1.3.1 ASCII Coding Scheme

Definition

ASCII stands for American Standard Code for Information Interchange. It is a standard data coding scheme in computers. It assigns standard numeric values to alphabets, numerals, punctuation marks, and other characters used in computers.

Main Idea

Before ASCII was developed, different makes and models of computers could not communicate with one another. Each computer manufacturer represented alphabets, numerals, and other characters in its own way. ASCII solved this problem by providing a common standard that all computers could follow.

Core Concept

ASCII is a character encoding standard that assigns a unique numeric code (from 0 to 127 in the original 7-bit version, and 0 to 255 in the extended 8-bit version) to each character. These numeric codes are then stored in the computer as binary numbers.

Detailed Explanation

History of ASCII

On June 17, 1963, ASCII was approved as the American standard code. However, it did not gain wide acceptance because IBM chose to use EBCDIC (Extended Binary Coded Decimal Interchange Code) in its computers.

Initially, ASCII used 7-bit codes of various combinations of 0's and 1's to represent 128 different characters (2⁷ = 128).

ASCII-7 could not represent newly developed characters, so it underwent further developments and revisions, and ASCII-8 was developed.

Extended ASCII Code (ASCII-8 bit Code)

ASCII was extended to an 8-bit code that can represent (2⁸ = 256) different characters.

ASCII-8 became popular in 1981 when it was used for the first time in IBM's Personal Computer (PC), and soon it became the industry standard for personal computers.

Many Windows systems use an 8-bit ASCII encoding, and this Microsoft-specific encoding is known as ANSI code. ANSI stands for American National Standards Institute.

In extended ASCII-8, 32 code combinations are used for machine and control commands, such as "start of text," "carriage return," and "form feed," etc. Control commands do not represent printable information but rather help control devices, such as printers.

Just to Remember!

Table 1 (Annexure 1) shows non-printable ASCII control codes. Table 2 (Annexure 2) shows printable ASCII characters, and Table 3 (Annexure 3) shows extended ASCII printable characters.

Intermediate Level

At the intermediate level, you should understand the structure of ASCII codes.

• Range: 0 to 127
• Total characters: 128
• Includes: control characters (0–31), printable characters (32–126), and DEL (127)

• Range: 0 to 255
• Total characters: 256
• Includes all 7-bit ASCII characters plus additional symbols, accented letters, and special characters (128–255)

Important ASCII values:

Important Points

• - ASCII = American Standard Code for Information Interchange.
• - ASCII assigns numeric values to characters.
• - 7-bit ASCII: 128 characters (0–127).
• - 8-bit Extended ASCII: 256 characters (0–255).
• - ASCII-8 became popular in 1981 with IBM PC.
• - ANSI is a Microsoft-specific 8-bit encoding.
• - 32 code combinations are used for control commands.
• - ASCII is limited to English and some Western languages.

Remember

• - ASCII = character encoding standard.
• - 'A' = 65, 'a' = 97, '0' = 48.
• - 7-bit ASCII = 128 characters.
• - 8-bit Extended ASCII = 256 characters.
• - ASCII-8 became popular in 1981.
• - ANSI = Microsoft-specific 8-bit encoding.
• - ASCII is the foundation of Unicode.

Common Mistakes

• - Confusing ASCII value of uppercase and lowercase letters ('A' = 65, 'a' = 97).
• - Forgetting that ASCII values are decimal numbers, not binary.
• - Thinking that ASCII can represent all world languages (it cannot).
• - Confusing 7-bit ASCII (128 characters) with 8-bit Extended ASCII (256 characters).
• - Forgetting that control characters (0–31) are non-printable.
• - Mixing up the ASCII values of digits and letters.

Quick Revision

• ASCII is a standard coding scheme for characters.
• 7-bit ASCII: 128 characters (0–127).
• 8-bit Extended ASCII: 256 characters (0–255).
• ASCII-8 became popular in 1981.
• 'A' = 65, 'Z' = 90, 'a' = 97, 'z' = 122, '0' = 48, '9' = 57.
• Control characters: 0–31.
• ANSI is Microsoft-specific 8-bit encoding.
• ASCII cannot represent all world languages.

60-Second Revision

• ASCII = character encoding standard.
• 7-bit: 128 characters; 8-bit: 256 characters.
• 'A' = 65, 'a' = 97, '0' = 48.
• ASCII-8 became popular in 1981 with IBM PC.
• ANSI = Microsoft-specific 8-bit encoding.
• Warning: ASCII cannot represent all languages; Unicode is needed for that.