Unlike ISO 8859 character encodings which use 8 bits for every character, the ISO 2022 encodings are variable size encodings typically using either 8 or 16 bits per character. Several character encodings use ISO 2022 mechanisms. For example, ISO-2022-JP is a widely used character encoding for the Japanese language.
Many languages or language families not based on the Latin alphabet such as Greek, Russian, Arabic, or Hebrew have historically been represented on computers with 8-bit extended ASCII encodings including the ISO 8859 family of character sets. Written East Asian languages, specifically Chinese, Japanese, and Korean, use far more characters than can be represented in an 8-bit computer byte and were first represented on computers with language-specific double byte encodings.
ISO 2022 was developed as a technique to attack both of these problems: to represent characters in multiple character sets within a single character encoding, and to represent large character sets.
Being based on ISO 646, ISO 2022 exhibits many of ISO 646's properties. For example, the most significant bit of each byte does not carry any meaning; this allows ISO 2022 (like ISO 646) to be easily transmitted through 7-bit communication channels. (This 7-bit property also forms the basis of the EUC code.)
To represent multiple character sets, the ISO 2022 character encodings include escape sequences which indicate the character set for characters which follow. The escape sequences are registered with ISO and are often three characters long starting with the ASCII ESCAPE character (hexadecimal 1B, octal 33). These character encodings require data to be processed sequentially in a forward direction since the correct interpretation of the data depends on the most recently encountered escape sequence.
To represent large character sets, ISO 2022 builds on ISO 646's property that 1 byte can define 94 graphic (printable) characters (in addition to space and 33 control characters). Using two bytes, it is thus possible to represent up to 8836 (94×94) characters; and, using three bytes, up to 830584 (94×94×94) characters. For the two-byte character sets, the code point of each character is normally specified in so-called kuten form (sometimes called quwei, especially when dealing with GB2312 and related standards), which specifies a zone (ku or qu), and the point (ten) or position (wei) of that character within the zone.
The escape sequences therefore do not only declare which character set is being used, but also, by knowing the properties of these character sets, know whether a 94-, 8836-, or 830584-character (or some other sized) encoding is being dealt with.
In practice, the escape sequences declaring the national character sets may be absent if context or convention dictates that a certain national character set is to be used. For example, RFC 1922, which defines ISO-2022-CN, allows ASCII SHIFT characters to be used instead of escape sequences.
ISO/IEC 2022 coding specifies a two-layer mapping between character codes and displayed characters. Escape sequences allow any of a large registry of graphic character sets to be "designated" into one of four working sets, named G0 through G3, and shorter control sequences specify the working set that is "invoked" to interpret bytes in the stream.
Character codes from the 7-bit ASCII graphic range (0x20–0x7F) are referred to as "GL" codes, being on the left side of a character code table, while codes from the "high ASCII" range (0xA0–0xFF), if available, are referred to as the "GR" codes.
By default, GL codes specify G0 characters, and GR codes specify G1 characters, but this may be modified with control codes:
Locking shift zero
|GL encodes G0 from now on|
Locking shift one
|GL encodes G1 from now on|
|ESC 0x6E (n)||LS2||Locking shift two||GL encodes G2 from now on|
|ESC 0x6F (o)||LS3||Locking shift three||GL encodes G3 from now on|
ESC 0x4E (N)
|SS2||Single shift two||GL encodes G2 for next character only|
ESC 0x4F (O)
|SS3||Single shift three||GL encodes G3 for next character only|
|ESC 0x7E (~)||LS1R||Locking shift one right||GR encodes G1 from now on|
|ESC 0x7D (})||LS2R||Locking shift two right||GR encodes G2 from now on|
|ESC 0x7C (|)||LS3R||Locking shift three right||GR encodes G3 from now on|
Each of the four working sets may be a 94-character set or a 94n-character set. Additionally, G1 through G3 may be a 96- or 96n-character set. When one of the latter is invoked in the GL region, the space and delete characters (codes 0x20 and 0x7F) are not available.
There are additional (rarely used) features for switching control character sets, but this is a single-level lookup: the 0x00–0x1F range is the C0 control character set, the 0x80–0x9F range is the C1 control character set, and there are escape sequences which switch in various alternatives. It is required that any C0 character set include the ESC character at position 0x1B, so that further changes are possible.
As seen in the SS2 and SS3 examples above, single control
characters from the C1 control character set may be invoked using
only 7 bits using the sequences
ESC 0x40 (@) through
ESC 0x5F (_). Additional control functions are
assigned in the range
ESC 0x60 (`) through
0x7E (~). While this article describes escape sequences
using the corresponding ASCII characters, they are actually defined
in terms of byte values, and the graphic assigned to that byte
value may be altered without affecting the control sequence.
