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Character encoding

From Wikipedia, the free encyclopedia
(Redirected from Code unit)

Punched tape with the word "Wikipedia" encoded in ASCII. Presence and absence of a hole represents 1 and 0, respectively; for example, W is encoded as 1010111.

Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using computers.[1] The numerical values that make up a character encoding are known as code points and collectively comprise a code space, a code page, or character map.

Early character encodings that originated with optical or electrical telegraphy and in early computers could only represent a subset of the characters used in written languages, sometimes restricted to upper case letters, numerals and some punctuation only. Over time, character encodings capable of representing more characters were created, such as ASCII, the ISO/IEC 8859 encodings, various computer vendor encodings, and Unicode encodings such as UTF-8 and UTF-16.

The most popular character encoding on the World Wide Web is UTF-8, which is used in 98.2% of surveyed web sites, as of May 2024.[2] In application programs and operating system tasks, both UTF-8 and UTF-16 are popular options.[3]

History

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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 as Bacon's cipher, Braille, international maritime signal flags, and the 4-digit encoding of Chinese characters for a Chinese 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 a telegraph key and decipherable by ear, and persists in amateur radio and aeronautical 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, the Baudot code, the American 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 by Émile 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 name baudot 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.

Hollerith 80-column punch card with EBCDIC character set

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. Electromechanical tabulating machines represented date internally by the timing of pulses relative to the motion of the cards through the machine.

When IBM went to electronic processing, starting with the IBM 603 Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code. IBM used several binary-coded decimal (BCD) six-bit character encoding schemes, starting as early as 1953 in its 702[5] and 704 computers, and in its later 7000 Series and 1400 series, as well as in associated peripherals. Since the punched card code then in use only allowed 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 the UNIVAC I,[6] but usually had the ability to read tapes produced on IBM equipment. IBM's BCD encodings were the precursors of their Extended Binary-Coded Decimal Interchange Code (usually abbreviated as EBCDIC), an eight-bit encoding scheme developed in 1963 for the IBM System/360 that featured a larger character set, including lower case letters.

In 1959 the U.S. military defined its Fieldata 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 European ECMA-6 standard.[7] Eight-bit extended ASCII encodings, such as various vendor extensions and the ISO/IEC 8859 series, supported all ASCII characters as well as additional non-ASCII characters.

In 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's hard 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 and developed 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 called code 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 implement variable-length encodings where an escape sequence would signal that subsequent bits should be parsed as a higher code point.

Terminology

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Informally, the terms "character encoding", "character map", "character set" and "code page" are often used interchangeably.[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 these terms has become important.

  • A character is a minimal unit of text that has semantic value.[9][10]
  • A character set is a collection of elements used to represent text.[9][10] For example, the Latin alphabet and Greek alphabet are both character sets.
  • A coded character set is a character set mapped to a set of unique numbers.[10] For historical reasons, this is also often referred to as a code page.[9]
  • A character repertoire is the set of characters that can be represented by a particular coded character set.[10][11] 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 extent Windows code pages).[11]
  • A code point is a value or position of a character in a coded character set.[10]
  • A code space is the range of numerical values spanned by a coded character set.[10][12]
  • A code unit is the minimum bit combination that can represent a character in a character encoding (in computer science terms, it is the word size of the character encoding).[10][12] For example, common code units include 7-bit, 8-bit, 16-bit, and 32-bit. In some encodings, some characters are encoded using multiple code units; such an encoding is referred to as a variable-width encoding.

Code pages

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Main article: Code page

"Code page" is a historical name for a coded character set.

Originally, a code page referred to a specific page number in the IBM standard character set manual, which would define a particular character encoding.[13] Other vendors, including Microsoft, SAP, and Oracle Corporation, also published their own sets of code pages; the most well-known code page suites are "Windows" (based on Windows-1252) and "IBM"/"DOS" (based on code page 437).

Despite no longer referring to specific page numbers in a standard, many character encodings are still referred to by their code page number; likewise, the term "code page" is often still used to refer to character encodings in general.

