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Specifications
Standard Disk Format
The most widespread disk format
Basic Structure
The FAT file system structure contains the following
regions:
FAT File System Structure In A Hard Disk
| Region |
|
Reserved Region (incl. Boot Sector)=
(Sector 0+Hidden Sectors +BootSectors +Reserved Sectors (Generally
0+63+10+22 = 0--94 ) |
|
File Allocation Tables (FAT)
=(example : Sectors 95--10094) (FAT1 +FAT2) |
|
Root Directory( C:\)
=(example : sectors 10095-10102) |
|
Data Region =
remaining sectors of disk |
Introduction
The standard described in the document is used on nearly
all x86 PC's, for administration of diskspace on fixed disks. The kind of
diskspace administration this standard provide is very simple. The standard
allows the diskspace to be divided in up to 4 partitions (normaly refered to as
primary partitions). Each of these 4 partitions can contain a separate file
system. The heart of the standard is the first sector on the disk,sector zero, which is
reserved for the Master Boot Record (MBR).
Master Boot Record
The Master Boot Record is in first sector on the
disk. This is sector 0 in LBA mode, and sector 0:0:1 in CHS mode. This sector is checked by the
BIOS for the AA55h signature at offset 1FEh, to see if drive is bootable. This
signature lets the BIOS know that the code in the beginning of this sector is
executable. The structure of the Master Boot Record is:
Master Boot Record Structure
| Part |
Offset |
Size |
Description |
| Code |
0000h |
446 bytes |
Execution is transferred to here after the BIOS has finished. |
| Partition Table |
01BEh |
16 bytes |
1st Partition Entry |
| 01CEh |
16 bytes |
2nd Partition Entry |
| 01DEh |
16 bytes |
3rd Partition Entry |
| 01EEh |
16 bytes |
4th Partition Entry |
| Signature |
01FEh |
2 bytes |
AA55h signature. 55h at offset 1FEh and AAh at offset
1FFh |
Partition Entries
Each of the partition entries from the partition table
have the following structure:
Partition Entry Structure
| Offset |
Size |
Description |
| 0000h |
1 bytes |
A value of 80h means active, and 00h means inactive. |
| 0001h |
1 bytes |
Start of partition - Head |
| 0002h |
2 bytes |
Start of partition - Cylinder/Sector |
| 0004h |
1 bytes |
Partition
Type |
| 0005h |
1 bytes |
End of partition - Head |
| 0006h |
2 bytes |
End of partition - Cylinder/Sector |
| 0008h |
4 bytes |
LBA address for the first sector in the partition. |
| 000Ch |
4 bytes |
Number of sectors in the partition |
Packed CHS
The location information for the beginning and end of the
partition, consist of three bytes each. The first byte is the Head count. This
is an 8 bit value, which makes room for 256 different values. The next word
contains the Cylinder and Sector information and is packed together. The
cylinder number is a 10 bit value and the Sector number is a 6 bit value. They
are packed in the following way:
Structure of the packed cylinder/structure word
| 15 |
14 |
13 |
12 |
11 |
10 |
9 |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Cylinder [7-0] |
Cyl. [9+8] |
Sector [5-0] |
0000h |
Partition Types
The partition type value show what kind of file system
that has been installed in the partition. Partition Type = File System
Identifier. The list of partition types is long, and world wide there are
probably at least a few which override each other. I'm continously trying to
compile a pretty complete list of file system identifiers. The most up-to-date
list can be found here.
MBR Code
The code in the MBR sector has a very limited amount of
resources available. It only has a maximum of 446 bytes to get the job done.
What it has to do, is locate the partition which is marked active, load the
first sector from that partition into linear memory at 00007C00h, and transfer
execution to that code.
This is the execution enviroment under which the
MBR code must execute:
- CS:IP can be 07C0:0000 or 0000:7C00. Linear address is the same. Should be
initialized.
