1010111.Character encoding is a convention of using a numeric value to represent eachcharacter of awriting script. Not only can a character set includenatural languagesymbols, but it can also include codes that have meaning meaning or function outside of language, such ascontrol characters andwhitespace. Character encodings also have been defined for someartificial languages. When encoded, character data can be stored, transmitted, and transformed by acomputer.[1] The numerical values that make up a character encoding are known ascode points and collectively comprise a code space or acode page.
Early character encodings that originated with optical or electricaltelegraphy and in early computers could only represent a subset of the characters used in languages, sometimes restricted toupper case letters,numerals and limitedpunctuation. Over time, encodings capable of representing more characters were created, such asASCII,ISO/IEC 8859, andUnicode encodings such asUTF-8 andUTF-16.
Themost popular character encoding on theWorld Wide Web is UTF-8, which is used in 98.2% of surveyed web sites, as of May 2024.[2] Inapplication programs andoperating system tasks, both UTF-8 and UTF-16 are popular options.[3]
The history of character codes illustrates the evolving need for machine-mediated character-based symbolic information over a distance, using once-novel electrical means. The earliest codes were based upon manual and hand-written encoding and cyphering systems, such asBacon's cipher,Braille,international maritime signal flags, and the 4-digit encoding of Chinese characters for aChinese telegraph code (Hans Schjellerup, 1869). With the adoption of electrical and electro-mechanical techniques these earliest codes were adapted to the new capabilities and limitations of the early machines. The earliest well-known electrically transmitted character code,Morse code, introduced in the 1840s, used a system of four "symbols" (short signal, long signal, short space, long space) to generate codes of variable length. Though some commercial use of Morse code was via machinery, it was often used as a manual code, generated by hand on atelegraph key and decipherable by ear, and persists inamateur radio andaeronautical use. Most codes are of fixed per-character length or variable-length sequences of fixed-length codes (e.g.Unicode).[4]
Common examples of character encoding systems include Morse code, theBaudot code, theAmerican Standard Code for Information Interchange (ASCII) and Unicode. Unicode, a well-defined and extensible encoding system, has replaced most earlier character encodings, but the path of code development to the present is fairly well known.
The Baudot code, a five-bit encoding, was created byÉmile Baudot in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No. 2 (ITA2) in 1930. The namebaudot has been erroneously applied to ITA2 and its many variants. ITA2 suffered from many shortcomings and was often improved by many equipment manufacturers, sometimes creating compatibility issues.
Herman Hollerith invented punch card data encoding in the late 19th century to analyze census data. Initially, each hole position represented a different data element, but later, numeric information was encoded by numbering the lower rows 0 to 9, with a punch in a column representing its row number. Later alphabetic data was encoded by allowing more than one punch per column. Electromechanicaltabulating machines represented date internally by the timing of pulses relative to the motion of the cards through the machine.
WhenIBM went to electronic processing, starting with theIBM 603 Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code. IBM used severalbinary-coded decimal (BCD) six-bit character encoding schemes, starting as early as 1953 in its702[5] and704 computers, and in its later7000 Series and1400 series, as well as in associated peripherals. Since the punched card code then in use was limited to digits, upper-case English letters and a few special characters, six bits were sufficient. These BCD encodings extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping them easily to punch-card encoding which was already in widespread use. IBM's codes were used primarily with IBM equipment. Other computer vendors of the era had their own character codes, often six-bit, such as the encoding used by theUNIVAC I.[6] They usually had the ability to read tapes produced on IBM equipment. IBM's BCD encodings were the precursors of theirExtended Binary-Coded Decimal Interchange Code (usually abbreviated as EBCDIC), an eight-bit encoding scheme developed in 1963 for theIBM System/360 that featured a larger character set, including lower case letters.
In 1959 the U.S. military defined itsFieldata code, a six-or seven-bit code, introduced by the U.S. Army Signal Corps. While Fieldata addressed many of the then-modern issues (e.g. letter and digit codes arranged for machine collation), it fell short of its goals and was short-lived. In 1963 the first ASCII code was released (X3.4-1963) by the ASCII committee (which contained at least one member of the Fieldata committee, W. F. Leubbert), which addressed most of the shortcomings of Fieldata, using a simpler seven-bit code. Many of the changes were subtle, such as collatable character sets within certain numeric ranges. ASCII63 was a success, widely adopted by industry, and with the follow-up issue of the 1967 ASCII code (which added lower-case letters and fixed some "control code" issues) ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the EuropeanECMA-6 standard.[7] Eight-bitextended ASCII encodings, such as various vendor extensions and theISO/IEC 8859 series, supported all ASCII characters as well as additional non-ASCII characters.
