Character Codes - Alphanumeric Representation

A set of elements that comprises of the 10 decimal digits, the 26 letters of alphabets, a number of special characters, such as $, @, and = etc. form a alphanumeric character set. The standard alphanumeric binary code is the ASCII (American Standard Code for Information Interchange), which uses seven bits to code 128 characters. The decimal digits in ASCII can be converted to BCD (Binary Coded Decimal) by removing the three high-orders bits, 011. Since registers can only hold binary information, the code must be in binary form. Any set of discrete elements such as the musical notes and chess pieces and their positions on the chessboard can be expressed in binary code given to the fact that these codes only change the symbol not the meaning of the discrete elements. These codes are also used as tools for the formulation of instructions that specify control information for the computer. In ASCII scheme 65 represent 'A', 90 represent 'Z', 97 represent 'a' and 122 represent 'z'.

In digital computers at times the data to be processed may include not only numbers but letters also. For example, a banking company with millions of customers may use a digital computer to process its files. In a banking company, we may have to enter the customers name. To represent the customers name in the binary form, it is necessary to have a binary code for the alphabet. So the need arises for a binary code that can represent alphabets and some special characters in addition to decimal numbers.

An “alphabetic and numeric” (sometimes abbreviated as alphanumeric) code is a binary code of a group of elements inclusive of the 10 decimal digits, the 26 letters of the alphabet, and a certain number of special symbols such as @. The total number of elements in an alphanumeric group can be greater than 36. Therefore, it must be coded with a minimum of six bits (26 = 64, but 25 = 32 is insufficient).

For a better understanding of alphanumeric codes, one possible arrangement of a six-bit alphanumeric code is shown in the table. We can show alphanumeric characters with few variations. The need to represent more than 64 characters (the lowercase letters and special control characters for the transmission of digital information) gave rise to seven and eight-bit alphanumeric codes. One such code is known as ASCII (American Standard Code for Information Interchange); another is known as EBCDIC (Extended Binary Coded Decimal Interchange Code). EBCDIC uses 8 bits of each character and a 9th bit for parity. It has the same character symbols of ASCII but the bit assignment to characters is different. We have shown ASCII code in the table, which consists of seven bits but conventionally an eight-bit code is used wherein the eighth bit is invariably added for parity. When discrete information is transferred through punch cards, the alphanumeric characters use a 12-bit binary code. A punch card consists of 80 columns and 12 rows. In each column, an alphanumeric character is represented by holes punched in the appropriate rows.

A hole is sensed as a 1 and the absence of a hole is sensed as a 0. The 12 rows are marked, starting from the top, as the 12, 11, 0, 1, 2,..., 9 punches. The first three are called the zone punch and the last nine are called the numeric punch. The 12-bit card code shown in Table 1-5 lists the rows where a hole is punched (giving the 1's). The remaining unlisted rows are assumed to be 0's. The 12-bit card code is inefficient with respect to the number of bits used. Most computers translate the input code into an internal six-bit code. As an example, the internal code representation of the name “John Doe” is:

100001 100110 011000 100101 110000 010100 100110 010101
J O H N blank D O E