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DSN= & DISP= narrative

This describes how DD statements can be used to allocate existing data sets.

In the case of existing data sets the DSN= operand is mandatory so that the correct data set can be identified. As will be seen on subsequent pages the use of DSN= is optional.

Files within z/OS UNIX System Services can also be allocated but this is done via the PATH= operand, not DSN=.

DD Statement - existing data set narrative

DSN=

This identifies the specific data set, not to be confused with UNIX files, to be made available to the program via the filename (ddname) coded after the // on the statement. On System Managed Storage (SMS) volumes, all data sets must be catalogued. The system uses the data set name as an index to the catalogue to locate the unit type and volume holding the data set. Data set names are restricted to 44 characters in size. The ddname at the beginning of the statement is restricted to 8 characters, and must represent a file name embedded within the program.

DISP=

The DISP= operand is also mandatory for an existing data set, as if omitted the default is DISP=(NEW,DELETE). In the example illustrated on the previous page the value SHR is complimented by a default of KEEP, so it is not necessary to code DISP=(SHR,KEEP).

The DISP= operand indicates the current status of the data set, with SHR meaning the data set exists, and can be used concurrently by other jobs/users. (Using SHR when the program is to write to the data set could compromise the integrity of the data.)