1. Text Representation ASCII Collating Sequence
Coding System
Text Representation
ASCII
Collating Sequence
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2. GEN/OPM1300 Information & Communication Technologies
What is a BIT
As w know a bit is the smallest unit in a computer system
A bit can either be a 12 or a 02
This means that with one single bit we can only represent only 2 different values
12 = 110 and 02 = 010
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3. Representing Text, Pictures ect …
Text, pictures and videos are also stored in a computer system using a sequence of binary digits
Using one single bit is not enough due to its small size
If we want to store a large quantity of data many more bits are required.
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4. Combining a number of Bits
Each time a bit is combined with another bit, more different combinations can be stored.
If we will be using two bits to store data, there are a total of 22= 4 different combinations
Hence if we have 4 bits we would have 24= 16 different combinations
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5. GEN/OPM1300 Information & Communication Technologies
Working
With the following number of bits how many combinations could be created?
5 bits
3 bits
6 bits
8 bits
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6. GEN/OPM1300 Information & Communication Technologies
Answers
5 bits = 25  32 combinations
3 bits = 23  8 combinations
6 bits = 26  64 combinations
8 bits = 28  256 combinations
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7. Number of Combinations
List of Combination
Number of Bits
Calculation
Number of Combinations 1 21 2 22 4 3 23 8 24 16 5 25 32 6 26 64 7 27
128 28
256
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8. GEN/OPM1300 Information & Communication Technologies
In the Early Days
When computers were first introduced they were simply used for performing calculations
After a while scientists realized that computers can also be used to store and edit text in different ways
The problem was that a computer could only understand binary (1s & 0s)
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9. GEN/OPM1300 Information & Communication Technologies
ASCII Coding System
The scientists had to come up with a way for binary to be able to represent characters
ASCII (American Standard Code for Information Interchange) was developed
With ASCII every single character is assigned a binary code
ASCII also allows computers to communicate with each other (files containing text can be copied from one computer to another)
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10. GEN/OPM1300 Information & Communication Technologies
How ASCII Coding Works
The first ASCII system used a series of 7 Bits
This meant that 27=128 different characters could be used
These 128 characters are represented in the following table
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11. GEN/OPM1300 Information & Communication Technologies
Code
Char
NUL 32
Space 64 @ 96 ` 1
SOH 33 ! 65 A 97 a 2
STX 34 “ 66 B 98 b 3
ETX 35 # 67 C 99 c 4
EOT 36 $ 68 D
100 d 5
ENQ 37 % 69 E
101 e 6
ACK 38
& 70 F
102 f 7
BEL 39 ‘ 71 G
103 g 8 BS 40 ( 72 H
104 h 9
TAB 41 ) 73 I
105 i 10 LF 42 * 74 J
106 j 11 VT 43 + 75 K
107 k 12 FF 44 , 76 L
108 l 13 CR 45 - 77 M
109 m 14 SO 46 . 78 N
110 n 15 SI 47 / 79 O
111 o 16
DLE 48 80 P
112 p 17
DC1 49 81 Q
113 q 18
DC2 50 82 R
114 r 19
DC3 51 83 S
115 s 20
DC4 52 84 T
116 t 21
NAK 53 85 U
117 u 22
SYN 54 86 V
118 v 23
ETB 55 87 W
119 w 24
CAN 56 88 X
120 x 25 EM 57 89 Y
121 y 26
SUB 58 : 90 Z
122 z 27
ESC 59 ; 91 [
123 { 28 FS 60
< 92 \
124 | 29 GS 61 = 93 ]
125 } 30 RS 62
> 94 ^
126 ~ 31 US 63 ? 95 _
127
DEL
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12. GEN/OPM1300 Information & Communication Technologies
Now a Day’s
The first 32 characters were used for transmission purposes, they are no longer needed
After some time it was noted that ASCII consisted only of the English alphabet, so an extra bit was added to accommodate more languages, graphics and mathematical symbols
By adding an extra bit 128 more characters can be stored since 28=256.
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13. GEN/OPM1300 Information & Communication Technologies
Codes Available Today
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14. GEN/OPM1300 Information & Communication Technologies
Try it!
You can check the symbols on your computer by typing in its decimal code
Open Notepad (Start > Programs > Accessories > Notepad)
Make sure that NUM LOCK is switched on
Press and hold ALT
From the Numpad enter the 3 digits that you want (example 234 for Ω)
Release the ALT, and the symbol should appear.
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15. GEN/OPM1300 Information & Communication Technologies
Important Note
It is important that all computers use ASCII as otherwise different codes would represent different values
Example if a computer used the XYZ system (fictitious) and another uses ASCII the work HELLO would be represented differently on the two machines
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16. GEN/OPM1300 Information & Communication Technologies
Coding in ASCII
We must use the ASCII table to be able to code the following;
Letter H E L O
Decimal Code 72 69 76 79
Binary Code
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17. GEN/OPM1300 Information & Communication Technologies
Example of XYZ system
Remember this is fictitious
Code
Char 1 A 69 ! 76 * 2 B 70 “ 77 ( 3 C 71 78 ) 4 D 72 $ 79 - 5 E 73 % 80 _ 6 F 74 ^ 81 +
... 75
&
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18. GEN/OPM1300 Information & Communication Technologies
So HELLO would look different when decoded
Binary Code
Decimal 72 69 76 79
Letter $ ! * -
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19. GEN/OPM1300 Information & Communication Technologies
When the computer with the XYZ representation system receives the text file from the computer with the ASCII representation system instead of HELLO, one will see $!**-, which is completely wrong!
Since nowadays computers are used across the world a new system is being used which is UNICODE
UNICODE is compatible with ASCII but then it uses a different system. A list of UNICODE characters can be accessed on
ode_characters.
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20. GEN/OPM1300 Information & Communication Technologies
Work
Code the following word using ASCII;
Letter B Y E !
Decimal Code
Binary Code
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21. GEN/OPM1300 Information & Communication Technologies
Answer
Letter B Y E !
Decimal Code 66 89 69 33
Binary Code
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22. GEN/OPM1300 Information & Communication Technologies
Collating Sequence
The character codes assigned to the characters in ASCII are very important since they can be used to sort letters and distinguishing from upper to lower case letters
For instance if the letters C, V and B have to be sorted, they can be easily sorted by looking at their ASCII code and use it to sort text in ascending (AZ) or descending order (ZA).
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23. GEN/OPM1300 Information & Communication Technologies
Example
Using the ASCII codes the computer could sort the letters in ascending or descending order by sorting the ASCII code (numbers)
Character
Code C 67 V 86 B 66
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