Technical overview
The name of the file system originates from the file system's prominent usage of an index table, the '' File Allocation Table'', statically allocated at the time of formatting. The table contains entries for each ''Layout
A FAT file system is composed of four regions: ; Reserved sectors : The first reserved sector (logical sector 0) is the Boot Sector (also called '' Volume Boot Record'' or simply ''VBR''). It includes an area called the '' BIOS Parameter Block'' (''BPB'') which contains some basic file system information, in particular its type and pointers to the location of the other sections, and usually contains the operating system'sReserved sectors area
Boot Sector
On non-partitioned storage devices, such as floppy disks, the Boot Sector ( VBR) is the first sector (logical sector 0 with physical CHS address 0/0/1 or LBA address 0). For partitioned storage devices such as hard disks, the Boot Sector is the first sector of a partition, as specified in the partition table of the device. Common structure of the Boot Sector used by most FAT versions for IBM compatible x86 machines since DOS 2.0 are: FAT-formatted Atari ST floppies have a very similar boot sector layout: FAT12-formattedBIOS Parameter Block
Common structure of the first 25 bytes of the BIOS Parameter Block (BPB) used by FAT versions since DOS 2.0 (bytes at sector offset to are stored since DOS 2.0, but not always used before DOS 3.2, values at to are used since DOS 3.0): DOS 3.0 BPB: The following extensions were documented since DOS 3.0, however, they were already supported by some issues of DOS 2.11. MS-DOS 3.10 still supported the DOS 2.0 format, but could use the DOS 3.0 format as well. DOS 3.2 BPB: Officially, MS-DOS 3.20 still used the DOS 3.0 format, but SYS
and FORMAT
Format may refer to:
Printing and visual media
* Text formatting, the typesetting of text elements
* Paper formats, or paper size standards
* Newspaper format, the size of the paper page
Computing
* File format, particular way that informatio ...
were adapted to support a 6 bytes longer format already (of which not all entries were used).
DOS 3.31 BPB:
Officially introduced with DOS 3.31 and not used by DOS 3.2, some DOS 3.2 utilities were designed to be aware of this new format already. Official documentation recommends to trust these values only if the logical sectors entry at offset is zero.
A simple formula translates a volume's given cluster number CN
to a logical sector number LSN
:
# Determine (once) SSA=RSC+FN×SF+ceil((32×RDE)/SS)
, where the reserved sector count RSC
is stored at offset , the number of FATsFN
at offset , the sectors per FAT SF
at offset (FAT12/FAT16) or (FAT32), the root directory entries RDE
at offset , the sector size SS
at offset , and ceil(x)
rounds up to a whole number.
# Determine LSN=SSA+(CN−2)×SC
, where the sectors per cluster SC
are stored at offset .
On unpartitioned media the volume's number of hidden sectors is zero and therefore LSN
and LBA
addresses become the same for as long as a volume's logical sector size is identical to the underlying medium's physical sector size. Under these conditions, it is also simple to translate between CHS
addresses and LSNs
as well:
LSN=SPT×(HN+(NOS×TN))+SN−1
, where the sectors per track SPT
are stored at offset , and the number of sides NOS
at offset . Track number TN
, head number HN
, and sector number SN
correspond to Extended BIOS Parameter Block
Further structure used by FAT12 and FAT16 since OS/2 1.0 and DOS 4.0, also known as Extended BIOS Parameter Block (EBPB) (bytes below sector offset are the same as for the DOS 3.31 BPB):FAT32 Extended BIOS Parameter Block
In essence FAT32 inserts 28 bytes into the EBPB, followed by the remaining 26 (or sometimes only 7) EBPB bytes as shown above for FAT12 and FAT16. Microsoft and IBM operating systems determine the type of FAT file system used on a volume solely by the number of clusters, not by the used BPB format or the indicated file system type, that is, it is technically possible to use a "FAT32 EBPB" also for FAT12 and FAT16 volumes as well as a DOS 4.0 EBPB for small FAT32 volumes. Since such volumes were found to be created by Windows operating systems under some odd conditions, operating systems should be prepared to cope with these hybrid forms.Exceptions
Versions of DOS before 3.2 totally or partially relied on the media descriptor byte in the BPB or theFS Information Sector
