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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 LSn makes character set Gn the current one for codes with high
bit zero. The function LSnR makes character set Gn the current one for codes with high
bit one. The function SSn makes character set Gn (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 Gn 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 Gn 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 Gn 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.
SEE ALSO
iconv(1), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)