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FAT File System

The file allocation table or FAT stores information about the clusters on the disk in a table. There are three different varieties of this file allocation table, which vary based on their size. The system utility that you use to partition the disk will normally choose the correct type of FAT for the volume you are using, but sometimes you will be given a choice of which you want to use. Since each cluster has one entry in the FAT, and these entries are used to hold the cluster number of the next cluster used by the file, the size of the FAT is the limiting factor on how many clusters any disk volume can contain. The following are the three different FAT versions now in use:

(1) FAT12: The oldest type of FAT uses a 12-bit binary number to hold the cluster number. A volume formatted using FAT12 can hold a maximum of 4086 clusters, which is 2^12 minus a few values (to allow for reserved values to be used in the FAT). FAT12 is therefore most suitable for smaller volumes, and is used on floppy disks and hard disk partitions smaller than about 16 MB.

(2) FAT16: The FAT used for most hard disk partitions uses a 16-bit binary number to hold cluster numbers. When you see someone refer to a "FAT" volume generically, they are usually referring to FAT16, because it is the de facto standard for hard disks. A volume using FAT16 can hold a maximum of 65526 clusters, which is 2^16 less a few values (again for reserved values in the FAT). FAT16 is used for hard disk volumes ranging in size from 16 MB to 2 GB.

(3) FAT32: The newest FAT type, FAT32 is supported by Windows 95's OEM SR2 release, as well as Windows 98. FAT32 uses a 28-bit binary cluster number--not 32, because 4 of the 32 bits are "reserved". 28 bits is still enough to permit ridiculously huge volumes--FAT32 can theoretically handle volumes with over 268 million clusters, and will support (theoretically)drives up to 2 TB in size. However to do this the size of the FAT grows very large.

[Note]:

On volumes with fewer than 32,680 sectors, the cluster sizes can be up to 8 sectors per cluster. In this circumstance, the format program creates a 12-bit FAT. Volumes less than 16 MB are usually formatted for a 12-bit FAT, but the exact size depends on the disk geometry. The disk geometry also determines when a larger cluster size is needed because the number of clusters on the volume must fit into the number of bits used by the file system managing the volume. Therefore, you might have a 33-MB volume that has only 1 sector per cluster.

FAT12 is the original implementation of FAT and is intended for very small media. The file allocation table for FAT12 is smaller than the file allocation table for FAT16 and FAT32, because it uses less space for each entry, leaving more space for data. All 1.44-MB 3.5-inch floppy disks are formatted with FAT12.

The following are some file system size limitations that should also be considered:

1.)FAT volumes smaller than 16 MB are formatted as FAT12.

2.)FAT16 volumes larger than 2 GB are not accessible from computers running MS-DOS, Windows 95, Windows 98, and many other operating systems.

3.)While FAT32 volumes can theoretically be as large as 2 terabytes, Windows 2000 limits the maximum size FAT32 volume that it can format to 32 GB.

However, Windows 2000 can read and write to larger FAT32 volumes formatted

by other operating systems.

4.)The implementation of FAT32 in Windows 2000 limits the maximum number of clusters on a FAT32 volume that can be mounted by Windows 2000 to 4177918. This is the maximum number of clusters on a FAT32 volume that can be formatted by Windows 98.

Now we are ready to detect which file system is used

(1)If NumberOfClusters < 4087, then FAT12 is used.

(2)If 4087 <= NumberOfClusters < 65,527, then FAT16 is used.

(3)If 65,527 <= umberOfClusters < 268,435,457 then, FAT32 is used.

(4)Otherwise, none of the above is used.

illustrates how FAT12/FAT16 maps clusters on a volume. The file allocation tables identify each cluster in the volume as one of the following:

The only difference between the root folder and other folders is that the root folder is at a specified location and has a fixed number of entries (for a hard disk). The root folder exists at a specified location and has the maximum number of available entries fixed at 512. The maximum number of entries on a floppy disk depends on the size of the disk.

[Note]:

Each folder and 8.3 file name in the root folder counts as an entry. For example, since the maximum number of entries is fixed at 512, if you have 100 folders in the root folder, you can only create 412 more files or folders in the root folder. If those folders or files use names longer than the 8.3 format, fewer files and folders can be created.