- SS:SP is also unknown. Usually initialized.
- DL contain number of the drive from which the code was loaded from.
- The state of the other registers like AX, BX, CX, DH, SI, DI, BP, DS, ES,
FS and GS are unknown.
Remember to pass the drive
number along in DL.
Compatibility Advice
When adding new partitions to a disk, it is adviced
that the partition is aligned at sector 1. That means a partition would always
start at cylinder x, head y and sector 1, where x and y can be any given number
(but still within the limits of the disk).
The 8 Gb Limit
The CHS address mode has a limit of 16.515.072 sectors,
which is usually 8 Gb. To add partitions beyond this limits, the drive must
support the LBA address mode. With this addess mode the limit is 2 Tb. When
dealing with partitions beyond this limit, the packed CHS information in the
'Start of partition' and 'End of partition' fields should be set to the maximum
possible, and only the LBA address is used.
Specifications
FAT12 AND FAT16 File System
Used: On most x86 3.5" floppydisks
Introduction
This is the 16-bit version of the FAT file system. The
16-bit part describes the way units are allocated on the drive. The FAT16 file
system uses a 16-bit number to identify each allocation unit (called cluster),
and this gives it a total of 65.536 clusters. The size of each cluster is
defined in the boot sector of the volume (volume = partition). The File System
ID number usually associated with FAT16 volumes are 04h and 06h. The first is
used on volumes with less than 65536 sectors (typical this is on drives less
than 32 Mb in size), and the latter one is used on volumes with more than 65536
sectors. There is also another variant which is used with the LBA address mode,
that variant has a File System ID of 0Eh. I do not know if the
LBA variant is different from the CHS type. So far I don't se why anything
should be changed to support LBA addresses.
Basic Structure
The FAT16 file system structure contains the following
regions:
FAT16 File System Structure
| Region |
| Reserved Region (incl. Boot Sector) |
| File Allocation Table (FAT) |
| Root Directory |
| Data Region |
The first sector (boot
sector) contain information which is used to calculate the sizes and locations
of the other regions. The boot sector also contain code to boot the operating
system installed on the volume. The data region is split up into logical blocks
called clusters. Each of these clusters has an accompanying entry in the FAT
region. The cluster specific entry can either contain a value of the next
cluster which contain data from the file, or a so called End-of-file value which
means that there are no more clusters which contain data from the file. The root
directory and its sub-directories contain filename, dates, attribute flags and
starting cluster information about the filesystem objects.
Boot Sector
The first sector in the reserved region is the boot sector.
Though this sector is typical 512 bytes in can be longer depending on the media.
The boot sector typical start with a 3 byte jump instruction to where the
bootstrap code is stored, followed by an 8 byte long string set by the creating
operating system. This is followed by the BIOS Parameter Block, and then by an
Extended BIOS Parameter Block. Finally the boot sector contain boot code and a
signature.
BIOS Parameter Block
The BIOS Parameter Block contains basic information
about the overall structure of the FAT file system. That is information such as
sector and cluster size, location information of FAT copies, the root directory
size etc..
Bytes Per Sector
This value is the number of bytes in each physical
sector. The allowed values are: 512, 1024, 2048 or 4096 bytes. A lot of code
outthere are assuming 512 bytes per sectors, so any other values can give
compatibility problems.
Sectors Per Cluster
This is the number of sectors per cluster. The
allowed values are: 1, 2, 4, 8, 16, 32 or 128. But de-facto is that most
combinations of 'BytesPerCluster' * 'SectorsPerCluster' which gives a total
value over 32 Kb per cluster, are not supported on many system.
Reserved Sectors
Since the reserved region always contain the boot
sector a zero value in this field is not allowed. The usual setting of this
value is 1. The value is used to calculate the location for the first sector
containing the FAT.
Number of FAT copies
This is the number of FAT copies in the file
system. The recommended value is 2 (and then have two FAT copies), but other
values are validm though they may not be supported on some system. The usage of
two copies are to prevent data loss if one or part of one FAT copy is
corrupted.
