What are Organization, Architecture, Structure, And Function
ORGANIZATION AND ARCHITECTURE
A distinction is often made between computer architecture
and computer organization in describing computers. Although it is difficult to
give precise definitions for these terms, a consensus exists about the general
areas covered by each (e.g., see [VRANSO (SIEW82], and [BELL78a).
Computer architecture refers to those attributes of a system
visible to a programmer or, put another way, those attributes that directly
impact 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.
As an example, it is an architectural design issue whether a
computer will have multiple instructions. 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 important. 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 several different computer models,
its organization changing with changing technology.
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The IBM System/370 architecture is a prominent example of both phenomena. This architecture was first introduced in 1970 and
included several 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 organizations but also result in the introduction
of more powerful and 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).
This book examines both computer organization and computer
architecture. The emphasis is perhaps more on the side of the organization.
However, because a computer organization must be designed to implement a
particular architectural specification, a thorough treatment of the organization
requires a detailed examination of architecture as well.
STRUCTURE AND FUNCTION
A computer is a complex system; contemporary computers
contain millions of elementary electronic components. How, then, can one
clearly describe them? The key is to recognize 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 the
lowest level of the elementary subsystem.
The hierarchical nature of complex systems is essential to
both their design and their description. The designer needs 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 interrelationships.
The behavior at each level depends only on a simplified,
abstracted characterization of the system at the next lower level. At each
level, the designer is concerned with structure and function:
• Structure: How the components are interrelated
• Function: The operation of each component as part of the
structure
In terms of description, we have two choices: starting at
the bottom and building up to a complete description, or beginning with a top
view and decomposing the system into its subparts. Evidence from several fields
suggests that the top-down approach is the clearest and most effective
[WEIN75].
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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.
Function
Both the structure and functioning of a computer are, in
essence, simple. Figure 1.1 depicts the basic functions that a computer can
perform. In general terms, there are only four.
- · Data processing
- · Data storage
- · Data movement
- · Control
