PC Hardware —

ASCII Code

The standard code for handling text characters on most modern computers is called ASCII (American Standard Code for Information Interchange). The basic ASCII standard consists of 128 codes representing the English alphabet, punctuation, and certain control characters. Most systems today recognize 256 codes: the original 128, plus an additional 128 codes called the extended character set.

Remember that a byte represents one character of information; four bytes are needed to represent a string of four characters. The following four bytes represent the text string 12AB (using ASCII code):

 00110001     00110010     01000001     01000010
  1            2            A            B

The following illustrates how the binary language spells the word "binary":

 B            I            N            A            R            Y
  01000010     01001001     01001110     01000001     01010010     01011001
NOTE
It is very important to understand that in computer processing the "space" is a significant character. All items in a code must be set out for the machine to process. Like any other character, the space has a binary value that must be included in the data stream. In computing, the absence or presence of a space is critical and sometimes causes confusion or frustration among new users. Uppercase and lowercase letters also have different values. Some operating systems (for example, UNIX) distinguish between them for commands, while others (for example, MS-DOS) translate the uppercase and lowercase into the same word no matter how it is cased.

The following table is a complete representation of the ASCII character set. Even in present-day computing, laden with multimedia and sophisticated programming, ASCII retains an honored and important position.

Symbol Binary 1 Byte Decimal Symbol Binary 1 Byte Decimal
0 00110000 48 V 01010110 86
1 00110001 49 W 01010111 87
2 00110010 50 X 01011000 88
3 00110011 51 Y 01011001 89
4 00110100 52 Z 01011010 90
5 00110101 53 A 01100001 97
6 00110110 54 B 01100010 98
7 00110111 55 C 01100011 99
8 00111000 56 D 01100100 100
9 00111001 57 E 01100101 101
A 01000001 65 F 01100110 102
B 01000010 66 G 01100111 103
C 01000011 67 H 01101000 104
D 01000010 68 I 0110100 105
E 01000101 69 J 01101010 106
F 01000110 70 K 01101011 107
G 01000111 71 L 01101100 108
H 01001000 72 M 01101101 109
I 01001001 73 N 01101110 110
J 01001010 74 O 01101111 111
K 01001011 75 P 01110000 112
L 01001100 76 Q 01110001 113
M 01001101 77 R 01110010 114
N 01001110 78 S 01110011 115
O 01001111 79 T 01110100 116
P 01010000 80 U 01110101 117
Q 01010001 81 V 01110110 118
R 01010010 82 W 01110111 119
S 01010011 83 X 01111000 120
T 01010100 84 Y 01111001 121
U 01010101 85 Z 01111010 122
NOTE
All letters have a separate ASCII value for uppercase and lowercase. The capital letter "A" is 65, and the lowercase "a" is 97.

Keep in mind that computers are machines, and they do not really perceive numbers as anything other than electrical charges setting a switch on or off. Like binary numbers, electrical charges can exist in only two states-positive or negative. Computers interpret the presence of a charge as one and the absence of a charge as zero. This technology allows a computer to process information.