UNIX 24-Bit Filesystem Structure
                     ================================
             http://mdfs.net/Docs/Comp/Disk/Format/Unix/24bit

The Unix 24-bit filesystem uses 512-byte disk blocks. Blocks are counted
with 24-bit numbers from &000000 at the start of the filesystem. The Unix
filesystem is also known as UFS and S5FS (System 5 filesystem).

Large values are stored 'NUXI' format, that is b16-b23, b0-b7, b8-b15 for
24-bit values and b16-b23, b24-b31, b0-b7, b8-b15 for 32-bit values.


Block &000000
-------------
The first block is not used by the Unix filesystem and can be used by the
disk system itself to hold data about the disk.

Block &000001
-------------
Disk superblock. This contains information about the whole disk.
<< details not checked>>

000-001 s_isize      : size in blocks of inode table
002-003 s_fsize      : size in blocks of entire volume
004-005 s_nfree      : number of in-core free blocks (0-100)
006-0CD s_free[100]  : in core free blocks
0CE-0CF s_ninode     : number of in-core i-nodes (0-100)
0D0-197 s_inode[100] : in-core free i-nodes

	When the free space list in the superblock is exhausted, the inode
	table is scanned to find 100 more blocks.

198     s_flock      : locked during free list manipulation
199     s_ilock      : locked during i-list manipulation
19A     s_fmod       : super block modified flag
19B     s_ronly      : disk is read-only
19C-19F s_time[2]    : date of last update, seconds since 1970
1A0-1FF s_pad[48]    : padding (defined as s_pad[50] in source code)

Block &000002
-------------
Block &000002 onwards holds the inode table. Each inode entry is 64 bytes
long. An inode is a 16-bit number. inode entries in the inode table are
numbered from &0001 upwards. inode 2 is the root directory.

If inode 1 has a valid 16-bit allocation vector (inode entry 8-9 is
nonzero), then the file system is a 16-bit Unix filesystem, and inode 1 is
the root. If inode 1 does not have a valid 16-bit allocation vector (inode
entry 8-9 is zero), then the file system is a 24-bit Unix filesystem, and
inode 2 is the root.

inode entry
-----------
000-001 : i_mode     - object mode, "adclugsrwxrwxrwx"
	  bit  0 : "x" - executable by world
	  bit  1 : "w" - writable by world
	  bit  2 : "r" - readable by world
	  bit  3 : "x" - executable by group
	  bit  4 : "w" - writable by group
	  bit  5 : "r" - readable by group
	  bit  6 : "x" - executable by owner
	  bit  7 : "w" - writable by owner
	  bit  8 : "r" - readable by owner
	  bit  9 : "s" - save swapped text
	  bit 10 : "g" - set group ID on execution
	  bit 11 : "u" - set user ID on execution
	  bit 12 : "l" - large object
	  bit 13 : "c" - character device  "c"+"d"="b" - block device
	  bit 14 : "d" - directory
	  bit 15 : "a" - space allocated on disk
002-003 : i_nlink    - number of directory entries for this object
                       if zero, this inode if free
004-005 : i_uid      - object owner user id
006-007 : i_gid      - object owner group id
008-009 : i_sizehi   - b16-b23 of object size
00A-00B : i_sizelo   - b0-b23 of object size
00C-029 : i_addr[10] - object allocation vector of ten 24-bit block
                       numbers for the first 5K of the file.

02A-02C : indirected block, block number of a block containing the next 128
	block numbers for the next 64K of the file. Each block contains:
	000-001 : b16-b31 of first block number
	002-003 : b0-b16 of first block number
	004-005 : b16-b31 of second block number
	006-007 : b0-b15 of second block number
	etc.

02D-02F : double indirected block, block number of a block containing a
	list of block numbers of blocks containing the next block numbers.

	24-bit block numbers in the inode entry are stored as b16-b23 in
	the first byte, b0-b7 in the second byte and b8-b15 in the third
	byte.

	A block number of zero indicates that that part of the object has
	not been allocated any disk space.

030-033 : spare

034-035 : i_ctimehi - b16-b31 of object creation time, seconds since 1970
036-037 : i_ctimelo - b0-b15 of object creation time, seconds since 1970
038-039 : i_mtimehi - b16-b31 of object last modifcation time, seconds since 1970
03A-03B : i_mtimelo - b0-b15 of object last modified time, seconds since 1970
03C-03D : i_atimehi - b16-b31 of object last access time, seconds since 1970
03E-03F : i_atimelo - b0-b15 of object last access time, seconds since 1970

Directory entry
---------------
000-001 : i_node     - inode of this object, if zero, this entry is free
002-00F : i_name[14] - object name, padded with NULs

This first two entries of a directory are always '..' pointing to the
parent, and '.' pointing to the same directory. The following directory
entries are not stored in any particular order.