Catalog functions are performed in the catalog address space (CAS). Most catalog modules and control blocks are located in the catalog address space above 16MB. This reduces the required virtual storage in a user's private area needed to perform catalog functions.

During the initialization of a z/OS® system, all user catalog names identified in the master catalog, their aliases, and their associated volume serial numbers are placed in tables in CAS. The system creates the number of CAS service tasks you specify in the CAS service task lower limit parameter. A table called the CRT keeps track of these service tasks.

Changes to the master catalog are automatically reflected in the CAS tables. The information in the master catalog is normally the same as the information in CAS. For shared catalogs, the catalog address spaces on all the sharing systems are updated, maintaining data integrity for your systems.

When a user requests a catalog function, a service task is assigned for that request. This task is assigned a CAS ID, which can later be used to end or abnormally end the request if, for any reason, the request is not satisfied.

CAS also maintains a number of special tasks for its own use: the mother task, allocate task, analysis task, modify task, and the asynchronous events task. The asynchronous events task processes asynchronous events as they are signalled.

The CAS mother task keeps track of all CAS service tasks and other functions of the catalog address space. If the mother task is ended or abnormally ended, then all service tasks are ended and CAS is restarted.

The CAS allocate task performs VVDS and catalog allocation as needed. This marks a resource as being used by the task requesting it. If the allocate task allocates a catalog to CAS, the catalog remains allocated until the next system IPL, or until MODIFY CATALOG,UNALLOCATE is used on the catalog.

The CAS analysis task is dedicated to checking CAS for errors. This error checking is performed periodically.

The CAS modify task is used by the MODIFY CATALOG operator command. Only one MODIFY command can be processed at one time. If the modify task is active and another MODIFY CATALOG command is entered, the second command is rejected.

While they are processing, some of the MODIFY CATALOG options require control of specific system resources. If the resource required by the modify task is not available, the task waits a limited time for the resource. If the request is not completed in the allotted time, the CAS modify task abnormally ends with abend code A1A. A new modify task is then attached, so that the MODIFY CATALOG command is still available.

The MODIFY command can be entered at any console that can submit operator commands.