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".
Early character codes associated with the optical or electrical telegraph could only represent a subset of the characters used in written languages, sometimes restricted to upper case letters, numerals and some punctuation only. The low cost of digital representation of data in modern computer systems allows more elaborate character codes (such as Unicode) which represent most of the characters used in many written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form.
History
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).[2]
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 supplanted 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. 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 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.[3]
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's Binary Coded Decimal (BCD) was a six-bit encoding scheme used by IBM as early as 1953 in its 702[4] 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. BCD extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping it easily to punch-card encoding which was already in widespread use. IBMs codes were used primarily with IBM equipment; other computer vendors of the era had their own character codes, often six-bit, but usually had the ability to read tapes produced on IBM equipment. BCD was the precursor of IBM's 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 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),[5] 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
Informally, the terms "character encoding", "character map", "character set" and "code page" are often used interchangeably.[6] 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.[6][7]
A character set is a collection of elements used to represent text.[6][7] For example, the Latin alphabet and Greek alphabet are both character sets.
A coded character set is a character set mapped to set of unique numbers.[7] For historical reasons, this is also often referred to as a code page.[6]
A character repertoire is the set of characters that can be represented by a particular coded character set.[7][8] 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).[8]
A code point is a value or position of a character in a coded character set.[7]
A code space is the range of numerical values spanned by a coded character set.[7][9]
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).[7][9] 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 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.[10] 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".[9]
Code units
The code unit size is equivalent to the bit measurement for the particular encoding:
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.[11]
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
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:[9]
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 1.4million) 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).
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.[9]
Unicode code points
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 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
Consider a string of the letters "ab̲c𐐀"—that is, a string containing a Unicode combining character (U+0332̲) as well a supplementary character (U+10400𐐀). 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:
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
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.
TRON, part of the TRON project, is an encoding system that does not use Han Unification; instead, it uses "control codes" to switch between 16-bit "planes" of characters.
ISO 8859-14 Celtic languages (Irish Gaelic, Scottish, Welsh)
ISO 8859-15 Added the Euro sign and other rationalisations to ISO 8859-1
ISO 8859-16 Central, Eastern and Southern European languages (Albanian, Bosnian, Croatian, Hungarian, Polish, Romanian, Serbian and Slovenian, but also French, German, Italian and Irish Gaelic)
Windows-1250 for Central European languages that use Latin script, (Polish, Czech, Slovak, Hungarian, Slovene, Serbian, Croatian, Bosnian, Romanian and Albanian)
While Hypertext Markup Language (HTML) has been in use since 1991, HTML 4.0 from December 1997 was the first standardized version where international characters were given reasonably complete treatment. When an HTML document includes special characters outside the range of seven-bit ASCII, two goals are worth considering: the information's integrity, and universal browser display.
Extended Binary Coded Decimal Interchange Code is an eight-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems. It descended from the code used with punched cards and the corresponding six-bit binary-coded decimal code used with most of IBM's computer peripherals of the late 1950s and early 1960s. It is supported by various non-IBM platforms, such as Fujitsu-Siemens' BS2000/OSD, OS-IV, MSP, and MSP-EX, the SDS Sigma series, Unisys VS/9, Unisys MCP and ICL VME.
ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. ISO/IEC 8859-1 encodes what it refers to as "Latin alphabet no. 1", consisting of 191 characters from the Latin script. This character-encoding scheme is used throughout the Americas, Western Europe, Oceania, and much of Africa. It is the basis for some popular 8-bit character sets and the first two blocks of characters in Unicode.
Unicode, formally The Unicode Standard, is a text encoding standard maintained by the Unicode Consortium designed to support the use of text written in all of the world's major writing systems. Version 15.1 of the standard defines 149813 characters and 161 scripts used in various ordinary, literary, academic, and technical contexts.
UTF-8 is a variable-length character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode Transformation Format – 8-bit.
UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as code points are encoded with one or two 16-bit code units. UTF-16 arose from an earlier obsolete fixed-width 16-bit encoding now known as UCS-2 (for 2-byte Universal Character Set), once it became clear that more than 216 (65,536) code points were needed, including most emoji and important CJK characters such as for personal and place names.
