Extended UNIX code (EUC) encoding scheme
The EUC encoding scheme defines a set of encoding rules that can support one to four character sets. The encoding rules are based on the ISO2022 definition for the encoding of 7-bit and 8-bit data. The EUC encoding scheme uses control characters to identify some of the character sets. The following table shows the basic structure of all EUC encoding.
| EUC | Character Encoding |
|---|---|
| CS0 | 0xxxxxxx |
| CS1 | 1xxxxxxx
1xxxxxxx 1xxxxxxxx 1xxxxxxx 1xxxxxxxx 1xxxxxxx ... |
| CS2 | 10001110 1xxxxxxx
10001110 1xxxxxxx 1xxxxxxxx 10001110 1xxxxxxx 1xxxxxxxx 1xxxxxxxx ... |
| CS3 | 10001111 1xxxxxxx
10001111 1xxxxxxx 1xxxxxxxx 10001111 1xxxxxxx 1xxxxxxxx 1xxxxxxxx ... |
The term EUC denotes these general encoding rules. A code set based on EUC conforms to the EUC encoding rules but also identifies the specific character sets associated with the specific instances. For example, IBM-eucJP for Japanese refers to the encoding of the Japanese Industrial Standard characters according to the EUC encoding rules.
The first set (CS0) always contains an ISO646 character set. All of the other sets must have the most significant bit (MSB) set to 1 and can use any number of bytes to encode the characters. In addition, all characters within a set must have the following:
- Same number of bytes to encode all characters
- Same column display width (number of columns on a fixed-width terminal)
All characters in the third set (CS2) are always preceded with the control character SS2 (single-shift 2, 0x8e). Code sets that conform to EUC do not use the SS2 control character other than to identify the third set.
All characters in the fourth set (CS3) are always preceded with the control character SS3 (single-shift 3, 0x8f). Code sets that conform to EUC do not use the SS3 control character other than to identify the fourth set.