The "FS Information Sector" was introduced in FAT32 for speeding up access times of certain operations (in particular, getting the amount of free space). It is located at a logical sector number specified in the FAT32 EBPB boot record at position (usually logical sector 1, immediately after the boot record itself). The sector's data may be outdated and not reflect the current media contents, because not all operating systems update or use this sector, and even if they do, the contents is not valid when the medium has been ejected without properly unmounting the volume or after a power-failure. Therefore, operating systems should first inspect a volume's optional shutdown status bitflags residing in the FAT entry of cluster 1 or the FAT32 EBPB at offset and ignore the data stored in the FS information sector, if these bitflags indicate that the volume was not properly unmounted before. This does not cause any problems other than a possible speed penalty for the first free space query or data cluster allocation; see fragmentation. If this sector is present on a FAT32 volume, the minimum allowed logical sector size is 512 bytes, whereas otherwise it would be 128 bytes. Some FAT32 implementations support a slight variation of Microsoft's specification by making the FS information sector optional by specifying a value of (or ) in the entry at offset .File Allocation Table
Cluster map
A volume's data area is divided into identically sized ''clusters''—small blocks of contiguous space. Cluster sizes vary depending on the type of FAT file system being used and the size of the drive; typical cluster sizes range from 2 to . Each file may occupy one or more clusters depending on its size. Thus, a file is represented by a chain of clusters (referred to as aSpecial entries
The first two entries in a FAT store special values: The first entry (cluster 0 in the FAT) holds the FAT ID sinceCluster values
FAT entry values: FAT32 uses 28 bits for cluster numbers. The remaining 4 bits in the 32-bit FAT entry are usually zero, but are reserved and should be left untouched. A standard conformant FAT32 file system driver or maintenance tool must not rely on the upper 4 bits to be zero and it must strip them off before evaluating the cluster number in order to cope with possible future expansions where these bits may be used for other purposes. They must not be cleared by the file system driver when allocating new clusters, but should be cleared during a reformat.Size limits
The FAT12, FAT16, FAT16B, and FAT32 variants of the FAT file systems have clear limits based on the number of clusters and the number of sectors per cluster (1, 2, 4, ..., 128). For the typical value of 512 bytes per sector::Legend: 268435444+3 is , because FAT32 version 0 uses only 28 bits in the 32-bit cluster numbers, cluster numbers up to flag bad clusters or the end of a file, cluster number 0 flags a free cluster, and cluster number 1 is not used. Likewise 65524+3 is for FAT16, and 4084+3 is for FAT12. The number of sectors per cluster is a power of 2 fitting in a single byte, the smallest value is 1 (), the biggest value is 128 (). Lines in square brackets indicate the unusual cluster size 128, and for FAT32 the bigger than necessary cluster sizes 32 or 64. Because each FAT32 entry occupies 32 bits (4 bytes) the maximal number of clusters (268435444) requires 2097152 FAT sectors for a sector size of 512 bytes. 2097152 is , and storing this value needs more than two bytes. Therefore, FAT32 introduced a new 32-bit value in the FAT32 boot sector immediately following the 32-bit value for the total number of sectors introduced in the FAT16B variant. The boot record extensions introduced with DOS 4.0 start with a magic 40 () or 41 (). Typically FAT drivers look only at the number of clusters to distinguish FAT12, FAT16, and FAT32: the human readable strings identifying the FAT variant in the boot record are ignored, because they exist only for media formatted with DOS 4.0 or later. Determining the number of directory entries per cluster is straightforward. Each entry occupies 32 bytes; this results in 16 entries per sector for a sector size of 512 bytes. The DOS 5FAT12 requirements : 3 sectors on each copy of FAT for every 1,024 clusters FAT16 requirements : 1 sector on each copy of FAT for every 256 clusters FAT32 requirements : 1 sector on each copy of FAT for every 128 clusters FAT12 range : 1 to 4,084 clusters : 1 to 12 sectors per copy of FAT FAT16 range : 4,085 to 65,524 clusters : 16 to 256 sectors per copy of FAT FAT32 range : 65,525 to 268,435,444 clusters : 512 to 2,097,152 sectors per copy of FAT FAT12 minimum : 1 sector per cluster × 1 clusters = 512 bytes (0.5 KiB) FAT16 minimum : 1 sector per cluster × 4,085 clusters = 2,091,520 bytes (2,042.5 KB) FAT32 minimum : 1 sector per cluster × 65,525 clusters = 33,548,800 bytes (32,762.5 KB) FAT12 maximum : 64 sectors per cluster × 4,084 clusters = 133,824,512 bytes (≈ 127 MB) AT12 maximum : 128 sectors per cluster × 4,084 clusters = 267,694,024 bytes (≈ 255 MB) FAT16 maximum : 64 sectors per cluster × 65,524 clusters = 2,147,090,432 bytes (≈2,047 MB) AT16 maximum : 128 sectors per cluster × 65,524 clusters = 4,294,180,864 bytes (≈4,095 MB) FAT32 maximum : 8 sectors per cluster × 268,435,444 clusters = 1,099,511,578,624 bytes (≈1,024 GB) FAT32 maximum : 16 sectors per cluster × 268,173,557 clusters = 2,196,877,778,944 bytes (≈2,046 GB) AT32 maximum : 32 sectors per cluster × 134,152,181 clusters = 2,197,949,333,504 bytes (≈2,047 GB) AT32 maximum : 64 sectors per cluster × 67,092,469 clusters = 2,198,486,024,192 bytes (≈2,047 GB) AT32 maximum : 128 sectors per cluster × 33,550,325 clusters = 2,198,754,099,200 bytes (≈2,047 GB)
