ABCs of z/OS System Programming Volume 3 - IBM Redbooks

6.21 Catalog address space (CAS)
User
Address
Space
Catalog request
SVC 26
Return data
CATALOG
Address
Space
(CAS)
System 1 System 2
Figure 6-35 Catalog address space (CAS)
User catalog
CATALOG
Address
Space
(CAS)
Catalog request
SVC 26
Return data
User
Address
Space
The catalog address space
Catalog functions are performed in the catalog address space (CAS). The jobname of the
catalog address space is CATALOG.
As soon as a user requests a catalog function (for example, to locate or define a data set), the
CAS gets control to handle the request. When it has finished, it returns the requested data to
the user. A catalog task which handles a single user request is called a service task. To each
user request a service task is assigned. The minimum number of available service tasks is
specified in the SYSCATxx member of SYS1.NUCLEUS (or the LOADxx member of
SYS1.PARMLIB). A table called the CRT keeps track of these service tasks.
The CAS contains all information necessary to handle a catalog request, like control block
information about all open catalogs, alias tables, and buffered BCS records.
During the initialization of an MVS system, all user catalog names identified in the master
catalog, their aliases, and their associated volume serial numbers are placed in tables in
CAS.
You can use the MODIFY CATALOG operator command to work with the catalog address space.
See also 6.22, “Working with the catalog address space” on page 371.
Since z/OS 1.8, the maximum number of parallel catalog requests is 999, as defined in the
SYSCAT parmlib member. Previously it was 180.
Chapter 6. Catalogs 369