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+.\" Copyright (c) 1996 Eric S. Raymond <esr@thyrsus.com>
+.\" and Copyright (c) Andries Brouwer <aeb@cwi.nl>
+.\"
+.\" SPDX-License-Identifier: GPL-2.0-or-later
+.\"
+.\" This is combined from many sources, including notes by aeb and
+.\" research by esr.  Portions derive from a writeup by Roman Czyborra.
+.\"
+.\" Changes also by David Starner <dstarner98@aasaa.ofe.org>.
+.\"
+.TH charsets 7 2023-03-12 "Linux man-pages 6.05.01"
+.SH NAME
+charsets \- character set standards and internationalization
+.SH 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.
+.PP
+Standards discussed include such as
+ASCII, GB 2312, ISO 8859, JIS, KOI8-R, KS, and Unicode.
+.PP
+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.
+.SS 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.
+.PP
+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.
+.PP
+As Unicode, when using UTF-8, is ASCII-compatible, plain ASCII text
+still renders properly on modern UTF-8 using systems.
+.SS 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\[en]255.
+.PP
+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.
+.PP
+Console support for the other 8859 character sets is available under
+Linux through user-mode utilities (such as
+.BR setfont (8))
+that modify keyboard bindings and the EGA graphics
+table and employ the "user mapping" font table in the console
+driver.
+.PP
+Here are brief descriptions of each character set:
+.TP
+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.
+.TP
+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.
+.TP
+8859-3 (Latin-3)
+Latin-3 was designed to cover of Esperanto, Maltese, and Turkish, but
+8859-9 later superseded it for Turkish.
+.TP
+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.
+.TP
+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.
+.TP
+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.
+.TP
+8859-7
+Was created for Modern Greek in 1987, updated in 2003.
+.TP
+8859-8
+Supports Modern Hebrew without niqud (punctuation signs).
+Niqud and full-fledged Biblical Hebrew were outside the scope of this
+character set.
+.TP
+8859-9 (Latin-5)
+This is a variant of Latin-1 that replaces Icelandic letters with
+Turkish ones.
+.TP
+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.
+.TP
+8859-11
+Supports the Thai alphabet and is nearly identical to the TIS-620
+standard.
+.TP
+8859-12
+This character set does not exist.
+.TP
+8859-13 (Latin-7)
+Supports the Baltic Rim languages; in particular, it includes Latvian
+characters not found in Latin-4.
+.TP
+8859-14 (Latin-8)
+This is the Celtic character set, covering Old Irish, Manx, Gaelic,
+Welsh, Cornish, and Breton.
+.TP
+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.
+.TP
+8859-16 (Latin-10)
+This character set covers many Southeast European languages,
+and most importantly supports Romanian more completely than Latin-2.
+.SS 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.
+.PP
+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.
+.SS 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.
+.SS 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\[en]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.
+.\" Thanks to Tomohiro KUBOTA for the following sections about
+.\" national standards.
+.SS 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\[en]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.
+.SS 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.
+.SS 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
+.BR xterm (1).
+Several ISO 2022-based character encodings have been defined,
+especially for Japanese.
+.PP
+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\[en]0177 (041\[en]0176) or 0240\[en]0377 (0241\[en]0376).
+G0 always has size 94 and uses codes 041\[en]0176.
+.PP
+Switching between character sets is done using the shift functions
+\fB\[ha]N\fP (SO or LS1), \fB\[ha]O\fP (SI or LS0), ESC n (LS2), ESC o (LS3),
+ESC N (SS2), ESC O (SS3), ESC \[ti] (LS1R), ESC } (LS2R), ESC | (LS3R).
+The function LS\fIn\fP makes character set G\fIn\fP the current one
+for codes with high bit zero.
+The function LS\fIn\fPR makes character set G\fIn\fP the current one
+for codes with high bit one.
+The function SS\fIn\fP makes character set G\fIn\fP (\fIn\fP=2 or 3)
+the current one for the next character only (regardless of the value
+of its high order bit).
+.PP
+A 94-character set is designated as G\fIn\fP 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.
+.PP
+A 96-character set is designated as G\fIn\fP 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.
+.PP
+A multibyte character set is designated as G\fIn\fP 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.
+.PP
+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, \fB\[ha]N\fP and \fB\[ha]O\fP 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.
+.SS 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\[en]0xfe.
+.SS 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).
+.PP
+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.
+.PP
+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.
+.PP
+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.)
+.PP
+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.
+.PP
+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 (\[aq]\e0\[aq]) or \[aq]/\[aq]s
+that form part of some larger code.
+.PP
+Since ASCII, and, in particular, NUL and \[aq]/\[aq], 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.
+.PP
+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.
+.SH SEE ALSO
+.BR iconv (1),
+.BR ascii (7),
+.BR iso_8859\-1 (7),
+.BR unicode (7),
+.BR utf\-8 (7)