RMDIR
In computing, rmdir (or rd) is a command which will remove an empty directory on various operating systems.
Implementations
The command is available in Unix (e.g. macOS, Solaris, AIX, HP-UX), Unix-like (e.g. FreeBSD, Linux), DOS, Digital Resear ...
/RD
command removes the initial ".
" (this directory) and "..
" (parent directory) entries in subdirectories directly, therefore sector size 32 on a RAM disk is possible for FAT12, but requires 2 or more sectors per cluster. A FAT12 boot sector without the DOS 4 extensions needs 29 bytes before the first unnecessary FAT16B 32-bit number of hidden sectors, this leaves three bytes for the (on a RAM disk unused) boot code and the magic at the end of all boot sectors. On FORMAT
command options /A:128K
and /A:256K
correspond to the maximal cluster size 0x80
(128) with a sector size 1024 and 2048, respectively. For the common sector size 512 /A:64K
yields 128 sectors per cluster.
Both editions of each ECMA-107 and ISO/IEC 9293 specify a ''Max Cluster Number'' MAX
determined by the formula MAX=1+trunc((TS-SSA)/SC)
, and reserve cluster numbers MAX+1
up to 4086 (, FAT12) and later 65526 (, FAT16) for future standardization.
Microsoft's EFI FAT32 specification states that any FAT file system with less than 4085 clusters is FAT12, else any FAT file system with less than 65,525 clusters is FAT16, and otherwise it is FAT32. The entry for cluster 0 at the beginning of the FAT must be identical to the media descriptor byte found in the BPB, whereas the entry for cluster 1 reflects the end-of-chain value used by the formatter for cluster chains (, or ). The entries for cluster numbers 0 and 1 end at a byte boundary even for FAT12, e.g., for media descriptor .
The first data cluster is 2, and consequently the last cluster MAX
gets number MAX+1
. This results in data cluster numbers 2...4085 () for FAT12, 2...65525 () for FAT16, and 2...268435445 () for FAT32.
The only available values reserved for future standardization are therefore (FAT12) and (FAT16). As noted below "less than 4085" is also used for Linux implementations, or as ...when it says <, it does not mean <=. Note also that the numbers are correct. The first number for FAT12 is 4085; the second number for FAT16 is 65525. These numbers and the "<" signs are not wrong.
Fragmentation
The FAT file system does not contain built-in mechanisms which prevent newly written files from becoming scattered across the partition. On volumes where files are created and deleted frequently or their lengths often changed, the medium will become increasingly fragmented over time. While the design of the FAT file system does not cause any organizational overhead in disk structures or reduce the amount of free storage space with increased amounts of fragmentation, as it occurs with external fragmentation, the time required to read and write fragmented files will increase as the operating system will have to follow the cluster chains in the FAT (with parts having to be loaded into memory first in particular on large volumes) and read the corresponding data physically scattered over the whole medium reducing chances for the low-level block device driver to perform multi-sector disk I/O or initiate larger DMA transfers, thereby effectively increasing I/O protocol overhead as well as arm movement and head settle times inside the disk drive. Also, file operations will become slower with growing fragmentation as it takes increasingly longer for the operating system to find files or free clusters. Other file systems, e.g., HPFS or DIR
" operation, which always displays the free disk space as the last line. Displaying this line took longer and longer as the number of clusters increased. FAT32 therefore introduced a special file system information sector where the previously computed amount of free space is preserved over power cycles, so that the free space counter needs to be recalculated only when a removable FAT32 formatted medium gets ejected without first unmounting it or if the system is switched off without properly shutting down the operating system, a problem mostly visible with pre-FASTOPEN
In computing, FASTOPEN is a DOS terminate-and-stay-resident command, introduced in MS-DOS version 3.3, that provides accelerated access to frequently-used files and directories. The command is also available in SISNE plus.
