The smallest unit of computer data is a bit, or binary digit. A bit has only two possible values, 0 or 1. To represent character data, bits must be grouped. The most common grouping is the 8-bit byte, providing up to 256 different characters.
A named character set is a particular set of characters--"Latin-1", for example, is a set of Western uppercase and lowercase letters, numbers, and a selection of symbols.
A single-byte character set (SBCS) allows the representation of up to 256 characters, each character being represented by a single byte.
A double-byte character set (DBCS) allows the representation of more than 256 characters, each character being represented by a pair of bytes. Some languages require characters to be represented by multiple bytes in what is called a multi-byte character set (MBCS); here, each character is represented by up to 4 bytes.
The different languages in some countries (such as Japan) may use both SBCS and DBCS.
A computer program or operating system must assign a byte, 2-byte, or multi-byte value to each character that it wants to represent. Several conventions exist for character representation. In this chapter, these conventions are called interchange codes. Two different interchange codes are used in the hardware platforms on which CICS® products run:
A code page defines the code points for the characters in a particular character set. It consists of a list of byte values or 2-byte values and the characters they represent. The EBCDIC and ASCII interchange codes include more than one code page, so data conversion can be necessary even between two systems that use the same interchange code.
A Coded Character Set Identifier (CCSID) is a combination of a character set and its associated code page. A CCSID may be composite; that is, it may contain multiple character sets and code pages--for example, Katakana (1-byte) and Kanji (2-byte).
Most CICS products use in-built conversion tables to handle conversion between common code pages. Some products allow you to define your own conversion tables. For nonstandard conversion, you can supply a user-written conversion program.
For detailed information about data conversion, see the CICS Intercommunication manuals for your CICS products.