During a system initialization, the master catalog is read so that system data sets and catalogs can be located. Their catalog entries are placed in the cache that is selected by the user for the master catalog as they are read.

Catalog aliases are also read during system initialization, but they are placed in an alias table separate from the cache that is used for the master catalog.

Thus, if the master catalog only contains entries for system data sets, catalogs, and catalog aliases, the entire master catalog is in main storage by the completion of the system initialization.