# charsets

## NAME



           charsets - character set standards and internationalization
    


## DESCRIPTION



           This manual page gives an overview on different character set
           standards and how they were used on Linux before Unicode became
           ubiquitous.  Some of this information is still helpful for people
           working with legacy systems and documents.
    
           Standards discussed include such as ASCII, GB 2312, ISO/IEC 8859,
           JIS, KOI8-R, KS, and Unicode.
    
           The primary emphasis is on character sets that were actually used
           by locale character sets, not the myriad others that could be
           found in data from other systems.
    
       **ASCII**
           ASCII (American Standard Code For Information Interchange) is the
           original 7-bit character set, originally designed for American
           English.  Also known as US-ASCII.  It is currently described by
           the ISO/IEC 646:1991 IRV (International Reference Version)
           standard.
    
           Various ASCII variants replacing the dollar sign with other
           currency symbols and replacing punctuation with non-English
           alphabetic characters to cover German, French, Spanish, and others
           in 7 bits emerged.  All are deprecated; glibc does not support
           locales whose character sets are not true supersets of ASCII.
    
           As Unicode, when using UTF-8, is ASCII-compatible, plain ASCII
           text still renders properly on modern UTF-8 using systems.
    
       **ISO/IEC 8859**
           ISO/IEC 8859 is a series of 15 8-bit character sets, all of which
           have ASCII in their low (7-bit) half, invisible control characters
           in positions 128 to 159, and 96 fixed-width graphics in positions
           160–255.
    
           Of these, the most important is ISO/IEC 8859-1 ("Latin Alphabet
           No. 1" / Latin-1).  It was widely adopted and supported by
           different systems, and is gradually being replaced with Unicode.
           The ISO/IEC 8859-1 characters are also the first 256 characters of
           Unicode.
    
           Console support for the other ISO/IEC 8859 character sets is
           available under Linux through user-mode utilities (such as
           setfont(8)) that modify keyboard bindings and the EGA graphics
           table and employ the "user mapping" font table in the console
           driver.
    
           Here are brief descriptions of each character set:
    
           ISO/IEC 8859-1 (Latin-1)
                  Latin-1 covers many European languages such as Albanian,
                  Basque, Danish, English, Faroese, Galician, Icelandic,
                  Irish, Italian, Norwegian, Portuguese, Spanish, and
                  Swedish.  The lack of the ligatures Dutch Ĳ/ĳ, French œ,
                  and „German“ quotation marks was considered tolerable.
    
           ISO/IEC 8859-2 (Latin-2)
                  Latin-2 supports many Latin-written Central and East
                  European languages such as Bosnian, Croatian, Czech,
                  German, Hungarian, Polish, Slovak, and Slovene.  Replacing
                  Romanian ș/ț with ş/ţ was considered tolerable.
    
           ISO/IEC 8859-3 (Latin-3)
                  Latin-3 was designed to cover of Esperanto, Maltese, and
                  Turkish, but ISO/IEC 8859-9 later superseded it for
                  Turkish.
    
           ISO/IEC 8859-4 (Latin-4)
                  Latin-4 introduced letters for North European languages
                  such as Estonian, Latvian, and Lithuanian, but was
                  superseded by ISO/IEC 8859-10 and ISO/IEC 8859-13.
    
           ISO/IEC 8859-5
                  Cyrillic letters supporting Bulgarian, Byelorussian,
                  Macedonian, Russian, Serbian, and (almost completely)
                  Ukrainian.  It was never widely used, see the discussion of
                  KOI8-R/KOI8-U below.
    
           ISO/IEC 8859-6
                  Was created for Arabic.  The ISO/IEC 8859-6 glyph table is
                  a fixed font of separate letter forms, but a proper display
                  engine should combine these using the proper initial,
                  medial, and final forms.
    
           ISO/IEC 8859-7
                  Was created for Modern Greek in 1987, updated in 2003.
    
           ISO/IEC 8859-8
                  Supports Modern Hebrew without niqud (punctuation signs).
                  Niqud and full-fledged Biblical Hebrew were outside the
                  scope of this character set.
    
           ISO/IEC 8859-9 (Latin-5)
                  This is a variant of Latin-1 that replaces Icelandic
                  letters with Turkish ones.
    
           ISO/IEC 8859-10 (Latin-6)
                  Latin-6 added the Inuit (Greenlandic) and Sami (Lappish)
                  letters that were missing in Latin-4 to cover the entire
                  Nordic area.
    
           ISO/IEC 8859-11
                  Supports the Thai alphabet and is nearly identical to the
                  TIS-620 standard.
    
           ISO/IEC 8859-12
                  This character set does not exist.
    
           ISO/IEC 8859-13 (Latin-7)
                  Supports the Baltic Rim languages; in particular, it
                  includes Latvian characters not found in Latin-4.
    