Overview
The command w ...
by keeping track of the position of recently opened files or directories in various forms of lists (MS-DOS/PC DOS) or hash tables (DR-DOS), which can reduce file seek and open times significantly. Before DOS 5.0 special care must be taken when using such mechanisms in conjunction with disk defragmentation software bypassing the file system or disk drivers.
Windows NT will allocate disk space to files on FAT in advance, selecting large contiguous areas, but in case of a failure, files which were being appended will appear larger than they were ever written into, with a lot of random data at the end.
Other high-level mechanisms may read in and process larger parts or the complete FAT on startup or on demand when needed and dynamically build up in-memory tree representations of the volume's file structures different from the on-disk structures. This may, on volumes with many free clusters, occupy even less memory than an image of the FAT itself. In particular on highly fragmented or filled volumes, seeks become much faster than with linear scans over the actual FAT, even if an image of the FAT would be stored in memory. Also, operating on the logically high level of files and cluster-chains instead of on sector or track level, it becomes possible to avoid some degree of file fragmentation in the first place or to carry out local file defragmentation and reordering of directory entries based on their names or access patterns in the background.
Some of the perceived problems with fragmentation of FAT file systems also result from performance limitations of the underlying block device drivers, which becomes more visible the lesser memory is available for sector buffering and track blocking/deblocking:
While the single-tasking DOS had provisions for multi-sector reads and track blocking/deblocking, the operating system and the traditional PC hard disk architecture ( only one outstanding input/output request at a time and no DMA transfers) originally did not contain mechanisms which could alleviate fragmentation by asynchronously prefetching next data while the application was processing the previous chunks. Such features became available later. Later DOS versions also provided built-in support for look-ahead sector buffering and came with dynamically loadable disk caching programs working on physical or logical sector level, often utilizing EMS or XMS memory and sometimes providing adaptive caching strategies or even run in protected mode through DPMS or Directory table
A ''directory table'' is a special type of file that represents aA
–Z
* Numbers 0
–9
* Space (though trailing spaces in either the base name or the extension are considered to be padding and not a part of the file name; also filenames with space in them could not easily be used on the DOS command line prior to Windows 95 because of the lack of a suitable escaping system). Another exception are the internal commands MKDIR
The mkdir (make directory) command in the Unix, DOS, DR FlexOS, IBM OS/2, Microsoft Windows, and ReactOS operating systems is used to make a new directory. It is also available in the EFI shell and in the PHP scripting language. In DOS, OS/2, ...
/MD
and RMDIR
In computing, rmdir (or rd) is a command which will remove an empty directory on various operating systems.
Implementations
The command is available in Unix (e.g. macOS, Solaris, AIX, HP-UX), Unix-like (e.g. FreeBSD, Linux), DOS, Digital Resear ...
/RD
under DR-DOS which accept single arguments and therefore allow spaces to be entered.
* ! # $ % & ' ( ) - @ ^ _ ` ~
* Characters 128–228
* Characters 230–255
This excludes the following " * / : < > ? \ ,
+ , . ; = /code>
Allowed in long file names only
* Lower case letters a
–z
Stored as A
–Z
; allowed in long file names
* Control characters 0–31
* Character 127 (DEL)
Character 229 () was not allowed as first character in a filename in DOS 1 and 2 due to its use as free entry marker. A special case was added to circumvent this limitation with DOS 3.0 and higher.
