Download - Coding System
Coding System Text Representation ASCIICollating Sequence
What is a BITAs 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
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.
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
Working With the following number of bits
how many combinations could be created?
1. 5 bits2. 3 bits3. 6 bits4. 8 bits
Answers 1. 5 bits = 25 32 combinations
2. 3 bits = 23 8 combinations
3. 6 bits = 26 64 combinations
4. 8 bits = 28 256 combinations
List of Combination Number of Bits Calculation Number of Combinations
1 21 2
2 22 4
3 23 8
4 24 16
5 25 32
6 26 64
7 27 128
8 28 256
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)
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)
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
Code Char Code Char Code Char Code Char0 NUL 32 Space 64 @ 96 `1 SOH 33 ! 65 A 97 a2 STX 34 “ 66 B 98 b3 ETX 35 # 67 C 99 c4 EOT 36 $ 68 D 100 d5 ENQ 37 % 69 E 101 e6 ACK 38 & 70 F 102 f7 BEL 39 ‘ 71 G 103 g8 BS 40 ( 72 H 104 h9 TAB 41 ) 73 I 105 i10 LF 42 * 74 J 106 j11 VT 43 + 75 K 107 k12 FF 44 , 76 L 108 l13 CR 45 - 77 M 109 m14 SO 46 . 78 N 110 n15 SI 47 / 79 O 111 o16 DLE 48 0 80 P 112 p17 DC1 49 1 81 Q 113 q18 DC2 50 2 82 R 114 r19 DC3 51 3 83 S 115 s20 DC4 52 4 84 T 116 t21 NAK 53 5 85 U 117 u22 SYN 54 6 86 V 118 v23 ETB 55 7 87 W 119 w24 CAN 56 8 88 X 120 x25 EM 57 9 89 Y 121 y26 SUB 58 : 90 Z 122 z27 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
Now a Day’sThe 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.
Codes Available Today
Try it!You can check the symbols on your
computer by typing in its decimal code1. Open Notepad (Start > Programs >
Accessories > Notepad)2. Make sure that NUM LOCK is switched on3. Press and hold ALT 4. From the Numpad enter the 3 digits that
you want (example 234 for Ω)5. Release the ALT, and the symbol should
appear.
Important NoteIt 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
Coding in ASCIIWe must use the ASCII table to
be able to code the following;
Letter H E L L O
Decimal Code 72 69 76 76 79
Binary Code 1001000 1000101 1001100 1001100 1001111
Example of XYZ system Remember this is fictitious Code Char Code Char Code Char1 A 69 ! 76 *2 B 70 “ 77 (3 C 71 £ 78 )4 D 72 $ 79 -5 E 73 % 80 _6 F 74 ^ 81 +... 75 & ...
So HELLO would look different when decoded
Binary Code 1001000 1000101 1001100 1001100 1001111
Decimal 72 69 76 76 79
Letter $ ! * * -
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
http://en.wikipedia.org/wiki/List_of_Unicode_characters.
Work Code the following word using
ASCII;
Letter B Y E ! !
Decimal Code
Binary Code
Answer
Letter B Y E ! !
Decimal Code 66 89 69 33 33
Binary Code 1000010 1011001 1000101 0100001 0100001
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 (AZ) or descending order (ZA).
Example
Character Code
C 67
V 86
B 66
Using the ASCII codes the computer could sort the letters in ascending or descending order by sorting the ASCII code (numbers)