           ISO/IEC 8859-14 (Latin-8)
                  This is the Celtic character set, covering Old Irish, Manx,
                  Gaelic, Welsh, Cornish, and Breton.
    
           ISO/IEC 8859-15 (Latin-9)
                  Latin-9 is similar to the widely used Latin-1 but replaces
                  some less common symbols with the Euro sign and French and
                  Finnish letters that were missing in Latin-1.
    
           ISO/IEC 8859-16 (Latin-10)
                  This character set covers many Southeast European
                  languages, and most importantly supports Romanian more
                  completely than Latin-2.
    
       **KOI8-R / KOI8-U**
           KOI8-R is a non-ISO character set popular in Russia before
           Unicode.  The lower half is ASCII; the upper is a Cyrillic
           character set somewhat better designed than ISO/IEC 8859-5.
           KOI8-U, based on KOI8-R, has better support for Ukrainian.
           Neither of these sets are ISO/IEC 2022 compatible, unlike the
           ISO/IEC 8859 series.
    
           Console support for KOI8-R is available under Linux through user-
           mode utilities that modify keyboard bindings and the EGA graphics
           table, and employ the "user mapping" font table in the console
           driver.
    
       **GB 2312**
           GB 2312 is a mainland Chinese national standard character set used
           to express simplified Chinese.  Just like JIS X 0208, characters
           are mapped into a 94x94 two-byte matrix used to construct EUC-CN.
           EUC-CN is the most important encoding for Linux and includes ASCII
           and GB 2312.  Note that EUC-CN is often called as GB, GB 2312, or
           CN-GB.
    
       **Big5**
           Big5 was a popular character set in Taiwan to express traditional
           Chinese.  (Big5 is both a character set and an encoding.)  It is a
           superset of ASCII.  Non-ASCII characters are expressed in two
           bytes.  Bytes 0xa1–0xfe are used as leading bytes for two-byte
           characters.  Big5 and its extension were widely used in Taiwan and
           Hong Kong.  It is not ISO/IEC 2022 compliant.
    
       **JIS X 0208**
           JIS X 0208 is a Japanese national standard character set.  Though
           there are some more Japanese national standard character sets
           (like JIS X 0201, JIS X 0212, and JIS X 0213), this is the most
           important one.  Characters are mapped into a 94x94 two-byte
           matrix, whose each byte is in the range 0x21–0x7e.  Note that JIS
           X 0208 is a character set, not an encoding.  This means that JIS X
           0208 itself is not used for expressing text data.  JIS X 0208 is
           used as a component to construct encodings such as EUC-JP,
           Shift_JIS, and ISO/IEC 2022-JP.  EUC-JP is the most important
           encoding for Linux and includes ASCII and JIS X 0208.  In EUC-JP,
           JIS X 0208 characters are expressed in two bytes, each of which is
           the JIS X 0208 code plus 0x80.
    
       **KS X 1001**
           KS X 1001 is a Korean national standard character set.  Just as
           JIS X 0208, characters are mapped into a 94x94 two-byte matrix.
           KS X 1001 is used like JIS X 0208, as a component to construct
           encodings such as EUC-KR, Johab, and ISO/IEC 2022-KR.  EUC-KR is
           the most important encoding for Linux and includes ASCII and KS X
           1001.  KS C 5601 is an older name for KS X 1001.
    
       **ISO/IEC 2022 and ISO/IEC 4873**
           The ISO/IEC 2022 and ISO/IEC 4873 standards describe a font-
           control model based on VT100 practice.  This model is (partially)
           supported by the Linux kernel and by **xterm**(1).  Several
           ISO/IEC 2022-based character encodings have been defined,
           especially for Japanese.
    
           There are 4 graphic character sets, called G0, G1, G2, and G3, and
           one of them is the current character set for codes with high bit
           zero (initially G0), and one of them is the current character set
           for codes with high bit one (initially G1).  Each graphic
           character set has 94 or 96 characters, and is essentially a 7-bit
           character set.  It uses codes either 040–0177 (041–0176) or
           0240–0377 (0241–0376).  G0 always has size 94 and uses codes
           041–0176.
    
           Switching between character sets is done using the shift functions
           **^N**(SO or LS1), **^O**(SI or LS0), ESC n (LS2), ESC o (LS3), ESC N
           (SS2), ESC O (SS3), ESC ~ (LS1R), ESC } (LS2R), ESC | (LS3R).  The
           function LS _n_ makes character set G _n_ the current one for codes with
           high bit zero.  The function LS _n_ R makes character set G _n_ the
           current one for codes with high bit one.  The function SS _n_ makes
           character set G _n_ (_n_ =2 or 3) the current one for the next character
           only (regardless of the value of its high order bit).
    
