DD Statement

Enhanced Definition

A Data Definition (DD) statement in JCL (Job Control Language) is used to define and describe the characteristics of a data set or other I/O resources required by a job step or program. It specifies how a program will access input and output data, including the data set name, disposition, device type, and space allocation. A Job Control Language (JCL) statement that defines input/output files and datasets used by a job step or program. DD statements connect logical file names in programs to physical datasets or devices. A DD (Data Definition) statement in JCL (Job Control Language) is used to describe a dataset or device that a program will use during job execution. It associates a symbolic name (DDname) with the physical characteristics and location of data, allowing programs to access data without needing to know its specific storage details.

Key Characteristics

- Resource Specification: Each DD statement specifies a single data set, file, or other I/O resource (like a a printer or terminal) that a program will use.
- JCL Component: It is a fundamental statement within JCL, typically following an EXEC statement and preceding the SYSIN or SYSOUT statements.
- Keyword Parameters: Uses various keyword parameters (e.g., DSN, DISP, UNIT, VOL, SPACE, DCB) to provide detailed attributes and instructions for the data set.
- Dynamic Allocation: While often static, DD statements can also be used for dynamic allocation of resources at runtime, especially in environments like CICS or TSO/ISPF.
- Symbolic Names (DDNAMEs): Each DD statement is identified by a unique symbolic name (DDNAME) within the job step, which the application program (e.g., COBOL SELECT statement) uses to refer to the data set.
- Input/Output Flexibility: Supports various data set organizations (sequential, partitioned, VSAM) and devices (disk, tape, printer), providing a flexible interface between programs and data.

Use Cases

- Batch Processing: Defining input files (e.g., transaction data), output files (e.g., reports), and work files for COBOL, PL/I, or Assembler batch programs.
- Utility Execution: Specifying input and output data sets for z/OS utilities like IEBGENER (copying data) or IDCAMS (VSAM management).
- Program Libraries: Pointing to program libraries (STEPLIB, JOBLIB) containing the executable modules for a job step.
- Sort/Merge Operations: Allocating work files (SORTWKxx) and specifying input/output for sort utilities.
- System Output: Directing program output, system messages, and dumps to specific output classes or data sets (SYSOUT, SYSPRINT, SYSUDUMP).