The format-1 DSCB address provided in the OIEXDSCB field points to the start of the DS1FMTID field for the DSCB in the common work and control block area. Format-2 and -3 DSCBs might be associated with the format-1 DSCB. For BDAM and concatenated partitioned data sets, other format-1 through -3 DSCBs might be associated with the DCB being opened.

A format-1 DSCB may have several fields set uniquely if the data set is a large format data set. The fields affected are DS1FLAG1 and DS1LSTAR. For specific values in these fields that are unique to large format data sets, see z/OS DFSMSdfp Advanced Services. For a large format data set, the DCBE might also have BLOCKTOKENSIZE=LARGE specified to indicate that the program conforms to interfaces that are required for some uses of large format data sets. These interfaces, which support increased track numbers, are required for EXCP, or BSAM with NOTE or POINT, or if the IGDSMSxx member of SYS1.PARMLIB specifies BLOCKTOKENSIZE(REQUIRE). For a mapping of the DSCB, see z/OS DFSMSdfp Advanced Services.

If the VTOC is being opened, OIEXDSCB points to a format-4 DSCB. You can determine if the DSCB is a format-4 by testing the DS1FMTID field for a value of X'F4', or by testing the data in the JFCBDSNM field for 44 bytes of X'04'. For IFG0EX0B to receive control when processing a VSAM data space, you must provide a DCB, not an access method control block (ACB).