View source for ITS on-disk file system layout

Description of ITS file system on-disk data structures.

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==Disk geometry==

A '''block''' is always 1024 words, and is the basic unit of allocation and data storage.  A block may not cross a cylinder boundary.  '''NBLKS''' is a symbolic constant in the system which is the number of (usable) blocks on a disk pack.

==MFD==

The '''MFD''' is the ''Master File Directory''.  It's a list of all directories.  The size of the MFD is a system built-in constant.  The location is in the logical middle of the disk, computed as ''NBLKS/2-1''.  All disk packs have an MFD; if the file system is more than one disk pack, all MFDs should be identical.

The MFD is one block.  It starts with a header of seven words, and the rest of the block holds entries of two words each.

Header:
{| class="wikitable"
! Offset || Description
|-
| 0 || 
|-
| 1 || Pointer to name area
|-
| 2 || Current year
|-
| 3 || Clock offset
|-
| 4 || Preferred writing disk
|-
| 5 || SIXBIT "M.F.D."
|-
| 6 || Number of user directories
|-
| 7 || 
|}

Entry:
{| class="wikitable"
! Offset || Description
|-
| 0 || SIXBIT directory name
|-
| 1 || Zero (DECUUO depends on this)
|}

==UFD==

A ''UFD'' is a ''User File Directory'' which lists all files in a directory.  A UFD is one block, located on a fixed disk address which is computed from the UFD index in the MFD.  It starts with a header of five words, and the rest of the block holds file entries of five words each.  Thus there is space for at most 203 files in an ITS directory.

Header:
{| class="wikitable"
! Offset || Description
|-
| 0 || Pointer to descriptor area
|-
| 1 || Pointer to name area
|-
| 2 || SIXBIT UFD name
|-
| 3 || Amount of space allocated,,Number of blocks used
|-
| 4 || 
|}

Entry:
{| class="wikitable"
! Offset || Description
|-
| 0 || SIXBIT first file name
|-
| 1 || SIXBIT second file name
|-
| 2 || Random information
|-
| 3 || Creation timestamp
|-
| 4 || Reference timestamp,,Byte size and file length
|}

==TUT==

The '''TUT''' is the ''Track Utilization Table''.  It keeps track of which disk areas are in use and which are free.  The size of the TUT depends on the size of the disk and is a system constant named '''NTUTBL'''.  The location is right before the MFD, and is computed as ''NBLKS/2-1-NTUTBL''.  All disk packs have a TUT which describes its utilization.