The information in the folder is used by all operating systems that support FAT. Windows 2000 can store additional timestamps in a FAT folder entry. These timestamps show when the file was created or last accessed.

Because all entries in a folder are the same size, the attribute byte for each entry in a folder describes what kind of entry it is. For example, one bit indicates that the entry is for a subfolder and another bit marks the entry as a volume. Typically, the operating system controls the settings of these bits.

The attribute byte includes four bits that can be turned on or off by the user — archive, system, hidden, and read-only.

File Description Block, FDB

Folders contain a 32-byte entry for each file and folder they contain. The entry includes the following information:

(1) Name in 8.3 format (11 bytes)

(2) Attribute (1 byte)

(3) Create time (3 bytes)

(4) Create date (2 bytes)

(5) Last access date (2 bytes)

(6) Last modified time (2 bytes)

(7) Last modified date (2 bytes)

(8) Starting cluster number in the file allocation table (2 bytes)

(9) File size (4 bytes)

[Note]:

Three bytes in each entry are held in reserve.

Introduction to FAT

In FAT file system, storage device is of a fixed size. It is logically divided into chunks of data of equal size called clusters . Any file takes up a natural number of clusters. Disk map, called File Allocation Table (FAT), is located at a known position on disk. FAT is an array of the entries of equal size. Number of entries is equal to number of clusters on disk, so there exists a unique relationship between FAT entries and clusters. FAT file chain. Although several values may serve as EOF, only -1 is used by DOS. Bad clusters are physically They should not be used by anyone but their creator. In every FAT, the very first entry is reserved "DOC", "INTEL", and "INTEL386.TXT" in this order. The first token tells us to select the first primary the entry named "DOC". As I said earlier, one of the fields of this entry is FirstCluster. Now we know entry is one of the following:

Some of the values in the table are in the BPB. Let us calculate the rest of them:

(1)RootStart = ReservedSectors + NumberOfFATs * SectorsPerFAT

(2)ClustersStart = RootStart + (RootEntries * 32) / BytesPerSector

(3)NumberOfClusters = 2 + (NumberOfSectors - ClustersStart) / SectorsPerCluster

To convert cluster address to LBA address use the formula

LBA = ClustersStart + (Cluster - 2) * SectorsPerCluster

Disk Sectors Critical to Startup

The two sectors critical to starting your computer are the master boot record (MBR), which is always located at sector 1 of cylinder 0, head 0, the first sector of a hard disk, and the boot sector, which resides at sector 1 of each volume. These sectors contain both executable code and the data required to run the code.

Master Boot Record

The MBR, the most important data structure on the disk, is created when the disk is partitioned. The MBR contains a small amount of executable code called the master boot code, the disk signature, and the partition table for the disk. At the end of the MBR is a 2-byte structure called a signature word or end of sector marker, which is always set to 0x55AA. A signature word also marks the end of an extended boot record (EBR) and the boot sector.

Master Boot Code

The master boot code performs the following activities:

(1) Scans the partition table for the active partition.

(2) Finds the starting sector of the active partition.

(3) Loads a copy of the boot sector from the active partition into memory.

(4) Transfers control to the executable code in the boot sector.

If the master boot code cannot complete these functions, the system displays one of the following error messages:

(1) Invalid partition table.

(2) Error loading operating system.

(3) Missing operating system.

[Note]:

There is no MBR on a floppy disk. The first sector on a floppy disk is the boot sector. Although every hard disk contains an MBR, the master boot code is used only if the disk contains the active, primary partition.

Boot Sector

The boot sector, located at sector 1 of each volume, is a critical disk structure for starting your computer. It contains executable code and data required by the code, including information that the file system uses to access the volume. The boot sector is created when you format a volume. At the end of the boot sector is a two-byte structure called a signature word or end of sector marker, which is always set to 0x55AA. On computers running Windows 2000, the boot sector on the active partition loads into memory and starts Ntldr, which loads the operating system.

Boot Sector Startup Processes

Computers use the boot sector to run instructions during startup. The initial startup process is summarized in the following steps:

(1) The system BIOS and the CPU initiate the power-on self test (POST).

(2) The BIOS searches for a boot device (typically a disk).

(3) The BIOS loads the first physical sector of the boot device into memory and transfers CPU execution to that memory address.

