ISO 2022

ISO/IEC 2022 Information technology—Character code structure and extension techniques, is an ISO standard (equivalent to the ECMA standard ECMA-35) specifying

Many of the character sets included as ISO/IEC 2022 encodings are 'double byte' encodings where two bytes correspond to a single character. This makes ISO-2022 a variable width encoding. But a specific implementation does not have to implement all of the standard; the conformance level and the supported character sets are defined by the implementation.

Many languages or language families not based on the Latin alphabet such as Greek, Cyrillic, Arabic, or Hebrew have historically been represented on computers with different 8-bit extended ASCII encodings. 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/IEC 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.

A second requirement of ISO-2022 was that it should be compatible with 7-bit communication channels. So even though ISO-2022 is an 8-bit character set any 8-bit sequence can be reencoded to use only 7-bits without loss and normally only a small increase in size.

To represent multiple character sets, the ISO/IEC 2022 character encodings include escape sequences which indicate the character set for characters which follow. The escape sequences are registered with ISO and follow the patterns defined within the standard. These character encodings require data to be processed sequentially in a forward direction since the correct interpretation of the data depends on previously encountered escape sequences. Note, however, that other standards such as ISO-2022-JP may impose extra conditions such as the current character set is reset to US-ASCII before the end of a line.

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