" are, of course, welcome. On the other hand, it is not very easy to see what kind of restrictions there exist to their implementation. In order to approach this problem systematically, cer tain lines of thought, disciplines, and criteria should be followed. It is the pur pose of the added Chapter 9 to reflect upon such problems from a general point of view. Another important thing is a boom of new hardware technologies for dis tributed associative memories, especially high-density semiconductor circuits, and optical materials and components. The era is very close when the parallel processors can be made all-optical. Several working associative memory archi tectures, based solely on optical technologies, have been constructed in recent years. For this reason it was felt necessary to include a separate chapter (Chap. 10) which deals with the optical associative memories. Part of its con tents is taken over from the first edition.

This book describes the physical operation of the Tunnel Field-effect Transistor (TFET) and circuits built with this device. Whereas the majority of publications on TFETs describe in detail the device, its characteristics, variants and performance, this will be the first book addressing TFET integrated circuits (TFET ICs). The authors describe the peculiarities of TFET ICs and their differences with MOSFETs. They also develop and analyze a number of logic circuits and memories. The discussion also includes complex circuits combining CMOS and TFET, as well as a potential fabrication process in Silicon.