Macintosh File System (MFS)

The Macintosh File System (MFS) is the first file system created for Mac OS, intended for 400 KiB floppy disks.

Overview

A MFS file system consists of:

The backup master directory block (MDB), is stored in the last 2 sectors of the volume.

Characteristics	Description
Byte order	big-endian
Date and time values	TODO
Character strings	Narrow character (Single Byte Character (SBC) or Multi Byte Character (MBC)) stored using a system defined codepage

Term	Description
Clump size	Size of the group of (allocation) blocks (or clump), in bytes, to avoid fragmentation

If a volume is bootable, the first 2 blocks of the volume contain boot block. The boot block consists of:

The boot block header is 138 or 144 bytes in size and consists of:

Offset	Size	Value	Description
0	2	"LK" (or "\x4c\x4b")	Boot block signature
2	4		Boot code entry point
6	1		Flags
7	1		Format version
8	2		Page flags (or Secondary Sound and Video Pages)
10	1		System file name size, with a maximum of 15
11	15		System file name
26	1		Finder (or shell) file name size, with a maximum of 15
27	15		Finder (or shell) file name, typically "Finder"
42	1		Debugger file name size, with a maximum of 15
43	15		Debugger file name, typically "Macsbug"
58	1		Disassembler (or second debugger) file name size, with a maximum of 15
59	15		Disassembler (or second debugger) file name, typically "Disassembler"
74	1		Startup screen file name size, with a maximum of 15
75	15		Startup screen file name, typically "StartUpScreen"
90	1		Startup (or bootup) file name size, with a maximum of 15
91	15		Startup (or bootup) file name, typically "Finder"
106	1		Clipboard (or scrap) file name size, with a maximum of 15
107	15		Clipboard (or scrap) file name, typically "Clipboard"
122	2		Number of allocated file control blocks (FCBs)
124	2		Number of elements in the event queue, typically 20
126	4		System heap size on Macintosh computer with 128 KiB of RAM
130	4		System heap size on Macintosh computer with 256 KiB of RAM
134	4		System heap size on Macintosh computer with +512 KiB of RAM
Newer boot block header format
138	4		Additional system heap space
140	4		Fraction of available RAM for the system heap

Note that “LK” presumably is short for “Larry Kenyon” who originally designed MFS.

The boot code entry point contains machine-language instructions that translate to:

BRA.S *+ 0x90

Or for older versions of the boot block header:

BRA.S *+ 0x88

BRA.W *+ 0x88

BRA     $88(PC)         * $6000,$0086

This instruction jumps to the main boot code following the boot block header.

This field is ignored, however, if bit 6 is clear in the high-order byte of the boot block version number or if the low-order byte contains 0x0d.

Bit(s)	Description
0 - 4	Unknown (Reserved), should contain 0
5	Use relative system heap sizing
6	Execute boot code
7	Newer boot block header format is used

If bit 7 of the flag byte is clear, then bits 5 and 6 are ignored and the version number is set in the format version value.

If the format version value is:

less than 21, the values in the system heap size on 128K Mac and 256K Mac should be ignored and the value in system heap size on all machines should be used.
13 the boot code should be executed using the value in boot code entry point.
greater than or equal to 21 the value in system heap size on all machines should be used.

If bit 7 of the flag byte is set

bit 6 should be used to determine whether to execute the boot code using the value in boot code entry point.
bit 5 should be used to determine whether to use relative System heap sizing. If bit 5 is
- clear the value in system heap size on all machines should be used.
- is set the System heap is extended by the value in the additional system heap space plus the fraction of available RAM for the system heap.

The Master Directory Block (MDB) is located at offset 1024 of the volume and consists of:

The Master Directory Block (MDB) header is 64 bytes in size and consists of:

Offset	Size	Value	Description
0	2	"\xd2\xd7"	Volume signature
2	4		Creation date and time, which contains a HFS timestamp in local time
6	4		Last modification date and time, which contains a HFS timestamp in local time
10	2		Volume attribute flags
12	2		Number of files in the root directory
14	2		File directory area sector number, contains a sector number relative from the start of the volume, where 0 is the first sector number
16	2		File directory area size, in number of sectors
18	2		Number of blocks
20	4		Block size, in bytes, must be a multitude of 512
24	4		Clump size, in bytes
28	2		Data area sector number, contains a sector number relative from the start of the volume, where 0 is the first sector number
30	4		Next available file identifier
34	2		Number of unused blocks
36	1		Volume label size, with a maximum of 27
37	27		Volume label

TODO: describe similar to FAT-12 block allocation table

The file directory area consists of:

one or more file directory entries, where an individual file directory entry does not span multiple blocks

A file directory entry is of variable size and consists of:

Offset	Size	Value	Description
0	1		Flags, where 0x80 indicates the file directory entry is in use
1	1	0	Format version
2	4	"\x3f\x3f\x3f\x3f"	File type
6	4		File creator
10	2		Finder flags
12	4		Window position and dimension (boundaries), which contains the top, left, bottom, right-coordinate values
16	2		Folder file identifier, where 0 represents the main volume, -2 the desktop, -3 the trash, otherwise, if positive, a file identifier
18	4		File identifier
22	2		Data fork block number, contains 0 if the file entry has no data fork
24	4		Data fork size, in bytes
28	4		Data fork allocated size, in bytes
32	2		Resource fork block number, contains 0 if the file entry has no resource fork
34	4		Resource fork size, in bytes
38	4		Resource fork allocated size, in bytes
42	4		Creation date and time, which contains a HFS timestamp in local time
46	4		(Content) modification date and time, which contains a HFS timestamp in local time
50	1		File name size, with a maximum of 255
51	...		File name
...	...		16-bit alignment padding