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10 CHAPTER 1 / INTRODUCTION

IBM System/370 architecture. This architecture was first introduced in 1970 and in-
cluded a number of models. The customer with modest requirements could buy a
cheaper, slower model and, if demand increased, later upgrade to a more expensive,
faster model without having to abandon software that had already been developed.
Over the years, IBM has introduced many new models with improved technology to
replace older models, offering the customer greater speed, lower cost, or both.These
newer models retained the same architecture so that the customer’s software invest-
ment was protected. Remarkably, the System/370 architecture, with a few enhance-
ments, has survived to this day as the architecture of IBM’s mainframe product line.
In a class of computers called microcomputers, the relationship between archi-
tecture and organization is very close. Changes in technology not only influence or-
ganization but also result in the introduction of more powerful and more complex
architectures. Generally, there is less of a requirement for generation-to-generation
compatibility for these smaller machines. Thus, there is more interplay between or-
ganizational and architectural design decisions. An intriguing example of this is the
reduced instruction set computer (RISC), which we examine in Chapter 13.
This book examines both computer organization and computer architecture.
The emphasis is perhaps more on the side of organization. However, because a com-
puter organization must be designed to implement a particular architectural specifi-
cation, a thorough treatment of organization requires a detailed examination of
architecture as well.

1.2 STRUCTURE AND FUNCTION

A computer is a complex system;contemporary computers contain millions of elemen-
tary electronic components.How,then,can one clearly describe them?The key is to rec-
ognize the hierarchical nature of most complex systems, including the computer
[SIMO96].A hierarchical system is a set of interrelated subsystems,each of the latter,in
turn,hierarchical in structure until we reach some lowest level of elementary subsystem.
The hierarchical nature of complex systems is essential to both their design and
their description.The designer need only deal with a particular level of the system at
a time. At each level, the system consists of a set of components and their interrela-
tionships.The behavior at each level depends only on a simplified, abstracted charac-
terization of the system at the next lower level. At each level, the designer is
concerned with structure and function:
• Structure: The way in which the components are interrelated
• Function: The operation of each individual component as part of the structure
In terms of description, we have two choices: starting at the bottom and build-
ing up to a complete description, or beginning with a top view and decomposing the
system into its subparts. Evidence from a number of fields suggests that the top-
down approach is the clearest and most effective [WEIN75].
The approach taken in this book follows from this viewpoint. The computer
system will be described from the top down.We begin with the major components of
a computer, describing their structure and function, and proceed to successively
lower layers of the hierarchy. The remainder of this section provides a very brief
overview of this plan of attack.