Root Entries Count
This value contain the number of entries in the root
directory. Its recommended that the number of entries is an even multiple of the
BytesPerSector values. The recommended value for FAT16 volumes is 512 entries
(compatibility reasons).
Small Number of Sectors
This field states the total number of sectors in
the volume. That includes the number of sectors occupied by the four regions
which the FAT16 file system consist of. For FAT16 volumes that use less than
65536 sectors this field is used. The File System Id in the MBR is then 04h. For
FAT16 volumes that use more the 65535 sectors the Large
Number of Sectors field is used and this one should be set to
0h.
Sectors Per FAT
This is the number of sectors occupied by one copy of
the FAT.
Sectors Per Track
This value is used when the volume is on a media which
have a geometry, that is when the LBA
number is broken down into a Cylinder-Head-Sector address. This field represents
the multiple of the max. Head and Sector value used when the volume was
formatted. The field itself is used to check if the LBA to CHS translation has changed, since the
formatting. And for calculating the correct Cylinder, Head and Sector values for
the translation algorithm.
Number of Heads
This value is used when the volume is on a media which
have a geometry, that is when the LBA
number is broken down into a Cylinder-Head-Sector address. This field represents
the Head value used when the volume was formatted. The field itself is used to
check if the LBA to CHS translation has
changed, since the formatting. And for calculating the correct Cylinder, Head
and Sector values for the translation algorithm.
Hidden Sectors
When the volume is on a media that is partitioned, this
value contains the number of sectors preceeding the first sector of the
volume.
Large Number of Sectors
This field states the total number of sectors in
the volume. That includes the number of sectors occupied by the four regions
which the FAT16 file system consist of. For FAT16 volumes that use more than
65535 sectors this field is used. The File System Id in the MBR is then 06h. For
FAT16 volumes that use less than 65536 sectors the Small
Number of Sectors field is used and this one should be set to
0h.
Extended BIOS Parameter Block
The Extended BIOS Parameter Block contains
information that is only used in the FAT16 file system.
Drive Number
This is the int 13h drive number of the drive. The value
00h is used for the first floppy drive and the value 80h is used for the first
harddrive. MS-DOS's bootstrap uses this value to find the correct
drive.
Reserved
Reserved byte. It was original used to store the cylinder on
which the boot sector is located. But Windows NT uses this byte to store two
flags. The lowest bit would indicates that a check disk should be run at boot
time, and the second lowest flag indicates that a surface scan should be
run.
Extended Boot Signature
If this byte contain a value of 29h it indicates
that the following three fields are available.
Volume Serial Number
This value is a 32 bit random number, which,
combined with the volume
label, makes is possible to track removable media and check if the correct
one is inserted.
Volume Label
This 11 byte long string should match the volume label
entry in the root directory. If no such entry is available this field should
contain the string 'NO NAME ' (11 bytes long string). When updating the volume
label, both, this field and the entry in the root directory should be
updated.
File System Type
This 8 byte long string should be used for
informational display only. Thats because its sometime incorrectly set. The
field should contain the string 'FAT16 ' (8 bytes long
string).
Bootstrap Code
The bootstrap code is different between operating system
and versions which are intended to be loaded of this FAT16 volume. The
responsability of the bootstrap code is to continue the boot sequence. If ex.
MS-DOS is installed the bootstrap code will locate the file IO.SYS in the file
system, load part of it into memory and then jump to a specific entrypoint in
IO.SYS. What the bootstrap code does is vary between operating
system.
Boot Sector Signature
The word at offset 1FEh should contain the
signature AA55h. This will indicate to the BIOS that the sector is executable.
The signature is also used by other applications when validating that the
correct sector has been loaded. No matter what the Bytes Per
Sector value is this signature should always be at offset
1FEh.
