1
Introduction
This chapter introduces the computer as a machine and discusses about its brief history
of evolution and some of its salient features. After completion of this chapter, the reader
would know about
R Generation-wise salient features of computers
R Classi cation of present-day computers and their application areas.
C HAPTER OBJECTIVES
1.1 INTRODUCTION
In the very early days of computers, common people had an illusion about its capabilities, that it can
do anything and everything (as if, it can perform any miracle like: the Indian Rope Trick! ). Of course,
these false notions about computers were observed only in those people who were quite far away from
the real application areas of the machine.
However, even among a few present-day people, the general awareness about the overall work-
ing mechanism of computer is not very clear, although these people may be observed using their own
laptops. They use touch pad s or pen drive s without the knowledge of how these units function.
Knowledge of computer architecture and organization is essential to those who belong to Computer
Science or Information Technology streams. However, this does not indicate that others should not peep
into these secrets. A computer is a tool for general purpose and any one using it (or may use it) should
know about its basic working mechanism. The subject, which deals with these details, is known as
Computer Architecture and Organization .
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2 Computer Architecture and Organization
1.1.1 Difference Between Architecture and Organization
At this juncture, it is expected that these three terms, namely, computer , architecture and organization ,
should be de ned. In a straightforward manner, this may be done by presenting the simplest de nitions
without jumping into any controversy. A computer may be taken as an electronic instrument capable
of very high speed numerical calculations and carry out some speci c control operations, completely
depending on the software, which it would be executing. Computer architecture deals with broad or
overall design issues related with it, while computer organization means the electronic implementation
(or details) of the architectural modules. Therefore, a computer cannot be a substitute for hot pancake,
computer architecture does not mean how the manufacturer’s logo is printed on the laptop or computer
organization does not imply what should be the layout of a QWERT keyboard.
To elaborate the difference between computer architecture and computer organization, we may con-
sider a simple example. Let us assume that for a new computer, ongoing with its design and development
stages and yet to be manufactured and marketed, its designers consider about the provision of its exter-
nal bus to offer the facility for its user to interface some external devices. The type of such a bus and its
signal details are nalized from the architectural considerations. However, whether there should be any
external pull-up resistors, or noise-reduction capacitors or buffers for that bus, would be an organizational
issue. Broadly speaking, reduced instruction set computing (RISC) and complex instruction set comput-
ing (CISC), pipeline, multiple instruction, multiple data stream (MIMD) and single instruction, multiple
Do not be surprised if you overhear two naïve persons discussing about computer architecture
and organization in the following manner:
Person 1 : What is this organization about?
Person 2 : It decides about the layout pattern of the keys on the keyboard.
Person 1 : And what is the architecture?
Person 2 : Design of the chassis of the system.
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This discussion reminds me the following conversation between two persons regarding the
difference between architecture and organization.
–‘Can you please draw a clear demarcation line between architecture and organization?’
–‘No. I can only place at either side if someone draws a line.’
–‘That would also do.’
–‘But someone should first draw a line without any thickness.’
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