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أستاذ المادة بهيجة خضير شكر الغانمي       18/12/2016 09:18:01
BASIC COMPUTER OPERATIONS
A computer as shown in Fig. 2.1 performs basically five major operations or functions irrespective of
their size and make. These are 1) it accepts data or instructions by way of input, 2) it stores data, 3) it can
process data as required by the user, 4) it gives results in the form of output, and 5) it controls all operations
inside a computer. We discuss below each of these operations.
1. Input: This is the process of entering data and programs in to the computer system. You should know
that computer is an electronic machine like any other machine which takes as inputs raw data and
performs some processing giving out processed data. Therefore, the input unit takes data from us to
the computer in an organized manner for processing.
Fig. 2 Basic computer Operations
2. Storage: The process of saving data and instructions permanently is known as storage. Data has to be fed
into the system before the actual processing starts. It is because the processing speed of Central Processing
Unit (CPU) is so fast that the data has to be provided to CPU with the same speed. Therefore the data is first
stored in the storage unit for faster access and processing. This storage unit or the primary storage of the
computer system is designed to do the above functionality. It provides space for storing data and instructions.
Lec. (3)
Computer Fundamentals
1
st
stage
College of Computer Technology
Dept.Information Networks
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The storage unit performs the following major functions:
? All data and instructions are stored here before and after processing.
? Intermediate results of processing are also stored here.
3. Processing: The task of performing operations like arithmetic and logical operations is called processing.
The Central Processing Unit (CPU) takes data and instructions from the storage unit and makes all sorts of
calculations based on the instructions given and the type of data provided. It is then sent back to the storage
unit.
4. Output: This is the process of producing results from the data for getting useful information. Similarly the
output produced by the computer after processing must also be kept somewhere inside the computer before
being given to you in human readable form. Again the output is also stored inside the computer for further
processing.
5. Control: The manner how instructions are executed and the above operations are performed. Controlling
of all operations like input, processing and output are performed by control unit. It takes care of step by step
processing of all operations in side the computer.
Data representation.
The basic building block of personal computers is the transistor. A transistor is an electronic device for
controlling the flow of electrons in an electrical circuit. If electrons are allowed to flow, the circuit is on;
conversely, if electrons are not allowed to flow, the circuit is off. Thinking of a transistorized circuit as a
switch like a light switch at a home. The switch is either on or off and stays that way until it is flipped again.
When a circuit is on, we say it is in the marking state and assign a 1 to it. Conversely, when it is off, we
assign a 0 to it.
A modern digital computer is often said to be a binary computer because its most basic circuits can
remember either one of two states: 0 and 1. The binary digits 0 and 1 are called bits. Both the internal and
external memory of computer are nothing more than store-houses for bits.
Numbers systems.
The number systems that we discuss here are based on positional number systems. The decimal number
system that we are already familiar with is an example of a positional number system. In contrast, the Roman
numeral system is not a positional number system. Every positional number system has a radix or base, and
an alphabet. The base is a positive number. For example, the decimal system is a base-10 system. The
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number of symbols in the alphabet is equal to the base of the number system. The alphabet of the decimal
system is 0 through 9, a total of 10 symbols or digits. There are four number systems that are relevant in the
context of computer systems and programming. These are the decimal (base-10), binary (base-2), octal
(base-8), and hexadecimal (base-16) number systems.
Digits Binary
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
A 1010
B 1011
C 1100
D 1101
E 1110
F 1111
System Base Possible digits
Binary 2 0 1
Octal 8 0 1 2 3 4 5 6 7
Decimal 10 0 1 2 3 4 5 6 7 8 9
Hexadecimal
16 0 1 2 3 4 5 6 7 8 9 A B C D E F
Note
Computers internally use the binary system. The remaining two
number systems—octal and hexadecimal—are used mainly for
convenience to write a binary number even though they are number
systems on their own. We would have ended up using these number
systems if we had 8 or 16 fingers instead of 10.
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In a positional number system, a sequence of digits is used to represent a number. Each digit in this
sequence should be a symbol in the alphabet. There is a weight associated with each position. If we count
position numbers from right to left starting with zero, the weight of position n in a base b number system is
b
n
. For example, the number 579 in the decimal system is actually interpreted as
(Of course, 10
0
=1). In other words, 9 is in unit’s place, 7 in 10’s place, and 5 in 100’s
place.
Binary System:
The key to understanding computers is the binary number system. The binary number system has only
two digits: 0 to 1. Just as decimal notation is based on places that represent powers of ten, binary notation is
based on power of two. For example, the decimal number 537 is really the sum of powers of ten:
537= (5×10
2
) + (3×10
1
) + (7×10
0
)
= (5×100) + (3×10) + (7×1)
= 500 + 30 + 7 = 537
Binary numbers are sums of powers of two in the same way that decimal numbers are sums of powers
of ten. The following table shows the decimal numbers represented by some of more important powers of
two in personal computing:
2
-2
= 0.25
2
-1
= 0.5
2
0
= 1
2
1
= 2
2
2
= 4
2
3
= 8
2
4
= 16
2
5
= 32
2
6
= 64
2
7
= 128
2
8
= 256

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