The term "code page" is not used in Unix or Linux, where "charmap" is preferred, usually in the larger context of locales. IBM's Character Data Representation Architecture (CDRA) designates entities with coded character set identifiers (CCSIDs), each of which is variously called a "charset", "character set", "code page", or "CHARMAP".[12]

Code units

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The code unit size is equivalent to the bit measurement for the particular encoding:

  • A code unit in ASCII consists of 7 bits;
  • A code unit in UTF-8, EBCDIC and GB 18030 consists of 8 bits;
  • A code unit in UTF-16 consists of 16 bits;
  • A code unit in UTF-32 consists of 32 bits.

Code points

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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:

  • UTF-8: code points map to a sequence of one, two, three or four code units.
  • UTF-16: code units are twice as long as 8-bit code units. Therefore, any code point with a scalar value less than U+10000 is encoded with a single code unit. Code points with a value U+10000 or higher require two code units each. These pairs of code units have a unique term in UTF-16: "Unicode surrogate pairs".
  • UTF-32: the 32-bit code unit is large enough that every code point is represented as a single code unit.
  • GB 18030: multiple code units per code point are common, because of the small code units. Code points are mapped to one, two, or four code units.[14]

Characters

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Main article: Character (computing)

Exactly what constitutes a character varies between character encodings.

For example, for letters with diacritics, 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 single glyph. The former simplifies the text handling system, but the latter allows any letter/diacritic combination to be used in text. Ligatures pose similar problems.

Exactly how to handle glyph variants is a choice that must be made when constructing a particular character encoding. Some writing systems, such as Arabic and Hebrew, need to accommodate things like graphemes that are joined in different ways in different contexts, but represent the same semantic character.

Unicode encoding model

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Unicode and its parallel standard, the ISO/IEC 10646 Universal Character Set, together constitute a unified standard for character encoding. Rather than mapping characters directly to bytes, 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 this model precisely, Unicode uses its own set of terminology to describe its process:[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 a function that maps characters to code 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 example ISO/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 to code 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 include UTF-8, UTF-16BE, UTF-32BE, UTF-16LE, and UTF-32LE; compound character encoding schemes, such as UTF-16, UTF-32 and ISO/IEC 2022, switch between several simple schemes by using a byte order mark or escape sequences; compressing schemes try to minimize the number of bytes used per code unit (such as SCSU and BOCU).

Although UTF-32BE and UTF-32LE are simpler CESes, most systems working with Unicode use either UTF-8, which is backward compatible with fixed-length ASCII and maps Unicode code points to variable-length sequences of octets, or UTF-16BE,[citation needed] which is backward compatible with fixed-length UCS-2BE and maps Unicode code points to variable-length sequences of 16-bit words. See comparison of Unicode encodings for a detailed discussion.

Finally, there may be a higher-level protocol which supplies additional information to select the particular variant of a Unicode character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is the XML 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]

Unicode code points

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In Unicode, a character can be referred to as 'U+' followed by its codepoint value in hexadecimal. The range of valid code points (the codespace) for the Unicode standard is U+0000 to U+10FFFF, inclusive, divided in 17 planes, identified by the numbers 0 to 16. Characters in the range U+0000 to U+FFFF are in plane 0, called the Basic 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 called supplementary characters.

The following table shows examples of code point values:

Character Unicode 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 §

Example

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Consider a string of the letters "ab̲c𐐀"—that is, a string containing a Unicode combining character (U+0332 ̲ COMBINING LOW LINE) as well as a supplementary character (U+10400 𐐀 DESERET 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:

  • Four composed characters:
    a, b̲, c, 𐐀
  • Five graphemes:
    a, b, _, c, 𐐀
  • Five Unicode code points:
    U+0061, U+0062, U+0332, U+0063, U+10400
  • Five UTF-32 code units (32-bit integer values):
    0x00000061, 0x00000062, 0x00000332, 0x00000063, 0x00010400
  • Six UTF-16 code units (16-bit integers)
    0x0061, 0x0062, 0x0332, 0x0063, 0xD801, 0xDC00
  • Nine UTF-8 code units (8-bit values, or bytes)
    0x61, 0x62, 0xCC, 0xB2, 0x63, 0xF0, 0x90, 0x90, 0x80

Note 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.

