MineCraft FileSystem (MCFS) documentation


				Disk format

Each floppy disk capacity is limited to 2048 sectors by the disk drive.
Each sector is 128 bytes long so the maximum physical capacity of a disk
is 256 kilobytes. An MCFS formatted disk can hold (2048-16)*126 bytes
(250 kilobytes) of user data.

  Sector	Description
 0 -    3	MCFS boot loader
 4 -    5	Sector allocation map (2048 bits, one bit / sector)
 6 -   15	Directory (4 entries / sector, maximum 40 entries)
16 - 2047	Available for files



				 Boot loader

This small loader only job is to load the bootable programs which are
then executed. The boot loaders remaps itself after the execution into the
two stack area at $0100 and $0200 which makes possible to load the program
from $0500 like the original RPC8/e boot code does.

Most of the possible cases are handled and visible during the boot sequence,
these are the following:
- detects drive errors during the boot (for example: destroyed cable)
- handles read errors (for example: a truncated disk image)
- detects available memory and display and error message when a programs
  doesn't fit into the memory.
- warns if disk is not bootable

Sector    Byte
  0-3     0-511 Boot loader which is loaded to memory and started at
		address $0500

   0	122-123 Starting sector of the bootable file.
		If sector = 0 then the disk is not bootable.

   0	124-127	MCFS magic constant: $4d $43 $46 $53 "MCFS"



			   Sector allocation map

Sector allocation map (SAM) is a plain bitmap for each physical sector.
Bitmap is linear, however the least significant bit is on the left

Sector	Byte	Content
   4	  0	SAM for sector 0-7
		Example: the byte value is $a5 so the sector availability
		looks like this:

				 2^7 6 5 4 3 2 1 0
				-------------------
				   1 0 1 0 0 1 0 1

			sector:    0 1 2 3 4 5 6 7

		in this case sector 0,2,5 and 7 is occupied and sector
		1,3,4,6 is available.
		
   4	  1	SAM for sector 8-15
   4	  2	SAM for sector 15-23
  ...    ...
   4	 127	SAM for sector 1016-1023
   5	  0	SAM for sector 1024-1031
   5	  1	SAM for sector 1032-1039
  ...    ...		
   5	 126	SAM for sector 2032-2039
   5	 127	SAM for sector 2040-2047


			    Directory structure

The first entry of a directory structure is the header which contains the name
of the disk. Each entry is 32 bytes long, the maximum length of a filename
is 28. Each sector is 128 bytes long so 4 entries fits into one sector.
10 sectors are available for directory entries so the maximum capacity is 39
files plus the header (40 in total).

Sector	 Byte	Content
   6	  0- 3	Unused
   6	  4-31	Disk name. all characters MSB is set to 1 ($80)

   6     32-63	Directory entry #1
   6	 32-33	  Starting sector of the file.
		  If sector = 0 the directory entry is free.
   6     34-35	  Size of the file in sectors
   6     36-63	  File name

   6     64-95	Directory entry #2
   6	 64-65	  Starting sector of the file.
		  If sector = 0 the directory entry is free.
   6     66-67	  Size of the file in sectors
   6     68-95	  File name

  ...    ...

  15	 96-127	Directory entry #39
  15	 96- 97	  Starting sector of the file.
		  If sector = 0 the directory entry is free.
  15     98- 99	  Size of the file in sectors
  15    100-127	  File name


				File format

First two bytes of each sector were used as a pointer to the next sector
of the file, so only 126 out of the 128 bytes of a sector were used for file
contents. In the last sector of a file the higher value of a pointer is set
to $ff and the lower value of the pointer holds the remaining bytes used on
the actual sector.

Sector	Byte
  xx	0-1	Pointer to next sector #yy (word)
		If byte 1 is $ff then the number of bytes which used
		within the sector is contained in byte 0.
  xx	2-127	User data

  yy	0-1	Pointer to next sector #zz (word)
		If byte 1 is $ff then the number of bytes which used
		within the sector is contained in byte 0.
  yy	2-127	User data

  zz	...