ASCII: Encyclopedia - ASCII

ASCII

ASCII (American Standard Code for Information Interchange), generally pronounced [ˈæski], is a character encoding based on the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that work with text. Most modern character encodings have a historical basis in ASCII.

ASCII was first published as a standard in 1967 and was last updated in 1986. It currently defines codes for 33 non-printing, mostly obsolete control characters that affect how text is processed, plus the following 95 printable characters (starting with the space character):

 !"#$%&'()*+,-./0123456789:;<=>?
@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_
`abcdefghijklmnopqrstuvwxyz{|}~

ASCII - Overview

Like other character representation computer codes, ASCII specifies a correspondence between digital bit patterns and the symbols/glyphs of a written language, thus allowing digital devices to communicate with each other and to process, store, and communicate character-oriented information. The ASCII character encoding^[1] — or a compatible extension (see below) — is used on nearly all common computers, especially personal computers and workstations. The preferred MIME name for this encoding is "US-ASCII".^[2]

ASCII is, strictly, a seven-bit code, meaning that it uses the bit patterns representable with seven binary digits (a range of 0 to 127 decimal) to represent character information. At the time ASCII was introduced, many computers dealt with eight-bit groups (bytes or, more specifically, octets) as the smallest unit of information; the eighth bit was commonly used as a parity bit for error checking on communication lines or other device-specific functions. Machines which did not use parity typically set the eighth bit to zero, though some systems such as Prime machines running PRIMOS set the eighth bit of ASCII characters to one.

ASCII only defines a relationship between specific characters and bit sequences; aside from reserving a few control codes for line-oriented formatting, it does not define any mechanism for describing the structure or appearance of text within a document. Such concepts are within the realm of other systems such as the markup languages.

ASCII - History

ASCII developed from telegraphic codes and first entered commercial use as a seven-bit teleprinter code promoted by Bell data services. The Bell System had previously planned to use a six-bit code, derived from Fieldata, that added punctuation and lower-case letters to the earlier five-bit Baudot teleprinter code, but was persuaded instead to join the ASA subcommittee that had started to develop ASCII. Baudot helped in the automation of sending and receiving telegraphic messages, and took many features from Morse code; however, unlike Morse code, Baudot used constant-length codes. Compared to earlier telegraph codes, the proposed Bell code and ASCII both underwent re-ordering for more convenient sorting (especially alphabetization) of lists, and added features for devices other than teleprinters. Bob Bemer introduced features such as the 'escape sequence'.

The American Standards Association (ASA, later to become ANSI) first published ASCII as a standard in 1963. ASCII-1963 lacked the lowercase letters, and had an up-arrow (↑) instead of the caret (^) and a left-arrow (←) instead of the underscore (_). The 1967 version added the lowercase letters, changed the names of a few control characters and moved the two controls ACK and ESC from the lowercase letters area into the control codes area.

ASCII was subsequently updated and published as ANSI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.

Other international standards bodies have ratified character encodings that are identical or nearly identical to ASCII. These encodings are sometimes referred to as ASCII, even though ASCII is strictly defined only by the ASA/ANSI standards:

• The European Computer Manufacturers Association published editions of its ASCII clone, ECMA-6, in 1965, 1967, 1970, 1973, 1983, and 1991. The 1991 edition is the same as ANSI X3.4-1986.^[3]
• The International Organization for Standardization published its version, ISO 646 (later ISO/IEC 646) in 1967, 1972, 1983, and 1991. ISO 646:1972, in particular, established a set of country-specific versions with punctuation characters replaced with non-English letters. ISO/IEC 646:1991 International Reference Version is the same as ANSI X3.4-1986.
• The International Telecommunication Union published its version of ANSI X3.4-1986, ITU-T Recommendation T.50, in 1992. In the early 1970s, under the name CCITT, the same organization published a version as CCITT Recommendation V.3.
• DIN published a version of ASCII as DIN 66003 in 1974.
• The IETF published a versions in 1969 as RFC 20, and established the Internet's standard version, based on ANSI X3.4-1986, with the publication of RFC 1345 in 1992.
• IBM's version of ANSI X3.4-1986 is published in IBM technical literature as code page 367.