(4) If the boot device is on a hard disk, the BIOS loads the MBR. The master boot code in the MBR loads the boot sector of the active partition, and transfers CPU execution to that memory address. On computers that are running Windows 2000, the executable boot code in the boot sector finds Ntldr, loads it into memory, and transfers execution to that file.

If there is a floppy disk in drive A, the system BIOS loads the first sector (the boot sector) of the disk into memory. If the disk is startable — formatted by MS-DOS with core operating system files applied — the boot sector loads into memory and uses the executable boot code to transfer CPU execution to Io.sys, a core MS-DOS operating system file. If the floppy disk is not bootable, the executable boot code displays an error message such as:

(1) Non-System disk or disk error.

(2) Replace and press any key when ready.

Components of a Boot Sector

The MBR transfers CPU execution to the boot sector, so the first three bytes of the boot sector must be valid, executable x86-based CPU instructions. This includes a jump instruction that skips the next several nonexecutable bytes.

Following the jump instruction is the 8-byte OEM ID, a string of characters that identifies the name and version number of the operating system that formatted the volume. To preserve compatibility with MS-DOS, Windows 2000 records "MSDOS5.0" in this field on FAT16 and FAT32 disks. On NTFS disks, Windows 2000 records "NTFS."

Physical Sector: Cyl 0, Side 1, Sector 1 (Boot Sector)

0 1 2 3 4 5 6 7 8 9 A B C D E F

00000000: EB 3C 90 4D 53 44 4F 53 - 35 2E 30 00 02 40 01 00 .<.MSDOS5.0..@..

00000010: 02 00 02 00 00 F8 02 00 – 10 00 20 00 10 00 00 00 ........?.@.?...

00000020: 00 00 00 00 80 00 29 A8 - 8B 36 52 4E 4F 20 4E 41 ..>...)..6RNO NA

00000030: 4D 45 20 20 20 20 46 41 - 54 31 36 20 20 20 33 C0 ME FAT16 3.

00000040: 8E D0 BC 00 7C 68 C0 07 - 1F A0 10 00 F7 26 16 00 ....|h......&...

00000050: 03 06 0E 00 50 91 B8 20 - 00 F7 26 11 00 8B 1E 0B ....P.. ..&.....

00000060: 00 03 C3 48 F7 F3 03 C8 - 89 0E 08 02 68 00 10 07 ...H........h...

00000070: 33 DB 8F 06 13 02 89 1E - 15 02 0E E8 90 00 72 57 3.............rW

00000080: 33 DB 8B 0E 11 00 8B FB - 51 B9 0B 00 BE DC 01 F3 3.......Q.......

00000090: A6 59 74 05 83 C3 20 E2 - ED E3 37 26 8B 57 1A 52 .Yt... ...7&.W.R

000000A0: B8 01 00 68 00 20 07 33 - DB 0E E8 48 00 72 28 5B ...h. .3...H.r([

000000B0: 8D 36 0B 00 8D 3E 0B 02 - 1E 8F 45 02 C7 05 F5 00 .6...>....E.....

000000C0: 1E 8F 45 06 C7 45 04 0E - 01 8A 16 24 00 EA 03 00 ..E..E.....$....

000000D0: 00 20 BE 86 01 EB 03 BE - A2 01 E8 09 00 BE C1 01 . ..............

000000E0: E8 03 00 FB EB FE AC 0A - C0 74 09 B4 0E BB 07 00 .........t......

000000F0: CD 10 EB F2 C3 50 4A 4A - A0 0D 00 32 E4 F7 E2 03 .....PJJ...2....

00000100: 06 08 02 83 D2 00 A3 13 - 02 89 16 15 02 58 A2 07 .............X..

00000110: 02 A1 13 02 8B 16 15 02 - 03 06 1C 00 13 16 1E 00 ................

00000120: F7 36 18 00 FE C2 88 16 - 06 02 33 D2 F7 36 1A 00 .6........3..6..

00000130: 88 16 25 00 A3 04 02 A1 - 18 00 2A 06 06 02 40 3A ..%.......*...@:

00000140: 06 07 02 76 05 A0 07 02 - 32 E4 50 B4 02 8B 0E 04 ...v....2.P.....

