Editing Job Control Language (section)

===Data access (DD statement)===
The <code>DD</code> statement is used to reference data. This statement links a program's internal description of a dataset to the data on external devices: disks, tapes, cards, printers, etc.  The DD may provide information such as a device type (e.g. '181','2400-5','TAPE'), a [[Volume (computing)|volume serial]] number for tapes or disks, and the description of the data file, called the <code>DCB</code> subparameter after the [[Data Control Block]] (DCB)  in the program used to identify the file.

Information describing the file can come from three sources: The DD card information, the dataset label information for an existing file stored on tape or disk, and the DCB macro coded in the program.  When the file is opened this data is merged, with the DD information taking precedence over the label information, and the DCB information taking precedence over both.  The updated description is then written back to the dataset label. This can lead to unintended consequences if incorrect DCB information is provided.<ref>{{cite book|last1=IBM Corporation|title=OS/VS MVS Data Management Services Guide|date=August 1978|url=http://www.prycroft6.com.au/misc/download/GC26-3875-0_MVS_DataMgmtSrvcsGde_Aug78OCR.pdf|access-date=Oct 17, 2014|ref=DataMgmt}}</ref>

Because of the parameters listed above and specific information for various access methods and devices the DD statement is the most complex JCL statement. In one IBM reference manual description of the DD statement occupies over 130 pages—more than twice as much as the JOB and EXEC statements combined.<ref>{{cite book |last1=IBM Corporation |title=IBM System/360 Operating System: Job Control Language Reference |date=June 1971 |url=http://www.bitsavers.org/pdf/ibm/360/os/R20.1_Mar71/GC28-6704-1_OS_JCL_Reference_Rel_20.1_Jun71.pdf |access-date=June 25, 2019}}</ref>

The DD statement allows inline data to be injected into the job stream.  This is useful for providing control information to utilities such as IDCAMS, SORT, etc. as well as providing input data to programs.