UNIX 16-Bit Filesystem Structure
                     ================================
             http://mdfs.net/Docs/Comp/Disk/Format/Unix/16bit

The Unix 16-bit filesystem uses 512-byte disk blocks. Blocks are counted
with 16-bit numbers from &0000 at the start of the filesystem. The Unix
filesystem is also known as UFS and S5FS (System 5 filesystem).

Block &0000
-----------
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 &0001
-----------
Disk superblock. This contains information about the whole disk.

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 &0002
-----------
Block &0002 onwards holds the inode table. Each inode entry is 32 bytes
long. An inode is a 16-bit number. inode entries in the inode table are
numbered from &0001 upwards. inode 1 is the root directory.

If inode 1 have 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     : i_nlink - number of directory entries for this object
                    if zero, this inode if free
003     : i_uid   - object owner user id
004     : i_gid   - object owner groups id
005     : i_size0 - b16-b23 of object size
006-007 : i_size1 - b0-b15 of object size, low byte first
008-017 : i_addr[8] - object allocation vector of eight 16-bit block
                      numbers.

	If i_mode bit 12 is zero, then the eight blocks pointed to by the
	allocation vector contain the object's data. The first pointer
	points to the object's first 512 bytes, the second pointer to the
	second 512 bytes, etc, up to 8K in total.

	If i_mode bit 12 is one, then the eight blocks pointed to by the
	allocation vector each point to a block containing up to 256 block
	pointers that actually point to the object's contents. The first
	pointer points to a block that points to the data for the first
	128K, the second pointer points to a block that points to the data
	for the second 128K, etc, up to 1M in total.

	If i_mode bit 12 is one and the file is larger than 1M, then the
	eight blocks pointed to by the allocation vector are a double
	indirected allocation vector. [needs details]

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

018-01B : i_atime[2] - object last access time, seconds since 1970
01C-01F : i_mtime[2] - object last modified 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.