00000150: 02 C0 E5 06 0A 2E 06 02 - 86 E9 8B 16 24 00 CD 13 ............$...

00000160: 0F 83 05 00 83 C4 02 F9 - CB 58 28 06 07 02 76 11 .........X(...v.

00000170: 01 06 13 02 83 16 15 02 - 00 F7 26 0B 00 03 D8 EB ..........&.....

00000180: 90 A2 07 02 F8 CB 42 4F - 4F 54 3A 20 43 6F 75 6C ......BOOT: Coul

00000190: 64 6E 27 74 20 66 69 6E - 64 20 4E 54 4C 44 52 0D dn't find NTLDR.

000001A0: 0A 00 42 4F 4F 54 3A 20 - 49 2F 4F 20 65 72 72 6F ..BOOT: I/O erro

000001B0: 72 20 72 65 61 64 69 6E - 67 20 64 69 73 6B 0D 0A r reading disk..

000001C0: 00 50 6C 65 61 73 65 20 - 69 6E 73 65 72 74 20 61 .Please insert a

000001D0: 6E 6F 74 68 65 72 20 64 - 69 73 6B 00 4E 54 4C 44 nother disk.NTLD

000001E0: 52 20 20 20 20 20 20 00 - 00 00 00 00 00 00 00 00 R .........

000001F0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 55 AA ..............U.

FAT12/FAT16 Boot Sector

Table 32.6 describes the boot sector of a volume formatted with the FAT12/FAT16 file system.

Table 32.6 Boot Sector Sections on a FAT12/FAT16 Volume

Tables 32.7 and 32.8 illustrate the layout of the BPB and the extended BPB for FAT12/FAT16 volumes. The sample values correspond to the data in this example.

Table 32.7 BPB Fields for FAT12/FAT16 Volumes

Media Descriptor

For the Media Descriptor field, the following table lists some of the recognized media descriptor values and their associated media. A media descriptor value can be associated with more than one disk capacity.

Table 32.8 Extended BPB Fields for FAT12/FAT16 Volumes

FAT Type and Cluster Size Depends on Logical Drive Size

MS-DOS allocates disk space for files in units of one or more sectors; these units are called "clusters" or "allocation units." On any MS-DOS disk, a 1-byte file is allocated 1 cluster of disk space, wasting the unused area of the cluster. A file that is 3.2 clusters large is given 4 clusters. Overall, a smaller cluster size means less waste.

The cluster size for a drive is decided by FORMAT, depending on the size of the logical drive(see table, below). "Logical drive" refers to an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and so forth). Hard disk users may want to consider cluster size when choosing how to partition their drive(s).

The cluster size of a floppy drive cannot be changed. The cluster size of a hard drive can be changed only by changing the size of the logical drive, which is done by repartitioning the hard drive.

CHKDSK displays the allocation unit size for a logical drive. FDISK's option 4 displays the size(s) of logical drives on the hard drive(s).

[note]:

DoubleSpace-compressed drives appear to have 8K clusters, but internally vary the sectors-per-cluster as necessary (this information is recorded in the MDFAT). For example, a 10K file which compresses by a factor of 2:1 actually uses 5K, or 10 sectors, of drive space. The following is a table of logical drive sizes, FAT (File Allocation Table) types, and cluster sizes:

Drive Size FAT Type Sectors Cluster

(logical volume) Per Cluster Size

------------------ --------- ------------- ---------

(Floppy Disks) 360K 12bit 2 1K

720K 12bit 2 1K

1.2MB 12bit 1 512 bytes

1.44MB 12bit 1 512 bytes

2.88MB 12bit 2 1K

(Hard Disks) 0 MB - 15 MB 12bit 8 4K

16 MB - 127 MB 16bit 4 2K

128 MB - 255 MB 16bit 8 4K

256 MB - 511 MB 16bit 16 8K

512 MB - 1023 MB 16/32bit 32/ 16/4K

1024 MB - 2048 MB 16/32bit 64/ 32/4K

2 GB – 8 GB 32bit 4K

8 GB – 16 GB 32bit 8K

16 GB – 32 GB 32bit 16K

>32 GB 32bit 32K

[note]:

(1) Sectors are 512 bytes in size, except on some RAM drives.