Transcoding

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As a result of having many character encoding methods in use (and the need for backward compatibility with archived data), many computer programs have been developed to translate data between character encoding schemes, a process known as transcoding. Some of these are cited below.

Cross-platform:

  • Web browsers – most modern web browsers feature automatic character encoding detection. On Firefox 3, for example, see the View/Character Encoding submenu.
  • iconv – a program and standardized API to convert encodings
  • luit – a program that converts encoding of input and output to programs running interactively
  • International Components for Unicode – A set of C and Java libraries to perform charset conversion. uconv can be used from ICU4C.

Windows:

  • Encoding.Convert – .NET API[15]
  • MultiByteToWideChar/WideCharToMultiByte – to convert from ANSI to Unicode & Unicode to ANSI[16][17]

Common character encodings

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Main article: Popularity of text encodings

The most used character encoding on the web is UTF-8, used in 98.2% of surveyed web sites, as of May 2024.[2] In application programs and operating system tasks, both UTF-8 and UTF-16 are popular options.[3][18]

See also

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References

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  1. ^ "Character Encoding Definition". The Tech Terms Dictionary. 24 September 2010.
  2. ^ a b "Usage Survey of Character Encodings broken down by Ranking". W3Techs. Retrieved 29 April 2024.
  3. ^ a b "Charset". Android Developers. Retrieved 2 January 2021. Android note: The Android platform default is always UTF-8.
  4. ^ Tom Henderson (17 April 2014). "Ancient Computer Character Code Tables – and Why They're Still Relevant". Smartbear. Archived from the original on 30 April 2014. Retrieved 29 April 2014.
  5. ^ "IBM Electronic Data-Processing Machines Type 702 Preliminary Manual of Information" (PDF). 1954. p. 80. 22-6173-1. Archived (PDF) from the original on 9 October 2022 – via bitsavers.org.
  6. ^ "UNIVAC System" (PDF) (reference card).
  7. ^ Tom Jennings (20 April 2016). "An annotated history of some character codes". Sensitive Research. Retrieved 1 November 2018.
  8. ^ Strelho, Kevin (15 April 1985). "IBM Drives Hard Disks to New Standards". InfoWorld. Popular Computing Inc. pp. 29–33. Retrieved 10 November 2020.
  9. ^ a b c d Shawn Steele (15 March 2005). "What's the difference between an Encoding, Code Page, Character Set and Unicode?". Microsoft Docs.
  10. ^ a b c d e f g "Glossary of Unicode Terms". Unicode Consortium.
  11. ^ a b "Chapter 3: Conformance". The Unicode Standard Version 15.0 – Core Specification (PDF). Unicode Consortium. September 2022. ISBN 978-1-936213-32-0.
  12. ^ a b c d e Whistler, Ken; Freytag, Asmus (11 November 2022). "UTR#17: Unicode Character Encoding Model". Unicode Consortium. Retrieved 12 August 2023.
  13. ^ "VT510 Video Terminal Programmer Information". Digital Equipment Corporation (DEC). 7.1. Character Sets - Overview. Archived from the original on 26 January 2016. Retrieved 15 February 2017. In addition to traditional DEC and ISO character sets, which conform to the structure and rules of ISO 2022, the VT510 supports a number of IBM PC code pages (page numbers in IBM's standard character set manual) in PCTerm mode to emulate the console terminal of industry-standard PCs.
  14. ^ "Terminology (The Java Tutorials)". Oracle. Retrieved 25 March 2018.
  15. ^ "Encoding.Convert Method". Microsoft .NET Framework Class Library.
  16. ^ "MultiByteToWideChar function (stringapiset.h)". Microsoft Docs. 13 October 2021.
  17. ^ "WideCharToMultiByte function (stringapiset.h)". Microsoft Docs. 9 August 2022.
  18. ^ Galloway, Matt (9 October 2012). "Character encoding for iOS developers. Or UTF-8 what now?". Matt Galloway. Retrieved 2 January 2021. in reality, you usually just assume UTF-8 since that is by far the most common encoding.

Further reading

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Wikimedia Commons has media related to Encodings.
Wikiversity has learning resources about Character encoding
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