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 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 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 8859
ISO 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 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
8859-1 characters are also the first 256 characters of Unicode.
Console support for the other 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 set:
8859-1 (Latin-1)
Latin-1 covers many West 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 old-style „German' quotation marks was considered
tolerable.
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.
8859-3 (Latin-3)
Latin-3 was designed to cover of Esperanto, Maltese, and
Turkish, but 8859-9 later superseded it for Turkish.
8859-4 (Latin-4)
Latin-4 introduced letters for North European languages
such as Estonian, Latvian, and Lithuanian, but was
superseded by 8859-10 and 8859-13.
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.
8859-6 Was created for Arabic. The 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.
8859-7 Was created for Modern Greek in 1987, updated in 2003.
8859-8 Supports Modern Hebrew without niqud (punctuation signs).
Niqud and full-fledged Biblical Hebrew were outside the
scope of this character set.
8859-9 (Latin-5)
This is a variant of Latin-1 that replaces Icelandic
letters with Turkish ones.
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.
8859-11
Supports the Thai alphabet and is nearly identical to the
TIS-620 standard.
8859-12
This set does not exist.
8859-13 (Latin-7)
Supports the Baltic Rim languages; in particular, it
includes Latvian characters not found in Latin-4.
8859-14 (Latin-8)
This is the Celtic character set, covering Old Irish,
Manx, Gaelic, Welsh, Cornish, and Breton.
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.
8859-16 (Latin-10)
This 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 8859-5. KOI8-U,
based on KOI8-R, has better support for Ukrainian. Neither of
these sets are ISO-2022 compatible, unlike the ISO 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 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-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-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 2022 and ISO 4873
The ISO 2022 and 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 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 2375 International Register of Coded Character
Sets. For example, ESC ( @ selects the ISO 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 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 8859 series, Thai
characters are mapped into 0xa1–0xfe.
Unicode
Unicode (ISO 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 10646 then this progression continues up to 6-byte
codes.)
For most texts in ISO 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 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.