(2) In the past, some OEMs have modified their versions of MS-DOS to support other sector and/or cluster sizes. The Microsoft MS-DOS 5 Upgrade Setup will, if possible, convert the logical drive to MS-DOS 5.0 compatible. This entails converting the sector size to 512 bytes while retaining the nonstandard cluster size.

(3) MS-DOS determines the FAT size based on the number of clusters. If there are 4085 or fewer clusters, a 12-bit FAT is used. If there are 4086 or more clusters, a 16-bit FAT is used.

Partition Table

This document concerns only hard drives. Floppy disks, CD-ROM, and ZIP drives are not normally split into partitions. You should familiarize yourself with CHS and LBA sector addressing modes before reading this document.

The partition table, a 64-byte data structure used to identify the type and location of partitions on a hard disk, conforms to a standard layout independent of the operating system. Each partition table entry is 16 bytes long, with a maximum of four entries. Each entry starts at a predetermined offset from the beginning of the sector, as follows: Partition 1- 0x01BE, Partition 2 - 0x01CE, Partition 3 - 0x01DE, Partition 4 - 0x01EE.

Table 32.2 Partition Table Fields

Byte Offset	Field Length	Sample Value	Field Name and Definition
0x01BE	1 byte	0x80	Boot Indicator. Indicates whether the volume is the active partition. Legal values include : (00):Do not use for booting. (80):Active partition.
0x01BF	1 byte	0x01	Starting Head.
0x01C0	6 bits	0x01	Starting Sector. Only bits 0-5 are used. The upper two bits, 6 and 7, are used by the Starting Cylinder field.
0x01C1	10 bits	0x00 *	Starting Cylinder. Uses 1 byte in addition to the upper 2 bits from the Starting Sector field to make up the cylinder value. The Starting Cylinder is a 10-bit number, with a maximum value of 1023.
0x01C2	1 byte	0x07	System ID. Defines the volume type. See Table 32.3 for sample values.
0x01C3	1 byte	0xFE	Ending Head.
0x01C4	6 bits	0xBF *	Ending Sector. Only bits 0-5 are used. The upper two bits, 6 and 7, are used by the Ending Cylinder field.
0x01C5	10 bits	0x09 *	Ending Cylinder. Uses 1byte in addition to the upper 2 bits from the Ending Sector field to make up the cylinder value. The Ending Cylinder is a 10-bit number, with a maximum value of 1023.
0x01C6	4 bytes	0x3F000000	Relative Sectors. The offset from the beginning of the disk to the beginning of the volume, counting by sectors.
0x01CA	4 bytes	0x4BF57F00	Total Sectors. The total number of sectors in the volume.