In computing, a code page is a character encoding and as such it is a specific association of a set of printable characters and control characters with unique numbers. Typically each number represents the binary value in a single byte.
ISO/IEC 8859-7:2003, Information technology — 8-bit single-byte coded graphic character sets — Part 7: Latin/Greek alphabet, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. It is informally referred to as Latin/Greek. It was designed to cover the modern Greek language. The original 1987 version of the standard had the same character assignments as the Greek national standard ELOT 928, published in 1986. The table in this article shows the updated 2003 version which adds three characters. Microsoft has assigned code page 28597 a.k.a. Windows-28597 to ISO-8859-7 in Windows. IBM has assigned code page 813 to ISO 8859-7. (IBM CCSID 813 is the original encoding. CCSID 4909 adds the euro sign. CCSID 9005 further adds the drachma sign and ypogegrammeni.)
ISO/IEC 8859-9:1999, Information technology — 8-bit single-byte coded graphic character sets — Part 9: Latin alphabet No. 5, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1989. It is designated ECMA-128 by Ecma International and TS 5881 as a Turkish standard. It is informally referred to as Latin-5 or Turkish. It was designed to cover the Turkish language, designed as being of more use than the ISO/IEC 8859-3 encoding. It is identical to ISO/IEC 8859-1 except for the replacement of six Icelandic characters with characters unique to the Turkish alphabet. And the uppercase of i is İ; the lowercase of I is ı.
ISO/IEC 2022Information technology—Character code structure and extension techniques, is an ISO/IEC standard in the field of character encoding. It is equivalent to the ECMA standard ECMA-35, the ANSI standard ANSI X3.41 and the Japanese Industrial Standard JIS X 0202. Originating in 1971, it was most recently revised in 1994.
Shift JIS is a character encoding for the Japanese language, originally developed by the Japanese company ASCII Corporation in conjunction with Microsoft and standardized as JIS X 0208 Appendix 1.
Extended Unix Code (EUC) is a multibyte character encoding system used primarily for Japanese, Korean, and simplified Chinese (characters).
A variable-width encoding is a type of character encoding scheme in which codes of differing lengths are used to encode a character set for representation, usually in a computer. Most common variable-width encodings are multibyte encodings, which use varying numbers of bytes (octets) to encode different characters. (Some authors, notably in Microsoft documentation, use the term multibyte character set, which is a misnomer, because representation size is an attribute of the encoding, not of the character set.)
UTF-EBCDIC is a character encoding capable of encoding all 1,112,064 valid character code points in Unicode using one to five one-byte (8-bit) code units. It is meant to be EBCDIC-friendly, so that legacy EBCDIC applications on mainframes may process the characters without much difficulty. Its advantages for existing EBCDIC-based systems are similar to UTF-8's advantages for existing ASCII-based systems. Details on UTF-EBCDIC are defined in Unicode Technical Report #16.
Several 8-bit character sets (encodings) were designed for binary representation of common Western European languages, which use the Latin alphabet, a few additional letters and ones with precomposed diacritics, some punctuation, and various symbols. These character sets also happen to support many other languages such as Malay, Swahili, and Classical Latin.
Windows code pages are sets of characters or code pages used in Microsoft Windows from the 1980s and 1990s. Windows code pages were gradually superseded when Unicode was implemented in Windows, although they are still supported both within Windows and other platforms, and still apply when Alt code shortcuts are used.
Extended ASCII is a repertoire of character encodings that include the original 96 ASCII character set, plus up to 128 additional characters. There is no formal definition of "extended ASCII", and even use of the term is sometimes criticized, because it can be mistakenly interpreted to mean that the American National Standards Institute (ANSI) had updated its ANSI X3.4-1986 standard to include more characters, or that the term identifies a single unambiguous encoding, neither of which is the case.
KS X 1001, "Code for Information Interchange ", formerly called KS C 5601, is a South Korean coded character set standard to represent Hangul and Hanja characters on a computer.
Microsoft Windows code page 932, also called Windows-31J amongst other names, is the Microsoft Windows code page for the Japanese language, which is an extended variant of the Shift JIS Japanese character encoding. It contains standard 7-bit ASCII codes, and Japanese characters are indicated by the high bit of the first byte being set to 1. Some code points in this page require a second byte, so characters use either 8 or 16 bits for encoding.
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