Creating an Access Method Control Block

Before opening a data set for processing, you must create an access method control block (ACB) that:

Identifies the data set to be opened
Specifies the type of processing
Specifies the basic options
Indicates if a user exit routine is to be used while the data set is being processed

Include the following information in your ACB for OPEN to prepare the kind of processing your program requires:

The address of an exit list for your exit routines. Use the EXLST macro to construct the list.
If you are processing concurrent requests, the number of requests (STRNO) defined for processing the data set. For more information about concurrent requests see Making Concurrent Requests.
The size of the I/O buffer virtual storage space and/or the number of I/O buffers that you are supplying for VSAM to process data and index records.
The password required for the type of processing desired. Passwords are not supported for system-managed data sets. You must have RACF authorization for the type of operation to be performed.
The processing options that you plan to use:
- Keyed, addressed, or control interval, or a combination
- Sequential, direct, or skip sequential access, or a combination
- Retrieval, storage, or update (including deletion), or a combination
- Shared or nonshared resources.
The address and length of an area for error messages from VSAM.
If using RLS, see Using VSAM Record-Level Sharing.

You can use the ACB macro to build an access method control block when the program is assembled, or the GENCB macro to build a control block when the program is run. See Manipulating the Contents of Control Blocks for information about the advantages and disadvantages of using GENCB.