A BYTE is 8 bits, a WORD is 16 bits, a DWORD is 32 bits, and a LONGLONG is 64 bits. Sample values marked with an asterisk (*) do not accurately represent the value of the fields, because the fields are either 6 bits or 10 bits and the data is recorded in bytes. Numbers larger than one byte are stored in little endian format, or reverse-byte ordering. Little endian format is a method of storing a number so that the least significant byte appears first in the hexa 全站熱搜 創作者介紹 立你斯 立你斯學習記錄 立你斯 發表在 痞客邦 留言(0) 人氣() E-mail轉寄 全站分類：不設分類 個人分類：各種規格 此分類上一篇： FAT16 與 FAT32 的結構[轉貼自小木偶] 此分類下一篇： 檔案配置表（File Allocation Table） 上一篇： FAT16 與 FAT32 的結構[轉貼自小木偶] 下一篇： 檔案配置表（File Allocation Table） 歷史上的今天 2007: 檔案配置表（File Allocation Table） 2007: FAT16 與 FAT32 的結構[轉貼自小木偶] 2007: FAT12 ( 檔案配置表 ) 簡述[轉貼自小木偶] ▲top 留言列表 發表留言 熱門文章 文章分類 程式語言 (17) 8051 (5) openCL (2) C++ (1) GPS (4) BarCode (3) 多媒體 (7) 網路相關 (23) 程式設計 (20) ARM (22) u-boot (19) 各種規格 (41) 程式心得 (34) 程式工具 (18) 版本控制 (13) RTOS (5) uC/OS 筆記 (12) Objective-C (3) 其他 (8) 三國風雲 (4) SCRATCH (1) 產業消息 (2) 未分類資料夾 (12) 娛樂 (13) 語文學習 (8) 理財 (7) 幼教 (2) Linux Driver (9) PCI (6) driver 教學 (17) driver 問題修正 (11) 嵌入式 driver 相關 (10) USB (8) I2C (7) V4L2 (8) Driver 其他 (29) Alsa (15) Linux (19) WIFI (14) yocto (4) GUI (10) CentOS (4) Linux system programming (8) 工作使用 (2) 模擬-VM (3) 編譯相關 (26) tslib (9) 檔案系統 (18) 嵌入式相關 (10) 系統相關 (21) LINUX (57) 模擬-colinux (6) 模擬-cygwin (10) Android (21) Mplayer (11) tool chain (6) 系統指令 (6) NV (1) 未分類文章 (17) 最新文章 文章精選 文章精選 2024 二月 (1) 2023 八月 (1) 2023 六月 (1) 2023 三月 (2) 2022 七月 (2) 2019 十二月 (3) 2019 十月 (7) 2019 九月 (1) 2019 八月 (3) 2019 七月 (1) 2019 六月 (2) 2019 四月 (1) 2019 三月 (12) 2018 十月 (3) 2018 九月 (8) 2018 八月 (3) 2018 七月 (5) 2018 六月 (2) 2018 五月 (6) 2018 四月 (12) 2018 三月 (4) 2018 一月 (3) 2017 十一月 (4) 2017 九月 (1) 2017 七月 (7) 2017 六月 (1) 2017 五月 (3) 2017 三月 (4) 2017 二月 (3) 2017 一月 (3) 2016 十二月 (10) 2016 十一月 (10) 2016 八月 (1) 2016 七月 (5) 2016 六月 (1) 2016 五月 (5) 2016 三月 (11) 2015 十月 (2) 2015 九月 (1) 2015 七月 (1) 2015 六月 (2) 2015 四月 (6) 2015 三月 (4) 2015 二月 (5) 2015 一月 (3) 2014 十二月 (5) 2014 十一月 (5) 2014 十月 (1) 2014 九月 (3) 2014 八月 (1) 2014 七月 (5) 2014 六月 (2) 2014 五月 (5) 2014 四月 (5) 2014 三月 (5) 2014 二月 (1) 2014 一月 (5) 2013 十二月 (11) 2013 十一月 (6) 2013 十月 (11) 2013 五月 (1) 2013 四月 (1) 2013 三月 (4) 2013 二月 (3) 2013 一月 (1) 2012 十一月 (4) 2012 十月 (1) 2012 九月 (3) 2012 八月 (6) 2012 七月 (8) 2012 六月 (16) 2012 五月 (4) 2012 四月 (2) 2012 三月 (2) 2012 二月 (2) 2012 一月 (3) 2011 十二月 (13) 2011 十一月 (4) 2011 十月 (5) 2011 九月 (1) 2011 八月 (2) 2011 七月 (3) 2011 六月 (4) 2011 五月 (2) 2011 四月 (1) 2011 三月 (13) 2011 二月 (5) 2011 一月 (12) 2010 十二月 (5) 2010 十一月 (5) 2010 十月 (5) 2010 九月 (5) 2010 八月 (2) 2010 七月 (8) 2010 六月 (9) 2010 五月 (13) 2010 四月 (7) 2010 二月 (7) 2010 一月 (7) 2009 十二月 (5) 2009 十一月 (24) 2009 十月 (6) 2009 九月 (8) 2009 八月 (9) 2009 七月 (12) 2009 六月 (3) 2009 三月 (9) 2009 二月 (22) 2008 十二月 (8) 2008 十一月 (2) 2008 十月 (2) 2008 九月 (2) 2008 五月 (3) 2008 四月 (7) 2008 一月 (1) 2007 十二月 (7) 2007 十一月 (6) 2007 十月 (20) 2007 八月 (1) 2007 七月 (3) 2007 六月 (15) 2007 五月 (19) 2007 四月 (16) 所有文章列表 文章搜尋 最新留言 參觀人氣 本日人氣： 累積人氣： 我的好友 誰來我家 QR Code POWERED BY (登入) 回到頁首 回到主文 免費註冊 客服中心 痞客邦首頁 © 2003 - 2024 PIXNET 關閉視窗