Escape sequences to designate character sets take the form
ESC I [I...] F, where there
are one or more intermediate I bytes from the range
0x20–0x2F, and a final F byte from the range 0x40–0x7F.
(The range 0x30–0x3F is reserved for private-use F bytes.)
The I bytes identify the type of character set and the
working set it is to be designated to, while the F byte
identifies the character set itself.
|ESC ! F||1B 21 F||CZD||C0-designate||F selects a C0 control character set to be used.|
|ESC " F||1B 22 F||C1D||C1-designate||F selects a C1 control character set to be used.|
|ESC % F||1B 25 F||DOCS||Designate other coding system||F selects an 8-bit code; use
|ESC % / F||1B 25 2F F||DOCS||Designate other coding system||F selects an 8-bit code; there is no standard way to return.|
|ESC ( F||1B 28 F||GZD4||G0-designate 94-set||F selects a 94-character set to be used for G0.|
|ESC ) F||1B 29 F||G1D4||G1-designate 94-set||F selects a 94-character set to be used for G1.|
|ESC * F||1B 2A F||G2D4||G2-designate 94-set||F selects a 94-character set to be used for G2.|
|ESC + F||1B 2B F||G3D4||G3-designate 94-set||F selects a 94-character set to be used for G3.|
|ESC - F||1B 2D F||G1D6||G1-designate 96-set||F selects a 96-character set to be used for G1.|
|ESC . F||1B 2E F||G2D6||G2-designate 96-set||F selects a 96-character set to be used for G2.|
|ESC / F||1B 2F F||G3D6||G3-designate 96-set||F selects a 96-character set to be used for G3.|
|ESC $ ( F||1B 24 28 F||GZDM4||G0-designate multibyte 94-set||F selects a 94n-character set to be used for G0.|
|ESC $ ) F||1B 24 29 F||G1DM4||G1-designate multibyte 94-set||F selects a 94n-character set to be used for G1.|
|ESC $ * F||1B 24 2A F||G2DM4||G2-designate multibyte 94-set||F selects a 94n-character set to be used for G2.|
|ESC $ + F||1B 24 2B F||G3DM4||G3-designate multibyte 94-set||F selects a 94n-character set to be used for G3.|
|ESC $ - F||1B 24 2D F||G1DM6||G1-designate multibyte 96-set||F selects a 96n-character set to be used for G1.|
|ESC $ . F||1B 24 2E F||G2DM6||G2-designate multibyte 96-set||F selects a 96n-character set to be used for G2.|
|ESC $ / F||1B 24 2F F||G3DM6||G3-designate multibyte 96-set||F selects a 96n-character set to be used for G3.|
Note that the registry of F bytes is independent for
the different types. The 94-character graphic set designated by
ESC ( A through
ESC + A is not related in
any way to the 96-character set designated by
ESC - A
ESC / A. And neither of those are related to
the 94n-character set designated by
ESC $ (
ESC $ + A, and so on; the final bytes
must be interpreted in context. (Indeed, without any intermediate
ESC A is a way of specifying the C1 control
Also note that C0 and C1 control character sets are independent;
the C0 control character set designated by
A (which happens to be the NATS control set for newspaper
text transmission) is not the same as the C1 control character set
ESC " A (the CCITT attribute control set
Additional I bytes may be added before the F
byte to extend the F byte range. This is currently only
used with 94-character sets, where codes of the form
( ! F have been assigned. At the other
extreme, no multibyte 96-sets have been registered, so the
sequences above are strictly theoretical.
Character encodings using ISO 2022 mechanism include:
The character after the
ESC (for single-byte
character sets) or
ESC $ (for multi-byte character
sets) specifies the type of character set and working set that is
designated to. In the above examples, the character
(0x28) designates a 94-character set to the G0 character set. This
may be replaced by
(0x29–0x2B) to designate to the G1–G3 character sets.
Two of the codes above are 96-character codes, and in the above
examples, the character
- (0x2D) designates to the G1
character set. This may be replaced with
/ (0x2E or 0x2F) to designate to the G2 or G3
character sets. As mentioned earlier, a 96-character set may not be
designated to the G0 set.
There are three special cases for multi-byte codes. The code
ESC $ @,
ESC $ A, and
$ B were all registered before the ISO 2022 standard was
finalized, so must be accepted as synonyms for the sequences
ESC $ ( @ through
ESC $ ( B to designate
to the G0 character set. The latter form may also be used, and may
be adapated by changing the
( character to designate
to the G1 through G3 character sets.
The standard also defines a way to specify coding systems that
do not follow its own structure. Of particular interest, the
ESC % G designates the UTF-8 coding system, which does not reserve the
range 0x80–0xAF for control characters.