The following additional characters are allowed on Atari's GEMDOS, but should be avoided for compatibility with MS-DOS/PC DOS:
* " + , ; < = > ,
The semicolon (;
) should be avoided in filenames under DR DOS 3.31 and higher, PalmDOS, Novell DOS, OpenDOS, Concurrent DOS, Multiuser DOS, System Manager and REAL/32, because it may conflict with the syntax to specify file and directory passwords: "...\DIRSPEC.EXT;DIRPWD\FILESPEC.EXT;FILEPWD
". The operating system will strip off one (and also two—since DR-DOS 7.02) semicolons and pending passwords from the filenames before storing them on disk. (The command processor 4DOS
4DOS is a command-line interpreter by JP Software, designed to replace the default command interpreter COMMAND.COM in Microsoft DOS and Windows. It was written by Rex C. Conn and Tom Rawson and first released in 1989. Compared to the default, ...
uses semicolons for include lists and requires the semicolon to be doubled for password protected files with any commands supporting wildcards.)
The at-sign character (@
) is used for filelists by many DR-DOS, PalmDOS, Novell DOS, OpenDOS and Multiuser DOS, System Manager and REAL/32 commands, as well as by 4DOS and may therefore sometimes be difficult to use in filenames.
Under Multiuser DOS and REAL/32, the exclamation mark (!) is not a valid filename character since it is used to separate multiple commands in a single command line.
Under IBM 4680 OS and 4690 OS, the following characters are not allowed in filenames:
* ? * : . ; , ! + = < > " - / \ ,
Additionally, the following special characters are not allowed in the first, fourth, fifth and eight character of a filename, as they conflict with the host command processor (HCP) and input sequence table build file names:
* @ # ( ) $ &
The DOS file names are in the current OEM character set
Windows code pages are sets of characters or code pages (known as character encodings in other operating systems) used in Microsoft Windows from the 1980s and 1990s. Windows code pages were gradually superseded when Unicode in Microsoft Windows, Un ...
: this can have surprising effects if characters handled in one way for a given code page are interpreted differently for another code page (DOS command CHCP
) with respect to lower and upper case, sorting, or validity as file name character.
Directory entry
Before Microsoft added support for long filenames and creation/access time stamps, bytes – of the directory entry were used by other operating systems to store additional metadata, most notably the operating systems of the Digital Research family stored file passwords, access rights, owner IDs, and file deletion data there. While Microsoft's newer extensions are not fully compatible with these extensions by default, most of them can coexist in third-party FAT implementations (at least on FAT12 and FAT16 volumes).
32-byte directory entries, both in the Root Directory Region and in subdirectories, are of the following format (see also 8.3 filename
An 8.3 filename (also called a short filename or SFN) is a filename convention used by old versions of DOS and versions of Microsoft Windows prior to Windows 95 and Windows NT 3.5. It is also used in modern Microsoft operating systems as an alterna ...
):
The FlexOS
FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...
-based operating systems IBM 4680 OS
FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...
and IBM 4690 OS support unique distribution attributes stored in some bits of the previously reserved areas in the directory entries:
# Local: Don't distribute file but keep on local controller only.
# Mirror file on update: Distribute file to server only when file is updated.
# Mirror file on close: Distribute file to server only when file is closed.
# Compound file on update: Distribute file to all controllers when file is updated.
# Compound file on close: Distribute file to all controllers when file is closed.
Some incompatible extensions found in some operating systems include:
VFAT long file names
VFAT
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
Long File Names (LFNs) are stored on a FAT file system using a trick: adding additional entries into the directory before the normal file entry. The additional entries are marked with the Volume Label, System, Hidden, and Read Only attributes (yielding ), which is a combination that is not expected in the MS-DOS environment, and therefore ignored by MS-DOS programs and third-party utilities. Notably, a directory containing only volume labels is considered as empty and is allowed to be deleted; such a situation appears if files created with long names are deleted from plain DOS. This method is very similar to the DELWATCH method to utilize the volume attribute to hide pending delete files for possible future undeletion since DR DOS 6.0 (1991) and higher. It is also similar to a method publicly discussed to store long filenames on Ataris and under Linux in 1992.
Because older versions of DOS could mistake LFN names in the root directory for the volume label, VFAT was designed to create a blank volume label in the root directory before adding any LFN name entries (if a volume label did not already exist).
