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محاضره 1

الكلية كلية تكنولوجيا المعلومات     القسم قسم شبكات المعلومات     المرحلة 1
أستاذ المادة بهيجة خضير شكر الغانمي       18/12/2016 09:06:12
1-Introduction.
Computing machines have been around for a long time, hundreds of
years. The Chinese abacus, the calculators with gears and wheels and the
first analog computers are all examples of computing machinery; in some
cases quite complex, that predates the introduction of digital computing
systems. The computing machines that we’re interested in came about in
the 1940s because World War II artillery needed a more accurate way to
calculate the trajectories of the shells fired from battleships. Today, the
primary reason that computers have become so pervasive is the advances
made in integrated circuit manufacturing technology. The modern
computer has become faster and more powerful but the basic architecture
of a computing machine has essentially stayed the same for many years.
Most of us use computers for a variety of tasks, from serious
scientific computations to entertainment. The computer system can be
divided into computer hardware and computer software. Computer
hardware is the electronic circuitry that performs the actual work.
Hardware includes things with which you are already familiar such as the
processor, memory, keyboard, CD burner, and so on.
Computer software can be divided into application software and
system software. A user interacts with the system through an application
program. For the user, the application is the computer! For example, if
you are interested in browsing the Internet, you interact with the system
through a Web browser such as the Internet Explorer. For you, the system
appears as though it is executing the application program (i.e., Web
browser), as shown in Figure 1.
The computer hardware, as represented by a desktop PC, can be thought
of as being comprised of four basic parts:
1. Input devices can include components such as the mouse, keyboard,
microphone, disks, modem and the network.
2. Output devices are components such as the display, disk, modem,
sound card and speakers, and the network.
3. The memory system is comprised of internal and external caches , main
memory, video memory and disk.
4. The central processing unit, or CPU, is comprised of the arithmetic and
logic unit (ALU), control system and busses.
Computer architecture deals with the functional behavior of a
computer system as viewed by a programmer. This view includes aspects
such as the sizes of data types (e.g. using 16 binary digits to represent an
integer), and the types of operations that are supported (like addition and
subtraction). Also deals with the selection of the basic functional units
such as the processor and memory, and how they should be
interconnected into a computer system.
Computer organization is concerned with how the various hardware
components operate and how they are interconnected to implement the
architectural specifications. Computer organization deals with structural
relationships that are not visible to the programmer, such as interfaces to
peripheral devices and the technology used for the memory.
Computers are complex systems. How do we manage complexity of
these systems? We can get clues from looking at how we manage
complex systems in life. Think of how a large corporation is managed.
We use a hierarchical structure to simplify the management: president at
the top and employees at the bottom. Each level of manag ement filters
out unnecessary details into the lower levels and presents only an
abstracted version to the higher-level management. This is what we refer
to as abstraction.
Different people view computer systems differently depending on
the type of their interaction. We use the concept of abstraction to look at
only the details that are necessary from a particular viewpoint. For
example, if you are a computer architect, you are interested in the internal
details that do not interest a normal user of the system. One can look at
computer systems from several different perspectives. We have already
talked about the user’s view. We concentrate on the following views: (i) a
programmer’s view, (ii) an architect’s view, and (iii) an implementer’s
view.
A programmer’s view of a computer system depends on the type
and level of language she intends to use. From the programmer’s
viewpoint, there exists a hierarchy from low-level languages to high-level
languages.
A computer architect looks at the design aspect from a h igh level.
She uses higher-level building blocks to optimize the overall system
performance. A computer architect is much like an architect who designs
buildings. For example, when designing a building, the building architect
is not concerned with designing the elevator; as far as the architect is
concerned, the elevator is a building block someone else designs.
Similarly, a computer architect does not focus on low-level issues. From
the architect’s viewpoint, a computer system consists of three main
components: a processor or central processing unit (CPU), a memory
unit, and input/output (I/O) devices.
