DCB - Data Control Block

Key Characteristics

• • An in-memory data structure residing within the program's address space, allocated when a data set is opened and freed upon closure.
  • Contains critical data set attributes like DSORG (Data Set Organization), RECFM (Record Format), LRECL (Logical Record Length), and BLKSIZE (Block Size).
  • Populated from various sources, including JCL DD statement parameters, data set labels (DSCB on volume), and program-specified values (e.g., DCB macro in Assembler, OPEN statement in COBOL).
  • Serves as the primary interface between an application program and the z/OS access methods (e.g., QSAM, BSAM, VSAM) for performing I/O.
  • Can be dynamically modified by a program before the OPEN call to override or supplement JCL-specified attributes, providing flexibility.
  • Essential for both input and output operations, ensuring data is read or written correctly according to its defined structure and preventing data integrity issues.

Use Cases

• • Sequential File Processing: A COBOL program reading or writing a sequential data set (PS or PDS member) will use a DCB to define the record format, length, and block size for efficient I/O operations.
  • VSAM Access: When accessing a VSAM KSDS, ESDS, or RRDS, the DCB (or its VSAM-specific equivalents, ACB/RPL) holds the necessary control information for the VSAM access method services to locate and manage records.
  • Dynamic Allocation: Programs that dynamically allocate data sets (e.g., using SVC 99 or DYNALLOC services) construct a DCB or equivalent control blocks to specify the data set attributes at runtime without requiring JCL.
  • Utility Programs: System utilities like IEBGENER or DFSORT implicitly or explicitly use DCBs to manage the input and output data sets they process, ensuring correct data handling and transformation.

Related Concepts