When you are using improved control interval processing, the CBIC option lets you have multiple address spaces that address the same data and use the same control block structure. The VSAM control blocks associated with a VSAM data set are placed into the common service area (CSA). The control block structure and VSAM I/O operations are essentially the same whether the CBIC option is invoked, except for the location of the control block structure. The user-related control blocks are generated in the protect key (0 - 7). The system-related control blocks are generated in protect key 0. The VSAM control block structure generated when the CBIC option is invoked retains normal interfaces to the address space that opened the VSAM data set (for example, the DEB is chained to the address space's TCB).

The CBIC option is invoked when a VSAM data set is opened. To invoke the CBIC option, you set the CBIC flag (located at offset X'33' (ACBINFL2) in the ACB, bit 2 (ACBCBIC)) to one. When your program opens the ACB with the CBIC option set, your program must be in supervisor state with a protect key from 0 to 7. Otherwise, VSAM will not open the data set.