ISO/IEC 2022 - Code Structure

Code Structure

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 or by prior agreement:

Code Abbr. Name Effect
0x0F SI
LS0
Shift In
Locking shift zero
GL encodes G0 from now on
0x0E SO
LS1
Shift Out
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
0x8E
ESC 0x4E
(N)
SS2 Single shift two GL encodes G2 for next character only
0x8F
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 ESC 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 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.

Code Hex Abbr. Name Effect
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 % @ to return to ISO/IEC 2022.
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 through ESC / A. And neither of those is related to the 94n-character set designated by ESC $ ( A through ESC $ + A, and so on; the final bytes must be interpreted in context. (Indeed, without any intermediate bytes, ESC A is a way of specifying the C1 control code 0x81.)

Also note that C0 and C1 control character sets are independent; the C0 control character set designated by ESC ! A (which happens to be the NATS control set for newspaper text transmission) is not the same as the C1 control character set designated by ESC " A (the CCITT attribute control set for Videotex).

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 ESC ( ! F have been assigned. At the other extreme, no multibyte 96-sets have been registered, so the sequences above are strictly theoretical.

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