Extended Unix Code

Extended Unix Code (EUC) is a multibyte character encoding system used primarily for Japanese, Korean, and simplified Chinese.

The structure of EUC is based on the ISO-2022 standard, which specifies a way to represent character sets containing a maximum of 94 characters, or 8836 (94²) characters, or 830584 (94³) characters, as sequences of 7-bit codes. Only ISO-2022 compliant character sets can have EUC forms. Up to four coded character sets (referred to as G0, G1, G2, and G3 or as code sets 0, 1, 2, and 3) can be represented with the EUC scheme. G0 is almost always an ISO-646 compliant coded character set (e.g. US-ASCII/KS X 1003/ISO 646:KR in EUC-KR and US-ASCII/the lower half of JIS X 0201 in EUC-JP) that is invoked on GL (i.e. with the most significant bit cleared).

To get the EUC form of an ISO-2022 character, the most significant bit of each 7-bit byte of the original ISO 2022 codes is set (by adding 128 to each of these original 7-bit codes); this allows software to easily distinguish whether a particular byte in a character string belongs to the ISO-646 code or the ISO-2022 (EUC) code.