User:Mercury0x000d

Hey there! I'm Mercury, longtime computer enthusiast and creator/primary coder of the Night Kernel. This is my sandbox to flesh out ideas on which I'm working.

Filesystems
Virtual Filesystems
VFS
Disk Filesystems
FAT 12/16/32, VFAT, ExFAT Ext 2/3/4 LEAN HPFS NTFS HFS HFS+ MFS ReiserFS FFS (Amiga) FFS (BSD)/UFS BeFS BFS XFS SFS ZDSFS ZFS USTAR
CD/DVD Filesystems
ISO 9660 Joliet UDF
Network Filesystems
NFS RFS AFS
Flash Filesystems
JFFS2 YAFFS

This page or section is a work in progress and may thus be incomplete. Its content may be changed in the near future.

MFAT

That's right, it's Mercury's FAT file system! It's similar to exFAT, but slimmed down and simpler. In a way, FAT-based file systems are a great choice for solid state drives since fragmentation doesn't add to the overall load time of the file.

Features

Very low overhead
Minimal wasted drive space
Long filename support

Differences from exFAT

exFAT stores the case given to a file but internally ignores case when matching filenames, but MFAT honors case exactly as presented, eliminating the need for an upcase table.
MFAT uses the traditional method of determining free sectors by walking the FAT as opposed to using a volume allocation bitmap.
No 0x55AA signature at the end of the boot partition. Since regular FAT drivers should verify this signature prior to attempting to parse the file system structures, this is done to signify that they should not attempt to process an MFAT file system.

Partition Layout

Overview of the order of structures in an exFAT partition
Section	Contents	Offset (sector number)	Size (in sectors)
Main Boot Region	Boot Sector	0	1
	Extended Boot Sectors	1	8
	Reserved sectors	10	1
FAT Region	First FAT	FATOffset	FATLength
FAT Region	Second FAT	FATOffset + FATLength	FATLength * (NumberOfFATs - 1)
Data Region	Cluster Heap	ClusterheapOffset	ClusterCount * 2^{SectorsPerClusterShift}
Data Region	Excess Space (to end of partition or disk)	ClusterheapOffset + ClusterCount * 2^{SectorsPerClusterShift}	VolumeLength - (ClusterHeapOffset + ClusterCount * 2^{SectorsPerClusterShift})

Structures

Boot Sector Structure

Offset

Size

Name

Description

0x000

3

JumpBoot

Jump instruction to start of boot code. For x86 CPUs, the byte sequence is usually EB 76 90.

0x003

8

FileSystemName

The "MFAT " signature.

0x00B

53

MustBeZero

This range of bytes is set null to help existing FAT drivers from mistakenly parsing an MFAT partition under the assumption that it contains valid FAT16 or FAT32 data.

0x048

8

VolumeSize

The total number of sectors in this MFAT partition.

0x050

4

FATOffset

Offset in sectors from the start of the partition to the first FAT

0x054

4

FATSize

Number of sectors in each FAT.

0x058

4

ClusterHeapOffset

Offset in sectors from the start of the partition to the start of the Cluster Heap, where files and folders live.

0x05C

4

ClusterHeapSize

The total number of clusters in the Cluster Heap.

0x060

4

RootDirectoryChain

The starting cluster of the root directory's cluster chain. Usually, but not necessarily, zero.

0x064

4

VolumeSerialNumber

A serial number for this partition.

0x068

1

Revision

Specifies the version of MFAT specification to which the structures in this partition adhere.

0x06A

4

VolumeFlags

Bit	Name	Description
0	VolumeIntegrity	0 - Partition is not in a good state 1 - Partition is in a good state (the most recent file operation completed and all structures were written back to disk)
1 - 31	n/a	Reserved

0x06C

1

BytesPerSectorShift

The number of places one would need to shift to translate bytes to sectors or sectors to bytes. Serves as an assist to allow shifting instead of multiplication or division.

0x06D

1

SectorsPerClusterShift

Similar to the above, except used to calculate sectors per cluster.

0x06F

1

DriveSelect

Similar to the Drive Number field found at offset 0x24 in previous FAT versions. Usually 0x80 for hard disks.

0x070

1

PercentInUse

Percent of clusters in the Cluster Heap which are allocated, rounded down to the nearest whole number.

0x071

7

Reserved

Do not use

0x078

390

BootCode

Boot code pointed to by the initial jump instruction at the beginning of the structure

Extended Boot Sectors Structure

Offset	Size	Name	Description
0x00	2^{BytesPerSectorShift} - 4	ExtendedBootCode	Additional space for boot code.
2^{BytesPerSectorShift} - 4	4	ExtendedBootSignature	A series of bytes (0x00, 0x00, 0x55, 0xAA) which indicates this is a proper boot sector, similar to previous FAT versions, but now a 32-bit value instead of 16-bit. Should be verified prior to depending on any other fields in the Extended Boot Sector. Note that this signature will appear on each of the Extended Boot Sectors.

FAT Region Structure

Offset

Size

Name

Description

0x00

4

FatEntry[0]

Comprised of a Media Descriptor byte (always 0xF8) followed by three bytes of 0xFF each

0x04

4

FatEntry[1]

Always 0xFFFFFFFF. Do not use, interpret, or change.

0x08

4

FatEntry[2]

One of the following:

Value	Description
0x00000000	Denotes a free cluster available for allocation
0x00000002 through ClusterCount - 1	A pointer to the next cluster in the chain
0xFFFFFFF7	Bad cluster
0xFFFFFFFF	End-of-chain markeer

These are identical to the values used in previous FAT versions, with the difference that 0x00000001, 0xFFFFFFF0 - 0xFFFFFFF6, and 0xFFFFFFF8 - 0xFFFFFFFE are no longer ascribed any significance.

...

The Second FAT is structured identically to the first, and is only present in transaction-safe exFAT implementations.

Directory Hexdump Example

   00200000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200050  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   00200090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000A0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000B0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000C0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000D0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000E0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
   002000F0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|