Each phony entry can contain up to 13 UCS-2
The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), w ...
characters (26 bytes) by using fields in the record which contain file size or time stamps (but not the starting cluster field, for compatibility with disk utilities, the starting cluster field is set to a value of 0. See 8.3 filename
An 8.3 filename (also called a short filename or SFN) is a filename convention used by old versions of DOS and versions of Microsoft Windows prior to Windows 95 and Windows NT 3.5. It is also used in modern Microsoft operating systems as an alterna ...
for additional explanations). Up to 20 of these 13-character entries may be chained, supporting a maximum length of 255 UCS-2 characters.
After the last UCS-2
The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), w ...
character, a is added. The remaining unused characters are filled with .
LFN entries use the following format:
If there are multiple LFN entries required to represent a file name, the entry representing the ''end'' of the filename comes first. The sequence number of this entry has bit 6 () set to represent that it is the last logical LFN entry, and it has the highest sequence number. The sequence number decreases in the following entries. The entry representing the ''start'' of the filename has sequence number 1. A value of is used to indicate that the entry is deleted.
On FAT12 and FAT16 volumes, testing for the values at to be zero and at to be non-zero can be used to distinguish between VFAT LFNs and pending delete files under DELWATCH.
For example, a filename like "File with very long filename.ext" would be formatted like this:
A checksum
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...
also allows verification of whether a long file name matches the 8.3 name; such a mismatch could occur if a file was deleted and re-created using DOS in the same directory position. The checksum is calculated using the algorithm below. (pFCBName is a pointer to the name as it appears in a regular directory entry, i.e. the first eight characters are the filename, and the last three are the extension. The dot is implicit. Any unused space in the filename is padded with space characters (ASCII ). For example, "Readme.txt" would be "README␠␠TXT
".)
unsigned char lfn_checksum(const unsigned char *pFCBName)
If a filename contains only lowercase letters, or is a combination of a lowercase ''basename'' with an uppercase ''extension'', or vice versa; and has no special characters, and fits within the 8.3 limits, a VFAT entry is not created on Windows NT and later versions of Windows such as XP. Instead, two bits in byte of the directory entry are used to indicate that the filename should be considered as entirely or partially lowercase. Specifically, bit 4 means lowercase ''extension'' and bit 3 lowercase ''basename'', which allows for combinations such as "example.TXT
" or "HELLO.txt
" but not "Mixed.txt
". Few other operating systems support it. This creates a backwards-compatibility problem with older Windows versions (Windows 95 / 98 / 98 SE / ME) that see all-uppercase filenames if this extension has been used, and therefore can change the name of a file when it is transported between operating systems, such as on a USB flash drive. Current 2.6.x versions of Linux will recognize this extension when reading (source: kernel 2.6.18 /fs/fat/dir.c
and fs/vfat/namei.c
); the mount option shortname
determines whether this feature is used when writing.
See also
* Comparison of file systems
The following tables compare general and technical information for a number of file systems.
General information
Limits
Metadata
Features File capabilities
Block capabilities
Note that in addition to the below table, blo ...
* Drive letter assignment
In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, dri ...
* exFAT
exFAT (Extensible File Allocation Table) is a file system introduced by Microsoft in 2006 and optimized for flash memory such as USB flash drives and SD cards. exFAT was proprietary until 28 August 2019, when Microsoft published its specificati ...
* Extended Boot Record
An extended boot record (EBR), or extended partition boot record (EPBR), is a descriptor for a logical partition under the common DOS disk drive partitioning system. In that system, when one (and only one) partition record entry in the master boot ...
(EBR)
* FAT filesystem and Linux
* List of file systems
The following lists identify, characterize, and link to more thorough information on Computer file systems.
Many older operating systems support only their one "native" file system, which does not bear any name apart from the name of the operating ...
* Master Boot Record (MBR)
* Partition type
The partition type (or partition ID) in a partition's entry in the partition table inside a master boot record (MBR) is a byte value intended to specify the file system the partition contains or to flag special access methods used to access thes ...
* Timeline of DOS operating systems
This article presents a timeline of events in the history of 16-bit x86 DOS-family disk operating systems from 1980 to present. Non-x86 operating systems named "DOS" are not part of the scope of this timeline.
Also presented is a timeline of ...
* Transaction-Safe FAT File System Transaction-Safe FAT File System (TFAT) and Transaction-Safe Extended FAT File System (TexFAT) refer to two file systems used in Microsoft products to provide transaction-safety for data stored on a disk. The goal is to reduce the risk of data loss ...
