Initializing a Data Control Block (OPEN)

The OPEN macro instruction initializes one or more data control blocks so that their associated data sets can be processed. Issue OPEN for all data control blocks used by the direct access program. Some of the procedures performed when OPEN is executed include the following actions:

Constructing a data extent block
Transferring information from DD statements and data set labels to the DCB
Verifying or creating standard labels
Loading programmer-written appendage routines.

The two parameters of the OPEN macro instruction are the address or addresses of the data control blocks to be initialized and the method of I/O processing of the data set. The processing method can be specified as INPUT, OUTPUT, UPDAT, or EXTEND; however, if nothing is specified, INPUT is assumed. The parameters and different forms of the OPEN macro instruction are described in z/OS DFSMS Macro Instructions for Data Sets.