SJ Research MDFS Filesystem Structure
		    =====================================
		http://mdfs.net/Docs/Comp/Disk/Format/SJ/MDFS

The MDFS uses 1024-bytes disk blocks. Blocks are counted with 16-bit
numbers from &0000 at the start of the filesystem. Floppy disks are
formatted to 2 sides, 80 tracks, 5 sectors, 1024 bytes, and tracks are
interleaved, that is block=sector+side*5+track*10. Block zero is zero
bytes from the start of the physical disk (ie, sector &0000). Hard drives
have 512-byte sectors. Block zero is 1024 bytes from the start of the
physical disk (ie sector &0002).

Disk partitions are accessed by multiplying the partition number by the
configured drive size. For example, blocks on a system configured to use
60M partitions are accessed at partition*60M+block+1


Block zero
----------
The first disk block contains information about the disk as a whole, where
to find special objects such as "$" and "%PASSWORDS", and the printer
types defined on that disk.

000-03F: Directory entry for "$".
040-056: "SJ Research MDFS disk  " - string identifies this as a disk.
057-060: Disk title, terminated with <cr> if less than ten characters.
061    : ??? - usually set to &FF.
062-063: Disk size in blocks. Maximum disk size is &FFFF blocks, 1024
	  bytes less than 64M.
064-079: ???
07A-07B: Sometimes set to disk creation date in standard format.
07C-07D: Sometimes set to disk creation time, Hours, Minutes.
07E-07F: ???
080-0BF: Directory entry for "%PASSWORDS".
0C0-1BF: Sixteen 16-byte printer entries or eight 32-byte printer entries.
1C0-1FF: Directory entry for "%PRINTQ".
200-3FF: 256x16-bit account balances accounts for &000-&0FF
If more than 256 accounts are used, then the account balances continue in
blocks 1 onwards up to block 4.


Printer Entries
---------------
16-byte printer entry:
000-005: Printer name, padded with spaces, or six spaces for none.
006    : ???
007    : ???
008    : ???
009-00F: ??? - usually set to &00

32-byte printer entry:
000-005: Printer name, padded with spaces, or six spaces for none.
006    : ??? - port?
007    : ???
008    : ???
009-01F: Banner filename, terminated with <cr>


Directories
-----------
Directories contain a number of 64-byte entries. Entry zero is the
directory header containing information about the directory. Directory
entries are not stored in any particular order.


Directory Header
----------------
000-01F: Occupied entry bitmap. Each bit is set if the corresponding
	  directory entry is used, up to 256 directory entries. Byte 0, bit
	  0 represents entry 0, byte 0, bit 1 represents entry 1, and so
	  on. Entry zero is the directory header and will always be
	  occupied. 
020-03F: ???


Directory Entry
---------------
000    : Account number b0-b7.
001-00A: Object name, terminated by <cr> if less than ten characters.
00B    : ??? - usually <cr>
00C-00F: Load address.
010-013: Exec address.
	If the object is a directory:
		00C contains the default access.
		00D contains largest number of entries directory has had - cycle number?
		00E ???
		00F ???
		010 ???
		011 contains the number of entries.
014-016: Length. If object is a directory, length is zero.
017    : Access: Multiple:Private:Directory:Locked:OwnerW:OwnerR:PublicW:PublicR
018-019: Creation date in standard format.
01A-01B: Modification date in standard format.
01C-01D: Modification time, Hours, Minutes.
01E    : Auxilary account number b0-b7.
01F    : b0   :0=16xblock pointers, 1=16xAllocation block pointers
         b1-b3:account number b8-b10
         b4-b6:auxilary account number b8-b10       
         b7   :?
020-03F: Allocation vector of sixteen 16-bit block numbers.
	If byte &01F bit 0 is zero, then the sixteen blocks pointed to by
	the allocation vector contain the object's data. The first pointer
	points to the object's first 1024 bytes, the second pointer to the
	second 1024 bytes, etc, up to 16K in total.

	If byte &01F bit 0 is one, then the sixteen blocks pointed to by the
	allocation vector each point to a block containing up to 512 block
	pointers that actually point to the objects contents. The first
	pointer points to a block that points to the data for the first
	512K, the second pointer points to a block that points to the data
	for the second 512K, etc, up to 8M in total.

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

	If the object is a directory, a vector of block numbers is used.


Standard date format
--------------------
Byte 0  b0-b4: day of month.
        b5-b7: (year-1981) bit 4-bit 6.
Byte 1  b0-b3: month.
        b4-b7: (year-1981) bit 0-bit 3.


Parent directory
----------------
The parent directory does not appear to be specified in the directory. It
seems that the parent directory is found by scanning the filesystem for an
object that points to the child directory.


Version History
---------------
1.00 12-Sep-2004 First draft.
1.01 19-Apr-2007 Account balances, hard drive layout, disk creation date,
	32-byte printer entries.