DD Statement and its Parameters


For Input stream datasets SYSIN is traditionally used as ddname. Use SYSOUT parameter to indicate the Output stream dataset.

ddname - specifies the name of the DD statement. DSN= is used to specify the dataset name to OS, &&name is used to specify a temporary dataset. DISP=(status, norm, abnorm) is used to specify the Disposition of the DS, with the status at beginning and norm, abend after execution, valid options are new, old, shr, mod for status, keep, catalog, uncatalog, delete for normal & abnormal run's, In addition Pass is also used for normal run. UNIT specifies the I/O devices. VOL, SPACE and DCB are others.

The Dataset is Fixed Blocked, 1st column in the dataset is used for printer control, i.e to print the line in the next page, next line or 2 lines after.

UNIT parameter is used along with a DD statemnt. It conveys information input or output devices that will be used by the job.

The volume parameter is used to identify the volume serial number of the device on which a given dataset is to be written or on which a dataset resides. The Vol parameter can be used both for disk and tape datasets.

RECFM=U subparameter results in the operating system allowing records of undefined length to be created for the dataset specified. Eg: Load module dataset.

The BUFNO parameter is used to specify the number of buffers to be allocated to virtual storage for the dataset. A large number buffers can improve I/O performance.

The dummy parameter is coded on DD statement to test the execution of a program without actually reading or writing to a dataset.

The minimum parameters needed are DSN and DISP.

DCB info comes from :-
1) Program - FD: BLOCK CONTAINS 3 RECORDS RECORD CONTAINS 100                                                  CHARACTERS
2) The label - like a tape
3) From the VTOC - for Dasd
4) From the JCL - DCB=BLKSIZE=nnn. 
 
LABEL is a keyword parameter which can be specified on DD statement and consists of the following subparameters:
LABEL=(Relative File #, Type of Label Processing)
The LABEL parameter tells the type of label, the relative file number, and whether the data set is to be protected for input or output.  

NL should be specified when a program needs to process unlabeled tapes NL can also be specified when the pgm wants to create unlabeled tape because the system's default action, in cases when parameter is not specified, will create IBM standard label. Nonlabeled tapes are often used for sending tapes to another installation. That way you don't have to worry about the tape label corresponding to the standards at the other installation or about accidentally matching the volume serial number of an existing tape at the installation.
  
You should use either DD * or DD Data. 
 
DD * and DD DATA describe the input data which follows the above mention cards. If the input data contains record switch // in col 1 and 2 then DD Data should be used. 

It is a keyword parameter which should allocated on the DD statement for the output data sets stored on the disk. It consists of the following sub-parameters:
SPACE=(BLKS/CYL/TRK,(primary, secondary, index),RLSE,CONTIG)   

RLSE releases all unused space when the data set is closed. It permits you to allocate more space than perhaps, it needed without wasting space. Space is released only if the data set is not empty and if the data set is closed after being opened.  
 
CONTIG requests the primary space be allocated only on contiguous tracks and cylinders, that is all tracks on a cylinder are contiguous, and if more than one cylinder is needed, the cylinders are also contiguous. Always code CONTIG if track overflow is used.  

The OUTLIM parameter is coded on the DD statement and is used to limit the number of lines that are output to the destination specified in the sysout.

It is a keyword parameter which specifies a job initiation priority within its job class. When the job is initiated, the system will convert the job's priority into a dispatching priority so that job's task can complete with other tasks for use of main storage and CPU resources.

Using ACCEPT Statement

SYSOUT

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