The Mainframe Computer stores thousands of files(Datasets) in different areas of Memory(Storage). Each data-set is stored at a different location. Who keeps track of, where a data-set is stored? The System Catalog.
The System Catalog is special data-set (file) that acts as the address book of the MVS Operating System. The System Catalog contains an entry for all the datasets on a Mainframe Computer. It stores the name of the dataset, followed by the actual physical location of the dataset.
The DD (Data Definition) statement is used to identify the source of input and the location of the output information.
Those datasets whose location has been recorded in the System Catalog, are called Cataloged Datasets. Datasets whose location is not tracked by the System Catalog, are called Uncataloged Datasets.
Cataloged Datasets: When a DD Statement mentions an input or output file, that has been cataloged, MVS can easily find its place of residence (address) from the System Catalog.
Uncataloged Datasets: When a DD Statement refers to and Input or Output Dataset, whose location has not been registered in the System Catalog, you must tell the location of the dataset manually.
In order to process data, the operating system is given the necessary information about the input and output through the DD statement. The DD statement must have a name, so that it can be identified by the system, and referenced as necessary.
The source of the data that will be used must be specified. If this data is stored in a file, then the name of the file must be specified. The data may be created or stored in a file only for the duration of the job. Once the job is completed, it is deleted. The fact that a data set is temporary must be specified. The fact that data is not to be deleted upon the completion of the job must also be specified.
The data being processed may be created during the job step, can already exist, can be modified during the job step or can be shared between multiple users and jobs. On successful execution of a job the data can be deleted, kept, cataloged, send to a subsequent job step or uncataloged. On unsuccessful execution of a job, the data can be deleted, kept, cataloged or uncataloged.
The various parameters of the DD statement are given below:
The syntax of the DD statement is as follows://ddname DD positional/keyword parameters comments
The DD statement must immediately follow the EXEC statement. A valid name must be assigned to each data set used. A DD statement must exist for each data set used. Positional parameters (*, DATA, DUMMY, DYNAM) must be coded before the keyword parameters (DSN, DISP, UNIT, SPACE, DCB, VOLUME).
The ddname identifies the name of the DD statement. As a job executes, the system performs device and space allocations for each ddname specified. Each ddname should be unique within the job step. If there are duplicate names, even though the system will make the allocations specified, it will direct all related messages to the first ddname.
'ddname' must be 1 to 8 alphanumeric (A-Z, 0-9) or national characters (@,# and $). The first character should be an alphabet or a national character. The ddname must begin in column 3, after the two slashes (//). At least on blank space should follow the ddname. The following ddnames are reserved for system use and should not be used to define DD statements:
The DSN Parameter
The DSN parameter is a keyword parameter on the DD statement. It can also be coded as DSNAME. It is used to specify the name of the data set to the operating system.
The syntax of this parameter is DSN=data set-name, where data set-name can be non-qualified or qualified. It is a good practice to give meaningful names to the data sets. Non-qualified names are comprised of 1 to 8 alphanumeric or national characters. The first character must be alphabetic or national. Non
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