// [ddname             ] DD [positional-parameter][,keyword-parameter]...[comments]
   [procstepname.ddname]

// [ddname             ] DD
   [procstepname.ddname]
 

• The DD statement consists of the characters // in columns 1 and 2 and four fields: name, operation (DD), parameter, and comments. Do not code comments if the parameter field is blank.
• A DD statement is required for each data set.
• The maximum number of DD statements per job step is 3273, based on the number of single DD statements allowed for a TIOT (task input output table) control block size of 64K. This limit can be different depending on the installation-defined TIOT size. The IBM-supplied default TIOT size is 32K. For information about changing the size of the TIOT, see the ALLOCxx parmlib member information in z/OS MVS Initialization and Tuning Reference. For information about how dynamic allocation might cause changes to the task input/output table (TIOT), see z/OS MVS Programming: Authorized Assembler Services Guide.

  In a JES3 system, the installation might further reduce the maximum number of DD statements per job.