File Allocation Table
The FAT
structure contain linked lists of files in the file system. Any file or
directory entry in a (sub)directory list contain a cluster number for the first
chunk of the file/directory. This cluster number also has an associated entry in
the FAT. At this entry in the FAT a single word value either points to the next
cluster/chunk or it contain an End-of-file value. These are the valid
values:
Valid FAT16 values
| Value |
Description |
| 0000h |
Free cluster |
| 0001h - 0002h |
Not allowed |
| 0003h - FFEFh |
Number of the next cluster |
| FFF7h |
One or more bad sectors in cluster |
| FFF8h - FFFFh |
End-of-file |
Each FAT copy start with a
value of FFxxh for the first cluster, where xx is equal to the Media
Descriptor field in the BIOS Parameter Block. The FAT entry for the second
cluster is set to the End-of-file value (FFFFh). The two highest bits of this
value may be used for dirty volume flags in the following way.
FAT Entry Value for 2nd cluster
| 15 |
14 |
13 |
12 |
11 |
10 |
9 |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| C |
E |
These bits must be set |
0000h |
C = If clear then the volume may
be dirty because it was not cleanly dismounted. If the bit is set then the
volume is clean.
|
E = If clear a read/write error
was encountered during the last time the volume was mounted. If the bit is
set then no read/write error was
encountered.
|
It should be noted that the
last entry in the FAT should be calculated by taking the number of clusters in
the data area, and not relying on the Sectors
Per FAT field in the BIOS Parameter Block. This is because the number of
entries in the FAT doesn't have to be an even multiple of the bytes per sector
value.
The maximum size of each FAT copy is 128 Kb (2 *
65536).
Directory Entry Structure
Each of the entries in a directory list is 32
byte long. The only directory which is in a fixed location is the root
directory. This is also the only directory which may contain an entry with the
Volume Label attribute set. The size of the root
directory is defined in the BIOS Parameter Block.
Sub-directories are
created by allocating a cluster which then are cleared so it doesn't contain any
directory entries. Two default entries are then created: The '.' entry point to
the directory itself, and the '..' entry points to the parent directory. If the
contents of a sub-directory grows beyond what can be in the cluster a new
cluster is allocated in the same way as for files.
This is the format of
the directory entries:
Filename & Extension
The filename is 8 byte long. Shorter names must
be trailing padded with space bytes (ASCII: 20h). The extension is 3 byte long
and shorter names also have to be trailing padded. The characters allowed in the
filename and extension are basicly the uppercase letters of the english
alphabet, plus the digits 0 to 9.
The first byte in the filename is
treated special. The following rules apply:
- A value of 00h is interpertated as 'stop the search, there is no more
entries in this directory'.
- A value of 05h is should be replaced with the ASCII character E5h. The
character is used in Japan.
- Must not contain the value 20h.
- A value of E5h is interpertated as 'the entry is free'.
- Any of the values mentioned below are allowed.
The following
characters are not allowed in the filename or its extension:
- Any character below 20h, except the 05h character.
- Any character in the following list: 22h ("), 2Ah (*), 2Bh (+), 2Ch (,),
2Eh (.), 2Fh (/), 3Ah (:), 3Bh (;), 3Ch (<), 3Dh (=), 3Eh (>), 3Fh (?),
5Bh ([), 5Ch (\), 5Dh (]), 7Ch (|).
For compatibility reasons it
is recommended to limit the characters to:
- Any uppercase characters from A to Z.
- Any digit from 0 to 1.
- Any of the following characters: #, $, %, &, ', (, ), -, @
Attribute Byte
The attribute byte defines a set of flags which can be
set for directories, volume name, hidden files, system files, etc. These are the
flags:
Flags in the Attribute byte
| 7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Reserved |
A |
D |
V |
S |
H |
R |
0000h |
Read Only
This flag is used to prevent programs from not automatically
overwriting or deleting this entry.