* Turbo FAT
In computing, a NetWare File System (NWFS) is a file system based on a heavily modified version of FAT. It was used in the Novell NetWare operating system. It is the default and only file system for all volumes in versions 2.x through 4.x, and t ...
* Volume Boot Record (VBR)
Notes
If a volume's dirty shutdown flag is still cleared on startup, the volume was not properly unmounted. This would, for example, cause Windows 98 WIN.COM to start SCANDISK in order to check for and repair potential logical file system errors. If the bad sector flag is cleared, it will force a surface scan to be carried out as well. This can be disabled by setting AUTOSCAN=0 in the PTIONSsection in MSDOS.SYS
MSDOS.SYS is a system file in MS-DOS and Windows 9x operating systems.
In versions of MS-DOS from 1.1x through 6.22, the file comprises the MS-DOS kernel and is responsible for file access and program management. MSDOS.SYS is loaded by the DOS BI ...
file.
This is the reason, why had a special meaning in directory entries.
One utility providing an option to specify the desired format filler value for hard disks is DR-DOS' FDISK R2.31 with its optional wipe parameter /W:246
. In contrast to other FDISK
In computing, the fdisk command-line utility provides disk-partitioning functions, preparatory to defining file systems. fdisk features in the DOS, DR FlexOS, IBM OS/2, and Microsoft Windows operating systems, and in certain ports of FreeBS ...
utilities, DR-DOS FDISK is not only a partitioning tool, but can also format freshly created partitions as FAT12
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
, FAT16
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
or FAT32
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
. This reduces the risk to accidentally format wrong volumes.
For maximum compatibility with MS-DOS/PC DOS and DR-DOS, operating systems trying to determine a floppy disk's format should test on all mentioned opcode sequences at sector offset in ''addition'' to looking for a valid media descriptor byte at sector offset before assuming the presence of a BPB. Although PC DOS 1.0 floppy disks do not contain a BPB, they start with as well, but do not show a at offset . PC DOS 1.10 floppy disks even start with , although they still do not feature a BPB. In both cases, a test for a valid media descriptor at offset would fail (value instead of valid media descriptors and higher). If these tests fail, DOS checks for the presence of a media descriptor byte in the first byte of the first FAT in the sector following the boot sector (logical sector 1 on FAT12/FAT16 floppies).
The signature at offset in boot sectors is , that is at offset and at offset . Since little-endian
In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most si ...
representation must be assumed in the context of IBM PC compatible machines, this can be written as 16-bit word in programs for x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
processors (note the swapped order), whereas it would have to be written as in programs for other CPU architectures using a big-endian
In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most sig ...
representation. Since this has been mixed up numerous times in books and even in original Microsoft reference documents, this article uses the offset-based byte-wise on-disk representation to avoid any possible misinterpretation.
The checksum
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...
entry in Atari boot sectors holds the alignment value, not the magic value itself. The magic value is not stored anywhere on disk. In contrast to Intel x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
processors, the Motorola 680x0
The Motorola 68000 series (also known as 680x0, m68000, m68k, or 68k) is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and ...
processors as used in Atari machines use a big-endian
In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most sig ...
memory representation and therefore a big-endian representation must be assumed when calculating the checksum. As a consequence of this, for checksum verification code running on x86 machines, pairs of bytes must be swapped before the 16-bit addition.
DR-DOS is able to boot off FAT12/FAT16 logical sectored media with logical sector sizes up to 1024 bytes.
See other links for special precautions in regard to occurrences of a cluster value of on FAT12 volumes under MS-DOS/PC DOS 3.3 and higher.
To avoid potential misinterpretation of directory volume labels with VFAT LFN entries by non-VFAT aware operating systems, the DR-DOS 7.07 FDISK and FORMAT tools are known to explicitly write dummy "NO␠NAME␠␠␠␠
" directory volume labels if the user skips entering a volume label. The operating system would internally default to return the same string if no directory volume label could be found in the root of a volume, but without a real volume label stored as the first entry (after the directory entries), older operating systems could erroneously pick up VFAT LFN entries instead.
This IBM 4680 OS
FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...
and 4690 OS
4690 Operating System (sometimes shortened to 4690 OS or 4690) is a specially designed point of sale (POS) operating system, originally sold by IBM. In 2012, IBM sold its retail business, including this product, to Toshiba, which assumed support ...
distribution attribute type must have an on-disk bit value of 0 as files fall back to this type when attributes get lost accidentally.
