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 programmer
or, put another way, those attributes that have a direct impact on the logical
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 represent
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 computer
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 implemented
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 anticipated
frequency of use of the multiply instruction, the relative speed of the two approaches,
and the cost and physical size of a special multiply unit.
Historically, and still today, the distinction between architecture and organization
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 performance
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 IBM System/370 architecture. This architecture was first introduced in 1970 and included
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 investment
was protected. Remarkably, the System/370 architecture, with a few enhancements,
has survived to this day as the architecture of IBM’s mainframe product line.
In a class of computers called microcomputers, the relationship between architecture
and organization is very close. Changes in technology not only influence organization
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 organizational
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 computer
organization must be designed to implement a particular architectural specification,
a thorough treatment of organization requires a detailed examination of
architecture as well.
Source : Computer Organization and Architecture Designing for Performance