Incomputing andtelecommunications, acharacter is theencoded representation of anatural languagecharacter (includingletter,numeral andpunctuation),whitespace (space or tab), or acontrol character (controlscomputer hardware that consumes character-based data). A sequence of characters is called astring.
Some character encoding systems represent each character using a fixed number ofbits whereas other systems use varying sizes. Various fixed-length sizes were used for now obsolete systems such as thesix-bit character code,[1][2] thefive-bit Baudot code and even 4-bit systems (with only 16 possible values).[3] The more modernASCII system uses the 8-bitbyte for each character. Today, theUnicode-basedUTF-8 encoding uses a varying number of byte-sizedcode units to define acode point which combine to encode a character.
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In general, a character is a symbol (such as a letter or number) that represents information, and in the context of computing is a representation of such a symbol that may be accepted by a computer.[4] A character implies an encoding of information, often as defined by a standard such asANSI orUnicode.
A character set identifies a repertoire of characters that are each mapped to a unique numeric value.
Glyph describes a particular visual appearance of a character. Many computerfonts consist of glyphs that are indexed by the numerical code of the corresponding character.
With the advent and widespread acceptance of Unicode[5] and bit-agnosticcoded character sets,[clarification needed] a character is increasingly being seen as a unit ofinformation, independent of any particular visual manifestation. TheISO/IEC 10646 (Unicode) International Standard definescharacter, orabstract character as "a member of a set of elements used for the organization, control, or representation of data". Unicode's definition supplements this with explanatory notes that encourage the reader to differentiate between characters, graphemes, and glyphs, among other things. Such differentiation is an instance of the wider theme of theseparation of presentation and content.
For example, theHebrew letteraleph ("א") is often used by mathematicians to denote certain kinds ofinfinity (ℵ), but it is also used in ordinary Hebrew text. In Unicode, these two uses are considered different characters, and have two different Unicode numerical identifiers ("code points"), though they may be rendered identically. Conversely, theChineselogogram for water ("水") may have a slightly different appearance inJapanese texts than it does in Chinese texts, and localtypefaces may reflect this. But nonetheless in Unicode they are considered the same character, and share the same code point.
The Unicode standard differentiates between these abstract characters andcoded characters orencoded characters that have been paired with numeric codes that facilitate their representation in computers.
Thecombining character is addressed by Unicode which allocates a code point to each of:
This makes it possible to code the middle character of the word 'naïve' either as a single character 'ï' or as a combination of the character'i ' with the combining diaeresis: (U+0069 LATIN SMALL LETTER I + U+0308 COMBINING DIAERESIS); this is also rendered as'ï '.
InC,char (short for character) is adata type with size onebyte,[6][7] but unlike the de facto size of byte as 8 bits, this use of byte is less specific. Byte is defined to be large enough to contain any member of the "basic execution character set". The number of bits used by acompiler is accessible via theCHAR_BIT macro. By far the most common size is 8 bits, andPOSIXrequires it to be 8 bits.[8] In modern C standards,char is required to holdUTF-8 code units[6][7] which requires aminimum size of 8 bits.
Since aUnicode code point may require as many as 21 bits.[9] thechar type is generally not large enough for every character. Nonetheless, thechar type is well-suited for theUTF-8 encoding where each code point requires 1 to 4 bytes.
The fact that a character was historically stored in a single byte has led to the terms "char" and "character" being used interchangeably and this leads to confusion today when multibyte encodings such as UTF-8 are used. Modern POSIX documentation attempts to fix this by defining "character" as a sequence of one or more bytes representing a single graphic symbol or control code, and uses "byte" when referring to char data.[10][11] However it still contains errors such as defining an array ofchar as acharacter array (rather than abyte array).[12]
Unicode can be stored in strings of code units that are larger thanchar calledwide characters. The original C type was calledwchar_t. Due to some platforms definingwchar_t as 16 bits and others defining it as 32 bits, current versions provide unambiguouschar16_t andchar32_t. Even then the objects being stored might not be characters, for instance the variable-lengthUTF-16 is often stored in arrays ofchar16_t.
Other languages also have achar type. Many, includingC++, use 8-bit bytes like C.[7] Others, such asJava, use 2-byte, wide storage to more directly accommodate UTF-16.
[…] Internal data code is used: Quantitative (numerical) data are coded in a 4-bit decimal code; qualitative (alpha-numerical) data are coded in a 6-bit alphanumerical code. The internalinstruction code means that the instructions are coded in straight binary code.
As to the internal information length, the information quantum is called a "catena," and it is composed of 24 bits representing either 6 decimal digits, or 4 alphanumerical characters. This quantum must contain a multiple of 4 and 6 bits to represent a whole number of decimal or alphanumeric characters. Twenty-four bits was found to be a good compromise between the minimum 12 bits, which would lead to a too-low transfer flow from a parallel readout core memory, and 36 bits or more, which was judged as too large an information quantum. The catena is to be considered as the equivalent of a character in variableword length machines, but it cannot be called so, as it may contain several characters. It is transferred in series to and from the main memory.
Not wanting to call a "quantum" a word, or a set of characters a letter, (a word is a word, and a quantum is something else), a new word was made, and it was called a "catena." It is an English word and exists inWebster's although it does not in French. Webster's definition of the word catena is, "a connected series;" therefore, a 24-bit information item. The word catena will be used hereafter.
The internal code, therefore, has been defined. Now what are the external data codes? These depend primarily upon the information handling device involved. TheGamma 60 [fr] is designed to handle information relevant to any binary coded structure. Thus an 80-column punched card is considered as a 960-bit information item; 12 rows multiplied by 80 columns equals 960 possible punches; is stored as an exact image in 960 magnetic cores of the main memory with 2 card columns occupying one catena. […]
[…] Terms used here to describe the structure imposed by the machine design, in addition tobit, are listed below.
Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other thancharacter is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined frombite, but respelled to avoid accidental mutation tobit.)
Aword consists of the number of data bits transmitted in parallel from or to memory in one memory cycle.Word size is thus defined as a structural property of the memory. (The termcatena was coined for this purpose by the designers of theBullGAMMA 60 [fr] computer.)
Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program. […]
[…]Bit - The smallest unit of information which can be represented. (A bit may be in one of two states I 0 or 1). […]Byte - A group of 8 contiguous bits occupying a single memory location. […] Character - A group of 4 contiguous bits of data. […](NB. ThisIntel 4004 manual uses the termcharacter referring to4-bit rather than 8-bitdata entities. Intel switched to use the more common termnibble for 4-bit entities in their documentation for the succeeding processor4040 in 1974 already.)
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