ASCII has also become embedded in its probable replacement, Unicode, as the 'lowest' 128 characters. Some observers consider ASCII the most "successful" software standard ever promulgated.

ASCII - ASCII control characters

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to carry printable information, but rather to control devices (such as printers) that make use of ASCII. For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 27 represents the "escape" key often found in the top left corner of common keyboards.

Code 127 (all seven bits on), another special character, equates to "delete" or "rubout". Though its function resembles that of other control characters, the designers of ASCII used this pattern so that it could "erase" a section of paper tape (a popular storage medium until the 1980s) by punching all possible holes at a particular character position, thus effectively replacing any previous information. Since Code 0 (null,all bits off) was also ignored it was possible to leave gaps and then make corrections by blanking characters before or after the gap and then entering new characters in the gap.

Many of the ASCII control codes serve (or served) to mark data packets, or to control a data transmission protocol (e.g. ENQuiry [effectively, "any stations out there?"], ACKnowledge, Negative AcKnowledge, Start Of Header, Start of TeXt, End of TeXt, etc). ESCape and SUBstitute permit a communications protocol to, for instance, mark binary data so that if it contains codes with the same pattern as a protocol character, the recipient will process the code as data.

The designers of ASCII intended the separator characters ("Record Separator", etc.) for use with magnetic tape systems.

Two of the device control characters, commonly interpreted as XON and XOFF, generally function as flow control characters to throttle data flow to a slow device (such as a printer) from a fast device (such as a computer) - so data does not overrun and get lost.

Early users of ASCII adopted some of the control codes to represent "meta information" such as end-of-line, start/end of a data element, and so on. These assignments often conflict, so part of the effort in converting data from one format to another involves making the correct meta information transformations. For example, the character(s) representing end-of-line ("newline") in text data files/streams vary from operating system to operating system. When moving files from one system to another, the conversion process must recognize these characters as end-of-line markers and handle them appropriately.

Today, ASCII users use the control characters less and less—with the exception of "carriage return" and/or "line feed". Modern markup languages, modern communication protocols, the move from text-based to graphical devices, and the demise of teleprinters, punch-cards, and paper tapes have rendered most of the control characters obsolete.

1. Printable Representation, the Unicode glyphs reserved for representing control characters when it is necessary to print or display them rather than have them perform their intended function.
2. Control key Sequence, the traditional key sequences for inputting control characters. The caret (^) represents the "Control" or "Ctrl" key that must be held down while pressing the second key in the sequence. The caret-key representation is also used by some software to represent control characters.
3. The Backspace character can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
4. The Delete character can also be entered by pressing the "Delete" or "Del" key. It can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
5. The Escape character can also be entered by pressing the "Escape" or "Esc" key on some systems.
6. The Carriage Return character can also be entered by pressing the "Return", "Ret", "Enter", or ↵ key on most systems.
7. The ambiguity surrounding the Backspace key comes from systems that translated the DEL control character into a BS (backspace) before transmitting it. Some software was unable to process the character and would display "^H" instead. "^H" persists in messages today as a deliberate humorous device, e.g. "there's a sucker^H^H^H^H^H^H potential customer born every minute". A less common variant of this involves the use of "^W", which in some text editors means "delete previous word". The example sentence would therefore also work as "there's a sucker^W potential customer born every minute".

ASCII - ASCII printable characters

Code 32, the "space" character, denotes the space between words, as produced by the large space-bar of a keyboard. Codes 33 to 126, known as the printable characters, represent letters, digits, punctuation marks, and a few miscellaneous symbols.

Seven-bit ASCII provided seven "national" characters and, if the combined hardware and software permit, can use overstrikes to simulate some additional international characters: in such a scenario a backspace can precede a grave accent (which the American and British standards, but only those standards, also call "opening single quotation mark"), a tilde, or a breath mark (inverted vel).

