Page 32 -

1.1 / ORGANIZATION AND ARCHITECTURE 9

This book is about the structure and function of computers.Its purpose is to present,as
clearly and completely as possible, the nature and characteristics of modern-day com-
puters.This task is a challenging one for two reasons.
First, there is a tremendous variety of products, from single-chip microcomputers
costing a few dollars to supercomputers costing tens of millions of dollars, that can
rightly claim the name computer. Variety is exhibited not only in cost, but also in size,
performance,and application.Second,the rapid pace of change that has always charac-
terized computer technology continues with no letup. These changes cover all aspects
of computer technology,from the underlying integrated circuit technology used to con-
struct computer components to the increasing use of parallel organization concepts in
combining those components.
In spite of the variety and pace of change in the computer field, certain funda-
mental concepts apply consistently throughout.To be sure, the application of these con-
cepts depends on the current state of technology and the price/performance objectives
of the designer.The intent of this book is to provide a thorough discussion of the funda-
mentals of computer organization and architecture and to relate these to contemporary
computer design issues. This chapter introduces the descriptive approach to be taken.

1.1 ORGANIZATION AND ARCHITECTURE

In describing computers,a distinction is often made between computer architecture and
computer organization. Although it is difficult to give precise definitions for these
terms, a consensus exists about the general areas covered by each (e.g., see [VRAN80],
[SIEW82], and [BELL78a]); an interesting alternative view is presented in [REDD76].
Computer architecture refers to those attributes of a system visible to a pro-
grammer or, put another way, those attributes that have a direct impact on the logi-
cal execution of a program. Computer organization refers to the operational units
and their interconnections that realize the architectural specifications. Examples of
architectural attributes include the instruction set, the number of bits used to repre-
sent various data types (e.g., numbers, characters), I/O mechanisms, and techniques
for addressing memory. Organizational attributes include those hardware details
transparent to the programmer, such as control signals; interfaces between the com-
puter and peripherals; and the memory technology used.
For example, it is an architectural design issue whether a computer will have a
multiply instruction. It is an organizational issue whether that instruction will be im-
plemented by a special multiply unit or by a mechanism that makes repeated use of
the add unit of the system. The organizational decision may be based on the antici-
pated frequency of use of the multiply instruction, the relative speed of the two ap-
proaches, and the cost and physical size of a special multiply unit.
Historically, and still today, the distinction between architecture and organiza-
tion has been an important one. Many computer manufacturers offer a family of
computer models, all with the same architecture but with differences in organization.
Consequently, the different models in the family have different price and perfor-
mance characteristics. Furthermore, a particular architecture may span many years
and encompass a number of different computer models, its organization changing
with changing technology. A prominent example of both these phenomena is the