           A 94-character set is designated as G _n_ character set by an escape
           sequence ESC ( xx (for G0), ESC ) xx (for G1), ESC * xx (for G2),
           ESC + xx (for G3), where xx is a symbol or a pair of symbols found
           in the ISO/IEC 2375 International Register of Coded Character
           Sets.  For example, ESC ( @ selects the ISO/IEC 646 character set
           as G0, ESC ( A selects the UK standard character set (with pound
           instead of number sign), ESC ( B selects ASCII (with dollar
           instead of currency sign), ESC ( M selects a character set for
           African languages, ESC ( ! A selects the Cuban character set, and
           so on.
    
           A 96-character set is designated as G _n_ character set by an escape
           sequence ESC - xx (for G1), ESC . xx (for G2) or ESC / xx (for
           G3).  For example, ESC - G selects the Hebrew alphabet as G1.
    
           A multibyte character set is designated as G _n_ character set by an
           escape sequence ESC $ xx or ESC $ ( xx (for G0), ESC $ ) xx (for
           G1), ESC $ * xx (for G2), ESC $ + xx (for G3).  For example, ESC $
           ( C selects the Korean character set for G0.  The Japanese
           character set selected by ESC $ B has a more recent version
           selected by ESC & @ ESC $ B.
    
           ISO/IEC 4873 stipulates a narrower use of character sets, where G0
           is fixed (always ASCII), so that G1, G2, and G3 can be invoked
           only for codes with the high order bit set.  In particular, **^N** and
           **^O** are not used anymore, ESC ( xx can be used only with xx=B, and
           ESC ) xx, ESC * xx, ESC + xx are equivalent to ESC - xx, ESC . xx,
           ESC / xx, respectively.
    
       **TIS-620**
           TIS-620 is a Thai national standard character set and a superset
           of ASCII.  In the same fashion as the ISO/IEC 8859 series, Thai
           characters are mapped into 0xa1–0xfe.
    
       **Unicode**
           Unicode (ISO/IEC 10646) is a standard which aims to unambiguously
           represent every character in every human language.  Unicode's
           structure permits 20.1 bits to encode every character.  Since most
           computers don't include 20.1-bit integers, Unicode is usually
           encoded as 32-bit integers internally and either a series of
           16-bit integers (UTF-16) (needing two 16-bit integers only when
           encoding certain rare characters) or a series of 8-bit bytes
           (UTF-8).
    
           Linux represents Unicode using the 8-bit Unicode Transformation
           Format (UTF-8).  UTF-8 is a variable length encoding of Unicode.
           It uses 1 byte to code 7 bits, 2 bytes for 11 bits, 3 bytes for 16
           bits, 4 bytes for 21 bits, 5 bytes for 26 bits, 6 bytes for 31
           bits.
    
           Let 0,1,x stand for a zero, one, or arbitrary bit.  A byte
           0xxxxxxx stands for the Unicode 00000000 0xxxxxxx which codes the
           same symbol as the ASCII 0xxxxxxx.  Thus, ASCII goes unchanged
           into UTF-8, and people using only ASCII do not notice any change:
           not in code, and not in file size.
    
           A byte 110xxxxx is the start of a 2-byte code, and 110xxxxx
           10yyyyyy is assembled into 00000xxx xxyyyyyy.  A byte 1110xxxx is
           the start of a 3-byte code, and 1110xxxx 10yyyyyy 10zzzzzz is
           assembled into xxxxyyyy yyzzzzzz.  (When UTF-8 is used to code the
           31-bit ISO/IEC 10646 then this progression continues up to 6-byte
           codes.)
    
           For most texts in ISO/IEC 8859 character sets, this means that the
           characters outside of ASCII are now coded with two bytes.  This
           tends to expand ordinary text files by only one or two percent.
           For Russian or Greek texts, this expands ordinary text files by
           100%, since text in those languages is mostly outside of ASCII.
           For Japanese users this means that the 16-bit codes now in common
           use will take three bytes.  While there are algorithmic
           conversions from some character sets (especially ISO/IEC 8859-1)
           to Unicode, general conversion requires carrying around conversion
           tables, which can be quite large for 16-bit codes.
    
           Note that UTF-8 is self-synchronizing: 10xxxxxx is a tail, any
           other byte is the head of a code.  Note that the only way ASCII
           bytes occur in a UTF-8 stream, is as themselves.  In particular,
           there are no embedded NULs ('\0') or '/'s that form part of some
           larger code.
    
           Since ASCII, and, in particular, NUL and '/', are unchanged, the
           kernel does not notice that UTF-8 is being used.  It does not care
           at all what the bytes it is handling stand for.
    
           Rendering of Unicode data streams is typically handled through
           "subfont" tables which map a subset of Unicode to glyphs.
           Internally the kernel uses Unicode to describe the subfont loaded
           in video RAM.  This means that in the Linux console in UTF-8 mode,
           one can use a character set with 512 different symbols.  This is
           not enough for Japanese, Chinese, and Korean, but it is enough for
           most other purposes.
    


* * *