ASCII - Structural features

• The digits 0-9 are represented with their values in binary prefixed with 0011 (this means that bcd-ASCII is simply a matter of taking each bcd nibble separately and prefixing 0011 to it.
• Lowercase and uppercase letters only differ in bit pattern by a single bit simplifying case conversion to a range test (to avoid converting characters that are not letters) and a single bitwise operation.

ASCII - Aliases for ASCII

RFC 1345 (published in June 1992) and the IANA registry of character sets (ongoing), both recognize the following case-insensitive aliases for ASCII as suitable for use on the Internet:

• ANSI_X3.4-1968 (canonical name)
• ANSI_X3.4-1986
• ASCII
• US-ASCII (preferred MIME name)
• us
• ISO646-US
• ISO_646.irv:1991
• iso-ir-6
• IBM367
• cp367
• csASCII

Of these, only the aliases "US-ASCII" and "ASCII" have achieved widespread use. One often finds them in the optional "charset" parameter in the Content-Type header of some MIME messages, in the equivalent "meta" element of some HTML documents, and in the encoding declaration part of the prolog of some XML documents.

ASCII - Variants of ASCII

As computer technology spread throughout the world, different standards bodies and corporations developed many variations of ASCII in order to facilitate the expression of non-English languages that used Roman-based alphabets. One could class some of these variations as "ASCII extensions", although some mis-apply that term to cover all variants, including those that do not preserve ASCII's character-map in the 7-bit range.

ISO 646 (1972), the first attempt to remedy the pro-English-language bias, created compatibility problems, since it remained a 7-bit character-set. It made no additional codes available, so it reassigned some in language-specific variants. It thus became impossible to know what character a code represented without knowing which variant to work with, and text-processing systems could generally cope with only one variant anyway.

Eventually, improved technology brought out-of-band means to represent the information formerly encoded in the eighth bit of each byte, freeing this bit to add another 128 additional character-codes for new assignments. For example, IBM developed 8-bit code pages, such as code page 437, which replaced the control-characters with graphic symbols such as smiley faces, and mapped additional graphic characters to the upper 128 bytes. Operating systems such as DOS supported these code-pages, and manufacturers of IBM PCs supported them in hardware.

Eight-bit standards such as ISO/IEC 8859 and MacRoman developed as true extensions of ASCII, leaving the original character-mapping intact and just adding additional values above the 7-bit range. This enabled the representation of a broader range of languages, but these standards continued to suffer from incompatibilities and limitations. Still, ISO-8859-1 its variant Windows-1252 (often mislabeled as ISO-8859-1) and original 7-bit ASCII remain the most common character encodings in use today.

Unicode and the ISO/IEC 10646 Universal Character Set (UCS) have a much wider array of characters, and their various encoding forms have begun to supplant ISO/IEC 8859 and ASCII rapidly in many environments. While ASCII basically uses 7-bit codes, Unicode and the UCS use relatively abstract "code points": non-negative integer numbers that map, using different encoding forms and schemes, to sequences of one or more 8-bit bytes. To permit backward compatibility, Unicode and the UCS assign the first 128 code points to the same characters as ASCII. One can therefore think of ASCII as a 7-bit encoding scheme for a very small subset of Unicode and of the UCS. The popular UTF-8 encoding-form prescribes the use of one to four 8-bit code values for each code point character, and equates exactly to ASCII for the code values below 128. Other encoding forms such as UTF-16 resemble ASCII in how they represent the first 128 characters of Unicode, but tend to use 16 or 32 bits per character, so they require conversion for compatibility.

The portmanteau word ASCIIbetical has evolved to describe the collation of data in ASCII-code order rather than "standard" alphabetical order.^[4]

The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string.

See also

ANSI
ASCII art
ASCII games
Binary and text files
Bob Bemer
EBCDIC
Unicode

UTF-8
ISO 8859
ISCII
VISCII
Windows code pages

ISO 646
ATASCII
PETSCII
ZX Spectrum character set

Adapted from the Wikipedia article "ASCII", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki