Editing Files-11 (section)

== The Master File Directory ==
At the top level of an ODS file system is the ''master file directory'' (MFD), which contains all top-level directory files (including itself), and several system files used to store file system information. On ODS-1 volumes, a two-level directory structure is used: each ''[[user identification code]]'' (UIC) has an associated ''user file directory'' (UFD), of the form <kbd>[GROUP.USER]</kbd>. On ODS-2 and later volumes, the layout of directories under the MFD is free-form, subject to a limit on the nesting of directories (8 levels on ODS-2 and unlimited on ODS-5). On multi-volume sets, the MFD is always stored on the first volume, and contains the subdirectories of all volumes.

The following system files are present in the ODS MFD:
*<kbd>INDEXF.SYS;1</kbd>&mdash;Index file
*<kbd>BITMAP.SYS;1</kbd>&mdash;Storage bitmap file
*<kbd>BADBLK.SYS;1</kbd>&mdash;Bad block file
*<kbd>000000.DIR;1</kbd>&mdash;The MFD directory file itself
*<kbd>CORIMG.SYS;1</kbd>&mdash;Core image file
*<kbd>VOLSET.SYS;1</kbd>&mdash;Volume set list file (ODS-2/5 only)
*<kbd>CONTIN.SYS;1</kbd>&mdash;Continuation file (ODS-2/5 only)
*<kbd>BACKUP.SYS;1</kbd>&mdash;Backup log file (ODS-2/5 only)
*<kbd>BADLOG.SYS;1</kbd>&mdash;Pending bad block (ODS-2/5 only)
*<kbd>SECURITY.SYS;1</kbd>&mdash;Volume security profile (ODS-2/5 only)
*<kbd>QUOTA.SYS;1</kbd>&mdash;Quota file (optional and available under ODS-2/5 only)
*<kbd>GPT.SYS;1</kbd>&mdash;GUID Partitioning Table (GPT) (OpenVMS I64 EFI boot structures, optional on OpenVMS Alpha)

Note that the file system implementation itself does not refer to these files by name, but by their file IDs, which always have the same values. Thus, INDEXF.SYS is always the file with NUM = 1 and SEQ = 1.