While trying to develop universally interchangeable character encodings, researchers in the 1980s faced the dilemma that, on the one hand, it seemed necessary to add more bits to accommodate additional characters, but on the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). In 1985, the average personal computer user'shard disk drive could store only about 10 megabytes, and it cost approximately US$250 on the wholesale market (and much higher if purchased separately at retail),[8] so it was very important at the time to make every bit count.
The compromise solution that was eventually found anddeveloped into Unicode[vague] was to break the assumption (dating back to telegraph codes) that each character should always directly correspond to a particular sequence of bits. Instead, characters would first be mapped to a universal intermediate representation in the form of abstract numbers calledcode points. Code points would then be represented in a variety of ways and with various default numbers of bits per character (code units) depending on context. To encode code points higher than the length of the code unit, such as above 256 for eight-bit units, the solution was to implementvariable-length encodings where an escape sequence would signal that subsequent bits should be parsed as a higher code point.
The various terms related to character encoding are often used inconsistently or incorrectly.[9] Historically, the same standard would specify a repertoire of characters and how they were to be encoded into a stream of code units — usually with a single character per code unit. However, due to the emergence of more sophisticated character encodings, the distinction between terms has become important.
A character is the smallest unit of text that has semantic value.[9][10]
What constitutes a character varies between character encodings. For example, for letters withdiacritics, there are two distinct approaches that can be taken to encode them. They can be encoded either as a single unified character (known as a precomposed character), or as separate characters that combine into a singleglyph. The former simplifies the text handling system, but the latter allows any letter/diacritic combination to be used in text.Ligatures pose similar problems. Some writing systems, such as Arabic and Hebrew, need to accommodate things likegraphemes that are joined in different ways in different contexts, but represent the same semantic character.
A character set is a collection of characters used to represent text.[9][10] For example, theLatin alphabet andGreek alphabet are character sets.
A coded character set is a character set with each item uniquely mapped to a numberic value.[10]
Although generally antiquated, this is also known ascode page.[9] Originally,code page referred to apage number in an IBM manual that defined a particular character encoding.[11] Other vendors, includingMicrosoft,SAP, andOracle Corporation, also published their own code pages, including notableWindows code page andcode page 437. Despite no longer referring to specific pages in a manual, many character encodings are still identified to by the same number. Likewise, the termcode page is still used to refer to character encoding.
InUnix andUnix-like systems, the termcharmap is commonly used; usually in the larger context of locales.
IBM's Character Data Representation Architecture (CDRA) designates each entity with acoded character set identifier (CCSID), which is variously called acharset,character set,code page, orCHARMAP.[12]
A character repertoire is a set of characters that can be represented by a particular coded character set.[10][13] The repertoire may be closed, meaning that no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series); or it may be open, allowing additions (as is the case with Unicode and to a limited extentWindows code pages).[13]
A code point is the value or position of a character in a coded character set.[10] A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding:
Code space is the range of numerical values spanned by a coded character set.[10][12]
Code unit is the minimum bit combination that can represent a character in a character encoding (incomputer science terms, it is theword size of the character encoding).[10][12] Common code units include 7-bit, 8-bit, 16-bit, and 32-bit. In some encodings, some characters are encoded asmultiple code units.
For example:
Unicode and its parallel standard, the ISO/IEC 10646Universal Character Set, together constitute a unified standard for character encoding. Rather than mapping characters directly tobytes, Unicode separately defines a coded character set that maps characters to unique natural numbers (code points), how those code points are mapped to a series of fixed-size natural numbers (code units), and finally how those units are encoded as a stream of octets (bytes). The purpose of this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To describe the model precisely, Unicode uses existing terms and defines new terms.[12]
An abstract character repertoire (ACR) is the full set of abstract characters that a system supports. Unicode has an open repertoire, meaning that new characters will be added to the repertoire over time.
A coded character set (CCS) is afunction that maps characters tocode points (each code point represents one character). For example, in a given repertoire, the capital letter "A" in the Latin alphabet might be represented by the code point 65, the character "B" by 66, and so on. Multiple coded character sets may share the same character repertoire; for exampleISO/IEC 8859-1 and IBM code pages 037 and 500 all cover the same repertoire but map them to different code points.
A character encoding form (CEF) is the mapping of code points tocode units to facilitate storage in a system that represents numbers as bit sequences of fixed length (i.e. practically any computer system). For example, a system that stores numeric information in 16-bit units can only directly represent code points 0 to 65,535 in each unit, but larger code points (say, 65,536 to 1.4 million) could be represented by using multiple 16-bit units. This correspondence is defined by a CEF.
A character encoding scheme (CES) is the mapping of code units to a sequence of octets to facilitate storage on an octet-based file system or transmission over an octet-based network. Simple character encoding schemes includeUTF-8,UTF-16BE,UTF-32BE,UTF-16LE, andUTF-32LE; compound character encoding schemes, such asUTF-16,UTF-32 andISO/IEC 2022, switch between several simple schemes by using abyte order mark orescape sequences; compressing schemes try to minimize the number of bytes used per code unit (such asSCSU andBOCU).
AlthoughUTF-32BE andUTF-32LE are simpler CESes, most systems working with Unicode use eitherUTF-8, which isbackward compatible with fixed-length ASCII and maps Unicode code points to variable-length sequences of octets, orUTF-16BE,[citation needed] which isbackward compatible with fixed-length UCS-2BE and maps Unicode code points to variable-length sequences of 16-bit words. Seecomparison of Unicode encodings for a detailed discussion.
There may be a higher-level protocol which supplies additional information to select the particular variant of aUnicode character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is theXML attribute xml:lang.
The Unicode model uses the term "character map" for other systems which directly assign a sequence of characters to a sequence of bytes, covering all of the CCS, CEF and CES layers.[12]
A character is commonly documented as 'U+' followed by its code point value inhexadecimal. The range of valid code points (the code space) for the Unicode standard is U+0000 to U+10FFFF, inclusive, divided in 17planes, identified by the numbers 0 to 16. Characters in the range U+0000 to U+FFFF are in plane 0, called theBasic Multilingual Plane (BMP). This plane contains the most commonly used characters. Characters in the range U+10000 to U+10FFFF in the other planes are calledsupplementary characters.
The following table includes examples of code points:
| Character | Code point | Glyph |
|---|---|---|
| Latin A | U+0041 | Α |
| Latin sharp S | U+00DF | ß |
| Han for East | U+6771 | 東 |
| Ampersand | U+0026 | & |
| Inverted exclamation mark | U+00A1 | ¡ |
| Section sign | U+00A7 | § |
Consider, "ab̲c𐐀" – a string containing a Unicode combining character (U+0332̲COMBINING LOW LINE) as well as a supplementary character (U+10400𐐀DESERET CAPITAL LETTER LONG I). This string has several Unicode representations which are logically equivalent, yet while each is suited to a diverse set of circumstances or range of requirements:
a,b̲,c,𐐀a,b,_,c,𐐀U+0061,U+0062,U+0332,U+0063,U+104000x00000061,0x00000062,0x00000332,0x00000063,0x000104000x0061,0x0062,0x0332,0x0063,0xD801,0xDC000x61,0x62,0xCC,0xB2,0x63,0xF0,0x90,0x90,0x80Note in particular that 𐐀 is represented with either one 32-bit value (UTF-32), two 16-bit values (UTF-16), or four 8-bit values (UTF-8). Although each of those forms uses the same total number of bits (32) to represent the glyph, it is not obvious how the actual numeric byte values are related.
To support environments using multiple character encodings, software has been developed to translate text between character encoding schemes; a process known astranscoding. Notable software includes:
![[icon]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion with: Popularity and comparison:
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Themost used character encoding on theweb isUTF-8, used in 98.2% of surveyed web sites, as of May 2024.[2] Inapplication programs andoperating system tasks, both UTF-8 andUTF-16 are popular options.[3][18]
Android note: The Android platform default is always UTF-8.
In addition to traditionalDEC andISO character sets, which conform to the structure and rules ofISO 2022, theVT510 supports a number of IBM PC code pages (page numbers in IBM's standard character set manual) inPCTerm mode to emulate theconsole terminal of industry-standard PCs.
in reality, you usually just assume UTF-8 since that is by far the most common encoding.
| Early telecommunications | |
|---|---|
| ISO/IEC 8859 |
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| Bibliographic use | |
| National standards | |
| ISO/IEC 2022 | |
| Mac OSCode pages ("scripts") | |
| DOS code pages | |
| IBM AIX code pages | |
| Windows code pages | |
| EBCDIC code pages | |
| DEC terminals (VTx) | |
| Platform specific |
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| Unicode /ISO/IEC 10646 | |
| TeX typesetting system | |
| Miscellaneous code pages | |
| Control character | |
| Related topics | |
