X PREFACE

volumes, perhaps one on language translators and one on operating systems. We hope also that future volumes will be devoted to mass storage devices, terminals, and applications.

Two notations, ISP and PMS, were introduced in the book, Computer Structures [Bell and Newell, 1971]. We continue to use them in this book, especially since they have left the realm of notations and have become working design tools. ISP was introduced to describe the instruction set processor of a computer - the machine seen by the program (and programmer). ISP is now used for machine description, simulation, verification of diagnostics, microprogramming, automatic assembler generation, and the comparison of computer architectures. The evolution and improvement of ISP is principally due to needs of the Army/Navy Computer Family Architecture (CFA) project and the work of Mario Barbacci. The latest version, ISPS, is being used within DEC for implementing processors, simulators, etc. ISPS language descriptions of current DEC machines (PDP-8, PDP-l0, PDP-l1, VAX-l1) and several terminals have been made. We hope that these will be made widely available and so further stimulate the use of machine- description languages. The widespread application of good languages would help alleviate two current design problems: first, that of hand-crafted design tooling keeping up with the rate of introduction of new technologies and second, the problem of managing the ever-increasing complexity of computer structures. The PDP-8 description presented in Appendix 1 has been verified by machine diagnostics, in contrast to conventional descriptions.

PMS (processor-memory-switch) notation (given in Appendix 2) has not yet been widely used in formal methods to aid design. It has, however, been used extensively to describe computer structures. A prototype system which recognizes PMS and performs several performance analysis functions was constructed by Knudsen [1972]. Currently, ISPS is being extended to include the interconnection of computational blocks so that PMS and ISPS form a single system describing structure and behavior. In this book, we use PMS to describe functional blocks. However, all PMS components are enclosed to form a block diagram, unlike the original stick notation.

The book begins with three introductory chapters. The first presents the major themes to be illustrated by the book. We show that computer evolution has been based primarily on semiconductor and magnetic recording technologies. These technologies determine costs, and therefore price, performance, reliability, size, weight, power, and other dimensions which constitute the physical characteristics of the machines. The major theme of the book is that technology has enabled (or forced) three types of computers to be built:

1. Machines with constant performance and decreasing cost.

2. Machines with contant cost and increasing performance.

3. Radically new (large or small) structures, often research machines, which create new computer classes outside the evolution possibilities.