American Standard Code for Information Interchange (ASCII)

What is the American Standard Code for Information Interchange (ASCII)?[1]

The American Standard Code for Information Interchange, which everyone calls ASCII, is a character set that a computer uses to store characters. ASCII specifies a method for computers to store printable characters such as letters A to Z, a to z, digits 0 to 9, punctuation, and spaces. ASCII also includes control characters such as newline. Most computers either use ASCII or a superset of ASCII that adds more characters, such as accented letters, Cyrillic letters, CJK characters, or hieroglyphics.

ASCII (American Standard Code for Information Interchange) is the most common format for text files on computers and on the Internet. In an ASCII file, each alphabetic, numeric, or special character is represented with a 7-bit binary number (a string of seven 0s or 1s). 128 possible characters are defined.[2]

ASCII uses 7 bits to represent each character. For example, a capital "T" is represented by the number 84, and a lowercase "t" is represented by 116. Other keyboard keys are also mapped to standard ASCII values. For example, the Escape (ESC) key is represented as 27, and the Delete (DEL) key is represented as 32. ASCII codes may also be displayed as hexadecimal values instead of the decimal numbers listed above. For example, the ASCII value of the Escape key in hexadecimal is "1B" and the hexadecimal value of the Delete key is "7F." Since ASCII uses 7 bits, it only supports 2^7, or 128 values. Therefore, the standard ASCII character set is limited to 128 characters. While this is enough to represent all standard English letters, numbers, and punctuation symbols, it is not sufficient to represent all special characters or characters from other languages. Even Extended ASCII, which supports 8-bit values, or 256 characters, does not include enough characters to accurately represent all languages. Therefore, other character sets, such as Latin-1 (ISO-8859-1), UTF-8, and UTF-16 are commonly used for documents and webpages that require more characters.[3]

Historical Background of American Standard Code for Information Interchange (ASCII)[4]

The American Standard Code for Information Interchange (ASCII) was developed under the auspices of a committee of the American Standards Association (ASA), called the X3 committee, by its X3.2 (later X3L2) subcommittee, and later by that subcommittee's X3.2.4 working group (now INCITS). The ASA became the United States of America Standards Institute (USASI) and ultimately the American National Standards Institute (ANSI).

With the other special characters and control codes filled in, ASCII was published as ASA X3.4-1963, leaving 28 code positions without any assigned meaning, reserved for future standardization, and one unassigned control code. There was some debate at the time about whether there should be more control characters rather than the lowercase alphabet. The indecision did not last long: during May 1963 the CCITT Working Party on the New Telegraph Alphabet proposed to assign lowercase characters to sticks 6 and 7, and International Organization for Standardization TC 97 SC 2 voted during October to incorporate the change into its draft standard. The X3.2.4 task group voted its approval for the change to ASCII at its May 1963 meeting. Locating the lowercase letters in sticks 6 and 7 caused the characters to differ in bit pattern from the uppercase by a single bit, which simplified case-insensitive character matching and the construction of keyboards and printers.

The X3 committee made other changes, including other new characters (the brace and vertical bar characters), renaming some control characters (SOM became the start of header (SOH)), and moving or removing others (RU was removed). ASCII was subsequently updated as USAS X3.4-1967, then USAS X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.

Revisions of the ASCII standard:

  • ASA X3.4-1963
  • ASA X3.4-1965 (approved, but not published, nevertheless used by IBM 2260 & 2265 Display Stations and IBM 2848 Display Control)
  • USAS X3.4-1967
  • USAS X3.4-1968
  • ANSI X3.4-1977
  • ANSI X3.4-1986[
  • ANSI X3.4-1986 (R1992)
  • ANSI X3.4-1986 (R1997)
  • ANSI INCITS 4-1986 (R2002)
  • ANSI INCITS 4-1986 (R2007)
  • ANSI INCITS 4-1986 (R2012)

In the X3.15 standard, the X3 committee also addressed how ASCII should be transmitted (least significant bit first), and how it should be recorded on perforated tape. They proposed a 9-track standard for magnetic tape and attempted to deal with some punched card formats.

ASCII Sections[5]

The ASCII table is divided into three different sections.

  • Non-printable, system codes between 0 and 31.
  • Lower ASCII, between 32 and 127. This table originates from the older, American systems, which worked on 7-bit character tables.
  • Higher ASCII, between 128 and 255. This portion is programmable; characters are based on the language of your operating system or program you are using. Foreign letters are also placed in this section.

ASCII Table and Description (see Figures 1 and 2 below)[6]

ASCII stands for American Standard Code for Information Interchange. Computers can only understand numbers, so an ASCII code is the numerical representation of a character such as 'a' or '@' or an action of some sort. ASCII was developed a long time ago and now the non-printing characters are rarely used for their original purpose. Below is the ASCII character table and this includes descriptions of the first 32 non-printing characters. ASCII was actually designed for use with teletypes and so the descriptions are somewhat obscure. If someone says they want your CV however in ASCII format, all this means is that they want 'plain' text with no formatting such as tabs, bold or underscoring - the raw format that any computer can understand. This is usually so they can easily import the file into their own applications without issues. Notepad.exe creates ASCII text, or in MS Word you can save a file as 'text only'

American Standard Code for Information Interchange
Figure 1. source: VCampus

Extended Table
American Standard Code for Information Interchange
Figure 2.source: VCampus

See Also


Further Reading