IMS- ACB(Access control block)

IMS™ builds the ACB with the ACBGEN utility by merging information from the PSB and DBD. For execution in a batch environment, IMS can build ACBs either dynamically (PARM=DLI), or it can prebuild them using the ACB Maintenance utility (PARM=DBB).


ACBs must be prebuilt for use by online application programs. ACBs cannot be prebuilt for GSAM DBDs. However, ACBs can be prebuilt for PSBs that reference GSAM databases.

The ACB Maintenance utility,  gets the PSB and DBD information it needs from IMS.PSBLIB and IMS.DBDLIB.

You can have the utility prebuild ACBs for all PSBs in IMS.PSBLIB, for a specific PSB, or for all PSBs that reference a particular DBD.

Pre-built ACB Lib

Prebuilt ACBs are kept in the IMS.ACBLIB library. (IMS.ACBLIB is not used if ACBs are not prebuilt.) When ACBs are prebuilt and an application program is scheduled, the application program’s ACB is read from IMS.ACBLIB directly into storage.

This means that less time is required to schedule an application program. In addition, less storage is used if prebuilt ACBs are used. Another advantage of using the ACB Maintenance utility is the initial error checking it performs.

It checks for errors in the names used in the PSB and the DBDs associated with the PSB and, if erroneous cross-references are found, prints appropriate error messages.

IMS.ACBLIB has to be used exclusively. Because of this, the ACB Maintenance utility can only be executed using an IMS.ACBLIB that is not currently allocated to an active IMS system. Also, because IMS.ACBLIB is modified, it cannot be used for any other purpose during execution of the ACB Maintenance utility.

You can change ACBs or add ACBs in an “inactive” copy of ACBLIB and then make the changed or new members available to an active IMS online system by using the online change function. “Using the Online Change Function” in Modifying Databases describes how you effectively change ACBLIB for an online system.


Author: Srini

Experienced software developer. Skills in Development, Coding, Testing and Debugging. Good Data analytic skills (Data Warehousing and BI). Also skills in Mainframe.