Implementers are responsible for implementing the designs
produced by computer architects. This group works at the digital logic
level. At this level, logic gates and other hardware circuits are used to
implement the various functional units.
2-The generations of computers.
This section traces the history of computers from their mechanical
era. Our treatment is very brief.
1- The first generation.
Back to the beginning; the first generation of computing
engines was comprised of the mechanical devices (called
calculating machines). They were built using gears and powered by
a hand-operated crank. The abacus, the adding machine, the punch
card reader for textile machines fit into this category. Perhaps the
most well-known mechanical system, called the difference engine,
was built by Charles Babbage.
2- The second generation.
The next generation spanned the period from 1940–1960. Here
electronic devices—vacuum tubes—were used as the active device
or switching element. Even a vacuum tube is millions of times
larger than the transistor on a silicon wafer. It consumes millions of
times the power of the transistor, and its useful lifetime is hundreds
or thousands of times less then a transistor. Although the vacuum
tube computers were much faster than the mechanical computers of
the preceding generation, they are thousands of times slower than
the computers of today. Program instructions were given in
machine language, which is a code composed entirely of 0s and 1s.
These computers were slow, unreliable, expensive, and tedious to
program.
3- The third generation.
The third generation covered roughly the period of time from
1960 to 1968. Here the transistor replaced the vacuum tube, and
suddenly the computers began to be able to do real work.
Companies such as IBM®, Burroughs® and Univac® built large
mainframe computers. The IBM 360 family is a representative
example of the mainframe computer of the day. Also at this time,
Xerox® was carrying out some pioneering work on the
human/computer interface at their Palo Alto Research Center,
Xerox PARC. Here they studied what later would become
computer networks, Windows® operating system. Programmers
stopped programming in machine language and assembly language
and began to use FORTRAN, COBOL and BASIC.
4- The fourth generation.
The fourth generation, roughly 1969–1977 was the age of the
minicomputer. The minicomputer was the computer of the masses.
It wasn’t quite the PC, but it moved the computer out of the sterile
environment of the “computer room,” protected by technicians in
white coats, to a computer in your lab. The minicomputer also
represented the replacement of individual electronic parts, such as
transistors and resistors, mounted on printed circuit boards (called
discrete devices), with integrated circuits (IC), or collections of
logic functions in a single package. It is small, faster, and more
reliable than separate transistors. Here was the introduction of the
small and medium scale integrated circuits. Companies such as
Digital Equipment Company (DEC), Data General and HewlettPackard all built this generation of minicomputer. Also within this
timeframe, simple integrated-circuit microprocessors were
introduced and commercially produced by companies like Intel,
Texas Instruments, Motorola, MOS Technology and Zilog. Early
microcomputer devices that best represent this generation are the
4004, 8008 and 8080 from Intel, the 9900 from Texas Instruments
and the 6800 from Motorola. The computer languages of the fourth
generation were: assembly, C, Pascal, Modula, Smalltalk and
Microsoft BASIC.
5- The fifth generation.
We are currently in the fifth generation, although it could be
argued that the fifth generation ended with the Intel® 80486
microprocessor and the introduction of the Pentium® represents
the sixth generation. We’ll ignore that distinction until it is more
widely accepted. The advances made in semiconductor
manufacturing technology best characterize the fifth generation of
computers.
Today’s semiconductor processes typify what is referred to as
Very Large Scale Integration, or VLSI technology. Ever since ICs
were made possible, the density has been growing at a phenomenal
rate. By the mid-1970s, more than 10,000 components could be
fabricated on a single chip. The next step, Ultra Large Scale
Integration, or ULSI is either here today or right around the corner.
The fifth generation also saw the growth of the personal
computer and the operating system as the primary focus of the
machine. Standard hardware platforms controlled by standard
operating systems enabled thousands of developers to create
programs for these systems. In terms of software, the dominant
languages became ADA, C++, JAVA, HTML and XML. In
addition, graphical design language, based upon the universal
modeling language (UML), began to appear.

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