Coded Character Set

Enhanced Definition

A Coded Character Set (CCS) is a defined collection of characters (letters, numbers, symbols, control characters) where each character is assigned a unique numerical code point. In the mainframe and z/OS context, it primarily refers to the standard by which text data is represented and processed, ensuring consistent interpretation across different systems and applications. A Coded Character Set (CCS) is a defined mapping between a set of characters (letters, numbers, symbols, control characters) and unique numerical values, known as code points. In the mainframe and z/OS context, the most prevalent CCS is EBCDIC (Extended Binary Coded Decimal Interchange Code), though ASCII is also frequently encountered for interoperability. Its primary purpose is to enable consistent storage, processing, and display of textual data across systems and applications.

Key Characteristics

- Character-to-Code Mapping: It provides a one-to-one mapping between abstract characters and their corresponding numeric code points.
- EBCDIC Dominance: IBM mainframes historically and natively use Extended Binary Coded Decimal Interchange Code (EBCDIC) as their primary coded character set, distinct from ASCII used on most distributed systems.
- CCSID (Coded Character Set Identifier): Each specific implementation of a coded character set (often combined with an encoding scheme) is identified by a unique CCSID, such as 0037 for EBCDIC US English or 1208 for UTF-8.
- National Language Support: Different CCSIDs exist to support various national languages, special characters, and regional conventions (e.g., 0277 for EBCDIC Denmark/Norway).
- Data Integrity: Proper identification and handling of CCSIDs are crucial for maintaining data integrity, especially when exchanging data between systems with different native character sets.
- Encoding Scheme: While a CCS defines the characters and their code points, an encoding scheme specifies how these code points are represented as a sequence of bytes for storage and transmission.

Use Cases

- Data Storage: Defining the character set for text data stored in z/OS datasets (sequential, VSAM), DB2 tables, or IMS databases.
- Application Processing: COBOL, PL/I, and C programs use coded character sets to interpret and manipulate string data, ensuring correct display and processing of text.
- Data Exchange: Facilitating the transfer of text data between z/OS and distributed systems (e.g., via FTP, MQ, or database replication) by specifying source and target CCSIDs for conversion.
- Terminal Emulation: Ensuring that characters entered and displayed on 3270 terminals are correctly rendered according to the terminal's configured character set.
- Internationalization: Developing applications that support multiple languages by using appropriate CCSIDs and z/OS Unicode Services for character set conversions.

Related Concepts

A Coded Character Set is fundamental to National Language Support (NLS) on z/OS, enabling applications to handle diverse languages. It is closely tied to Code Pages, which are specific mappings of a coded character set to a sequence of bytes, identified by a CCSID. When data moves between systems or applications using different CCSIDs (e.g., EBCDIC on z/OS and ASCII or UTF-8 on a Linux server), data conversion services (like those provided by z/OS Unicode Services or database features) are required to translate the character representations correctly, preventing data corruption or misinterpretation.

Best Practices:

Explicitly Define CCSIDs: Always specify the CCSID for text data in file definitions, database schemas (e.g., CCSID EBCDIC in DB2 CREATE TABLE), and application configurations to avoid ambiguity.
Standardize Where Possible: Within a single application or system, strive to use a consistent CCSID to minimize the need for character set conversions, which can be resource-intensive.
Leverage z/OS Unicode Services: For modern applications and interoperability with distributed systems, utilize z/OS Unicode Services (ICONV, CUNUNI) for robust and reliable character set conversions, especially to and from UTF-8.
Test Conversions Thoroughly: When data is exchanged between different CCSIDs, rigorously test all conversion paths with a wide range of characters, including special characters and national language characters, to ensure data integrity.
Consider Unicode for New Development: For new applications or significant modernizations, consider storing and processing text data in Unicode (e.g., UTF-8 or UTF-16) on z/OS to simplify internationalization and interoperability.