Knowledge: A little light expels much darkness _ Bahya ibn Paquda, Duties of the Heart During the early 1970s digital computer techniques concentrated on the computational and interfacing aspects of digital systems and the decade began as the age of both the mainframe computer and the minicomputer. Engineers and system designers needed to know the fundamentals of computer operation and how the practical limitations of the architectures of the day, the memory size, cost and performance could be overcome; it was for this reason that this book was first written. By 1980 the microprocessor revolution had arrived. As a result the microprocessor became a component of a system, rather than a system itself, and the need to understand the behaviour of the device became of even greater importance to the system designer. New developments in mainframe computers were few, with networks of minicomputers taking over their role in many instarices. The 1980 revision of this book took into account the major advances in semiconductor technology that had occurred since it was first published in 1972, and included material relevant to the microprocessor.