Control Card
In the context of JCL and z/OS, a control card is a data record or set of records passed as input to a utility program or application program, containing parameters, commands, or instructions that direct the program's execution. It is not a JCL statement itself, but rather input *data* read by a program invoked by JCL, typically via the `SYSIN` DD statement.
Key Characteristics
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- Program-Specific Input: The format, syntax, and content of control cards are entirely defined by the specific utility program (e.g., IDCAMS, DFSORT) or application program that reads them.
SYSINAssociation: Control cards are most commonly associated with theSYSINDD statement, which defines the input source for these parameters or commands.- In-stream or Dataset: They can be provided directly within the JCL stream using
DD *orDD DATA, or referenced from an external sequential dataset or PDS/PDSE member usingDD DSN=.... - Parameterization: They enable users to specify runtime options, processing criteria, selection rules, or configuration settings to a program without requiring changes to the program's source code.
- Non-executable JCL: Unlike JCL statements which are interpreted by the Job Entry Subsystem (JES), control cards are data records that are read and processed by the application program itself.
- Utility Program Reliance: Many core IBM z/OS utility programs heavily rely on control cards for their functionality, making them indispensable for system administration and data management tasks.
Use Cases
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- Sorting and Merging Data: Providing sort keys, record formats, and output options to the DFSORT utility (e.g.,
SORT FIELDS=(1,10,CH,A)). - VSAM Dataset Management: Specifying commands for the IDCAMS utility to define, delete, alter, or list VSAM clusters, catalogs, or paths (e.g.,
DEFINE CLUSTER,DELETE). - Data Copying and Printing: Directing utilities like IEBGENER or IEBPTPCH on how to copy specific records, reformat data, or print selected portions of a dataset.
- Application Program Input: Supplying runtime parameters, configuration settings, or transaction data to custom COBOL, PL/I, or Assembler applications.
- Database Utilities: Providing instructions to DB2 or IMS utilities for tasks like reorganizing, loading, or unloading database segments or tables.
- Sorting and Merging Data: Providing sort keys, record formats, and output options to the DFSORT utility (e.g.,
Related Concepts
Control cards are fundamental for parameterizing job execution defined by JCL (Job Control Language). While JCL statements define the job steps, programs to execute, and datasets to use, control cards provide the *internal instructions* for those programs. They are typically read from an input source defined by a DD statement, often SYSIN, which links the program's input stream to the actual control card data. This mechanism allows for flexible, data-driven execution of utility programs and application programs without requiring recompilation, making them a cornerstone of z/OS batch processing.
- Externalize Complex Control Cards: For complex or frequently reused control cards, store them in separate PDS/PDSE members or sequential datasets rather than in-stream (
DD *) to improve maintainability, enable version control, and facilitate reuse across multiple jobs. - Use Clear Comments: Include comments within control cards (if supported by the utility or application) to explain their purpose, especially for intricate logic or critical parameters, enhancing readability and future maintenance.
- Validate Input Thoroughly: For custom application programs, implement robust validation routines to check the syntax and semantic correctness of control card input, preventing abends and ensuring data integrity.
- Understand Utility Syntax: Familiarize yourself with the specific syntax, keywords, and options required by the utility programs you are using, as incorrect control card syntax is a common source of job failures.
- Document Application Control Cards: For custom applications, thoroughly document the expected format, content, and purpose of all control cards to ensure proper usage by other developers, operators, and system administrators.