You must supply a DCB for each data set or device to open. The OPEN macro instruction finishes initializing one or more DCBs so that their associated data sets can be processed. Issue OPEN for all DCBs used by your channel programs. (A dummy data set cannot be opened for EXCP.) Some of the procedures performed by OPEN are:

• Checking data set access authorization
• Constructing the data extent block (DEB)
• Completing the fields in the DCB and DCBE
• Verifying or creating standard labels
• Positioning tape
• Loading your appendage routines.
• Capturing the UCB if the DCBE option "LOC" was set or defaulted to "BELOW", that is LOC=BELOW or not coded, or the "NON_VSAM_XTIOT" option of the DEVSUPxx member of PARMLIB was set or defaulted to "NO", that is NON_VSAM_XTIOT=NO or not coded. In other words, OPEN does not capture the UCB if LOC and NON_VSAM_XTIOT were specified as follows: LOC=ANY and NON_VSAM_XTIOT=YES.