Hidden
This flag indicates to the system that the file should be hidden
when doing normal directory listings. But in a lot of programs this can be
overwritten by the user.
System
This flag indicates that the file/directory is important for the
system, and shouldn't be manipulated without any concern.
Volume Name
When this flag is set, the directory entry is not pointing
to a file, but to nothing. Thus the the Starting cluster must point the cluster
0. The only information used from this entry is the filename (8 bytes) plus the
filename extension (3 bytes). These bytes form an 11 byte long volume label
(without any .) There may be only one valid entry in the entire volume
with this flag set. This entry must be in the root directory and preferably
among the first entries, if not, then MS-DOS can have trouble displaying the
right volume label if there are long file names present. This volume name should
be the same as the one in the boot sector.
Directory
This flag is set, when an entry in the directory table is not
pointing to the beginning of a file, but to another directory table. A
sub-directory. The sub-directory is placed in the cluster, which the Starting
Cluster field points to. The format of this sub-directory table is identical to
the root directory table.
Achieve Flag
This flag is used by backup utilities. The flag is set when
ever a file is created, renamed or changed. Backup utilities can then use this
flag to determine which files that has been modified since the last
backup.
Reserved for Windows NT
This byte is used by Windows NT. It set the
value to 0 when the file is created and then never look at it again. For what
purpose it uses it is unknown.
Creation Time - Millisecond
Due to size limitations this field (1 byte)
only contains the millisecond stamp in counts of 10 milliseconds. Therefore
valid values are between 0 and 199 inclusive.
Creation Time & Date
The 16 bit time field contain the time of day
when this entry was created. The 16 bit date field contain the date when the
entry was created. These two values never change. Both the time
field and the date
field are in a special format.
Last Access Date
This 16 bit field contain the date when the entry was
last read or written to. In case of writes this field of cause contain the same
value as the Last
Write Date field. The date
field is the same special format as the other dates.
Reserved for FAT32
This word is reserved for the FAT32 File System. When
used in that file system it will contain the high word of the starting cluster.
In the FAT16 File System this word should be set to 0.
Last Write Time
This 16 bit field is set to the time when the last write
occured. When the entry is create this field and the Creation
Time field should contain the same values. In case the entry is a directory
entry this field should change when the contents of the sub-directory
changes.
The field is in the special format described below:
Last Write Date
This 16 bit field is set to the date when the last write
occured. When the entry is create this field and the Creation
Date field should contain the same values. In case the entry is a directory
entry this field should change when the contents of the sub-directory
changes.
The field is in the special format described below:
First Cluster
This 16-bit field points to the starting cluster number of
entrys data. If the entry is a directory this entry point to the cluster which
contain the beginning of the sub-directory. If the entry is a file then this
entry point to the cluster holding the first chunk of data from the
file.
File Size
This 32-bit field count the total file size in bytes. For this
reason the file system driver must not allow more than 4 Gb to be allocated to a
file. For other entries than files then file size field should be set to
0.
Calculation Algorithms
How to calculate the starting location of each
region. The VolumeStart variable contain a LBA address of the first sector in the
volume. On drives which are not partitioned the VolumeStart value is 0. On
drives which are partitioned the VolumeStart variable contain the sector number
at which the partition start.
How to calculate other values
| Region Start |
Calculation |
| First sector of cluster N = |
DataRegion + ((N - 2) * SectorsPerCluster) |
| Previous calculated values which
are used again, are marked with this
color. |
Special Notes
When creating a FAT16 volume special care should be taken
to ensure best compatibility. Following these rules ensure the best
compatibility:
- A FAT16 partition may not have less than 4085 clusters or more than 65524
clusters.
-
-
-
-
-
-
-
-
Conclusion
The FAT family of file systems are relative simple file
systems. The complexity can be enhanced by adding support for long file names,
using the VFAT
Long File Names. Also have a look at the 32
bit version of the FAT file system.
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