In order to support the coexistence of DR-DOS with PC DOS and multiple parallel installations of DR-DOS, the extension of the default "IBMBIO␠␠COM
" boot file name can be changed using the SYS /DR:ext
option, where ext represents the new extension. Other potential DR-DOS boot file names to be expected in special scenarios are "DRBIOS␠␠SYS
", "DRDOS␠␠␠SYS
", "IO␠␠␠␠␠␠SYS
", "JO␠␠␠␠␠␠SYS
".
The following DOS functions return these register values:
INT 21h/AH=2Ah "Get system date" returned values: CX = year ( 1980..2099
In contemporary history, the third millennium of the anno Domini or Common Era in the Gregorian calendar is the current millennium spanning the years 2001 to 3000 (21st to 30th centuries). Ongoing futures studies seek to understand what is l ...
), DH = month (1..12), DL = day (1..31).
INT 21h/AH=2Ch "Get system time" returned values: CH = hour (0..23), CL = minute (0..59), DH = second (0..59), DL = 1/100 seconds (0..99).
Windows XP has been observed to create such hybrid disks when reformatting FAT16B formatted ZIP-100 disks to FAT32 format. The resulting volumes were FAT32 by format, but still used the FAT16B EBPB. (It is unclear how Windows determines the location of the root directory on FAT32 volumes, if only a FAT16 EBPB was used.)
Some versions of FORMAT
Format may refer to:
Printing and visual media
* Text formatting, the typesetting of text elements
* Paper formats, or paper size standards
* Newspaper format, the size of the paper page
Computing
* File format, particular way that informatio ...
since MS-DOS 1.25
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
and PC DOS 2.0
IBM PC DOS, an acronym for IBM Personal Computer Disk Operating System, is a discontinued disk operating system for IBM PC compatibles. It was manufactured and sold by IBM from the early 1980s into the 2000s. Developed by Microsoft, it was also ...
supported an option /O
(for ''old'') to fill the first byte of all directory entries with instead of utilizing the end marker . Thereby. the volume remained accessible under PC DOS 1.0- 1.1, while formatting took somewhat longer and newer versions of DOS could not take advantage of the considerable speed-up caused by using the end marker .
References
External links
ECMA-107 Volume and File Structure of Disk Cartridges for Information Interchange
identical to ISO/IEC 9293.
Microsoft Extensible Firmware Initiative FAT32 File System Specification, FAT: General Overview of On-Disk Format
* ttps://web.archive.org/web/20131220004435/http://users.iafrica.com/c/cq/cquirke/fat.htm Understanding FATincluding lots of info about LFNs
Detailed Explanation of FAT Boot Sector
Microsoft Knowledge Base Article 140418
Description of the FAT32 File System
Microsoft Knowledge Base Article 154997
FAT12/FAT16/FAT32 file system implementation for *nix
Includes libfat libraries and fusefat, a FUSE file system driver
MS-DOS: Directory and Subdirectory Limitations
Microsoft Knowledge Base Article 39927
Overview of FAT, HPFS, and NTFS File Systems
Microsoft Knowledge Base Article 100108
''Volume and file size limits of FAT file systems''
Microsoft Technet, copy made b
Internet Archive Wayback Machine
Microsoft TechNet: A Brief and Incomplete History of FAT32
by Raymond Chen
Raymond T. Chen (born 1968) is a United States circuit judge of the United States Court of Appeals for the Federal Circuit.
Biography
He joined the intellectual property law firm of Knobbe, Martens, Olson & Bear in Irvine, California. He prose ...
FAT32 Formatter
: allows formatting volumes larger than with FAT32 under Windows 2000
Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was officiall ...
, Windows XP
Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Windows 2000 for high-end and ...
and Windows Vista
Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
Fdisk does not recognize full size of hard disks larger than
Microsoft Knowledge Base Article 263044, copy made b
Internet Archive Wayback Machine
Explains inability to work with extremely large volumes under Windows 95/98.
Microsoft Windows XP: FAT32 File System
Copy made b
Internet Archive Wayback Machine
of an article with summary of limits in FAT32 which is no longer available on Microsoft website.
Visual Layout of a FAT16 drive
{{ISO standards
1980 software
Computer file systems
Disk file systems
DOS technology
Windows components
Windows disk file systems
Ecma standards
File systems supported by the Linux kernel
fr:File Allocation Table#Aspect technique