The in-storage catalog cache resides in main storage within the catalog address space. It is the default catalog cache. Each user catalog cached in ISC is given a fixed amount of space for cached records. When a user catalog uses its allotted space in the ISC, the least recently used record is removed from the ISC to make room for the new entry.

Catalogs that are not frequently updated or shared with other systems use the ISC most effectively. The performance of the ISC is affected if the catalog is shared with another system. See The Effect of Sharing Catalogs on Cache Usage for more information.

Master catalogs, unlike user catalogs, are not limited to a set amount of storage. All eligible records in the master catalog are cached in the ISC as they are read. Thus, you should keep the number of entries in the master catalog to a minimum, so that the ISC for the master does not use an excessive amount of main storage.

Since ISC is the default catalog cache, catalogs are cached in the ISC unless you specify that the catalog is to use CDSC, or unless you use the MODIFY CATALOG operator command to remove the catalog from the ISC.