Master file directory
The master file directory is the top directory on a volume. It is displayed as  in file specifications. It can also be omitted when you refer to a subdirectory specification: both DKA0:[000000.USERS] and DKA0:[USERS] are valid specifications and point to the same directory. The directory file for the master file directory is stored in the master file directory itself.
Usually, the master file directory is used to list the reserved files and users' file directories; users seldom enter files into the MFD, even on private volumes. In fact, on a private volume, it is most convenient for users to create a directory that has the same name as their default directory on a system disk.
When the Backup utility (BACKUP) creates sequential disk save sets, it stores the save-set file in the MFD.
A master file directory of a non-system disk stores the following system files:
- 000000.DIR is the master file directory directory file
- BACKUP.SYS is a backup log file reserved for future use
- BADBLK.SYS contains all the bad blocks on the volume
- BADLOG.SYS contains a list of suspected bad blocks on the volume that are not listed in the bad block file
- BITMAP.SYS stores the volume bitmap
- CONTIN.SYS is used as the extension file identifier in a volume set
- CORIMG.SYS is the core image file not used by the OS
- INDEXF.SYS contains the file headers for all files on the volume
- VOLSET.SYS contains a list of the labels of all volumes in the set and the name of the volume set
- QUOTA.SYS is used by the file system to keep track of the disk usage of each UIC on a volume
In addition to the files listed above, the master file directory of a system disk stores the following directories: