ISO/IEC 2022 Information technology—Character code structure and extension techniques, is an ISO/IEC standard in the field of character encoding. It is equivalent to the ECMA standard ECMA-35, the ANSI standard ANSI X3.41 and the Japanese Industrial Standard JIS X 0202. Originating in 1971, it was most recently revised in 1994.
ISO 2022 specifies a general structure which character encodings can conform to, dedicating particular ranges of bytes (0x00–1F and 0x7F–9F) to be used for non-printing control codes for formatting and in-band instructions (such as line breaks or formatting instructions for text terminals), rather than graphical characters. It also specifies a syntax for escape sequences, multiple-byte sequences beginning with the control code, which can likewise be used for in-band instructions. Specific sets of control codes and escape sequences designed to be used with ISO 2022 include ISO/IEC 6429, portions of which are implemented by ANSI.SYS and terminal emulators.
ISO 2022 itself also defines particular control codes and escape sequences which can be used for switching between different coded character sets (for example, between ASCII and the Japanese JIS X 0208) so as to use multiple in a single document, effectively combining them into a single stateful encoding (a feature less important since the advent of Unicode). It is designed to be usable in both 8-bit environments and 7-bit environments (those where only seven bits are usable in a byte, such as e-mail without 8BITMIME).
Encodings and conformance
Writing systems with relatively few characters, such as Greek, Cyrillic, Arabic, or Hebrew, as well as forms of the Latin alphabet using diacritics or letters absent in the ISO Basic Latin alphabet, have historically been represented on computers with different 8-bit, single byte, extended ASCII encodings. Some of these, such as the ISO 8859 series, conform to ISO 2022, while others such as DOS code page 437 do not, usually due to not reserving the bytes 0x80–9F for control codes.
Certain East Asian languages, specifically Chinese, Japanese, and Korean (collectively "CJK"), are written using far more characters than the maximum of 256 which can be represented in a single byte, and were first represented on computers with language-specific double-byte encodings or variable-width encodings; some of these (such as the Simplified Chinese encoding ) conform to , while others (such as the Traditional Chinese encoding Big5) do not. Control codes in ISO 2022 are always represented with a single byte, regardless of the number of bytes used for graphical characters. CJK encodings used in 7-bit environments which use mechanisms to switch between character sets are often given names starting with "ISO-2022-", most notably ISO-2022-JP, although some other CJK encodings such as EUC-JP also make use of ISO 2022 mechanisms.
Since the first 256 code points of Unicode were taken from ISO 8859-1, Unicode inherits the concept of C0 and C1 control codes from ISO 2022, although it adds other non-printing characters besides the ISO 2022 control codes. However, Unicode transformation formats such as UTF-8 generally deviate from the ISO 2022 structure in various ways, including: Using 8-bit bytes, but not representing the C1 codes in their single-byte forms specified in ISO 2022 (most UTFs, one exception being the obsolete UTF-1) Representing all characters, including control codes, with multiple bytes (e.g. UTF-16, UTF-32) Mixing bytes with the most significant bit set and unset within the coded representation for a single code point (e.g. UTF-1, )
ISO 2022 escape sequences do, however, exist for switching to and from UTF-8 as a "coding system different from that of ISO 2022", which are supported by certain terminal emulators such as xterm.
Overview
Elements
ISO/IEC 2022 specifies the following:
An infrastructure of multiple character sets with particular structures which may be included in a single character encoding system, including multiple graphical character sets and multiple sets of both primary (C0) and secondary (C1) control codes, A format for encoding these sets, assuming that 8 bits are available per byte, A format for encoding these sets in the same encoding system when only 7 bits are available per byte, and a method for transforming any conformant character data to pass through such a 7-bit environment, The general structure of ANSI escape codes, and Specific escape code formats for identifying individual character sets, for announcing the use of particular encoding features or subsets, and for interacting with or switching to other encoding systems.