The MVS Operating System keeps a diary/register, which contains name of each file, and its location in the computer memory. This diary/register is like your map around the Mainframe’s memory land. You want to find a file, just see the MVS operating system’s diary, and it will tell you the location of your file.
This diary or map, which you consult to find a file on a Mainframe computer, is called Catalog. The Catalog contains entries for each file, a file-name and a pointer to the physical memory location, where the file is present.
Not all the files present on a Mainframe may have an entry in the Catalog. If a given file’s entry is not present in the Catalog, you will have to find the file manually, by explicitly specifying the UNIT and VOLUME parameters on the DD Statements. Adding an entry for a file i.e. File name and its location to Catalog, is called cataloging a file. Deleting the entry of a file, from the Catalog is called uncataloging the file.
Actually, the Catalog itself is a VSAM File. So, you can use the ready-made IBM Supplied program IDCAMS to read the contents of the Catalog.
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