JCL statements: What does the DD statement do?

Data definition (DD) statements define the data sets that a program or procedure uses when it runs. You must code one DD statement for each data set that is used or created within a job step.

The order of DD statements within a job step is not usually significant.

This JCL example illustrates the format of a DD statement:

//PAY  DD  DSN=HLQ.PAYDS,DISP=NEW  VENDOR PAYROLL

The name field contains a one- through eight-character name, known as a ddname, that identifies the DD statement so that other JCL statements, programs, procedures, or the operating system can refer to it. The ddname of this DD statement is PAY.
The parameter field contains only two keyword parameters:
- DSN, which is an accepted abbreviation for the parameter DSNAME, which identifies the real name of a data set.
- DISP, which identifies the data set HLQ.PAYDS as a new data set; that is, one the system is to create when this job is submitted for processing.
The comment field contains the phrase VENDOR PAYROLL.

A DD statement describes a data set extensively, and can include the following information:

The name that the program uses to refer to the data set, known as the ddname
The actual name of the data set and its location
Physical characteristics of the data set, such as record format
The initial and final status of the data set, known as its disposition

You also can use DD statements to request I/O devices or specify storage allocation for new data sets.