In this example, a GENCB macro is used to identify a data set to open and to specify the types of processing to perform. This example specifies that the space for the control block be obtained above 16 megabytes. The access method control block generated by this example is built when the program is executed.

GENCB  GENCB   BLK=ACB,AM=VSAM,     One copy generated; VSAM gets the   x
               BUFND=4,BUFNI=3,     storage for it, because the         x
               BUFSP=19456,         WAREA LENGTH parameters have        x
               DDNAME=DATASETS,     been omitted.                       x
               EXLST=EXITS,                                             x
               LOC=ANY,                                                 x
               MACRF=(KEY,DIR,                                          x
               SEQ,OUT),                                                x
               RMODE31=ALL,                                             x
               STRNO=2
 
         ST    1,ACBADDR            Save the address of the access      x
                                    method control block.
ACBADDR  DS    A                    The address of the access method    x
                                    control block is saved in ACBADDR.

The GENCB macro’s parameters are:

• BUFND specifies four I/O buffers for data. BUFNI specifies three I/O buffers for index entries. BUFSP specifies 19456 bytes of buffer space, enough space to accommodate control intervals of data that are 4096 bytes and of index entries that are 1024 bytes.
• DDNAME specifies that this access method control block is associated with a DD statement named DATASETS.
• EXLST specifies that the exit list associated with this access method control block is named EXITS.
• LOC specifies that VSAM obtain virtual storage for the ACB from an area that may be above 16 megabytes.
• MACRF specifies keyed direct and keyed sequential processing for both insertion and update.
• RMODE31 specifies that VSAM obtain storage for the VSAM control blocks and I/O buffers in an area above 16 megabytes when the ACB is opened.
• STRNO specifies that two requests will require concurrent positioning.