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57-472: The FAT file system is a file system used on MS-DOS and Windows 9x family of operating systems . It continues to be used on mobile devices and embedded systems , and thus is a well-suited file system for data exchange between computers and devices of almost any type and age from 1981 through to the present. A FAT file system is composed of four regions: Important information from the Boot Sector

114-547: A DOS 3.31 BPB some GPT boot loaders (like BootDuet ) use 0x1FA – 0x1FD to store the high 4 bytes of the hidden sectors for volumes located outside the first 2-1 sectors. Since this location may contain code or other data in other boot sectors, it may not be written to when 0x1F9 – 0x1FD do not all contain zero.) If this belongs to a boot volume, the DR-DOS 7.07 enhanced MBR can be configured (see NEWLDR offset 0x014 ) to dynamically update this entry to

171-637: A boot sector depending on certain values detected here (e.g., NEWLDR offset 0x018 ). The boot ROM of the Wang Professional Computer will only treat a disk as bootable if the first four characters of the OEM label are " Wang ". Similarly, the ROM BIOS of the Philips :YES will only boot from a disk if the first four characters of the OEM label are " :YES ". If, in an FAT32 EBPB ,

228-557: A combined file manager and web browser, the ability to seamlessly open Microsoft Office documents from within Internet Explorer using ActiveX technology and a way to place dynamic web pages directly on the desktop in place of the regular static wallpaper . A leaked build had version number 4.10.999 (in comparison to Windows 95's 4.00.950, Windows 95 OSR2's 4.00.1111, Windows 98's 4.10.1998, Windows 98 Second Edition's 4.10.2222 A, and Windows ME 's 4.90.3000). The project

285-424: A device, device type, directory prefix, file path separator, or file type. File systems typically support organizing files into directories , also called folders , which segregate files into groups. This may be implemented by associating the file name with an index in a table of contents or an inode in a Unix-like file system. Directory structures may be flat (i.e. linear), or allow hierarchies by allowing

342-672: A directory to contain directories, called subdirectories. The first file system to support arbitrary hierarchies of directories was used in the Multics operating system. The native file systems of Unix-like systems also support arbitrary directory hierarchies, as do, Apple 's Hierarchical File System and its successor HFS+ in classic Mac OS , the FAT file system in MS-DOS 2.0 and later versions of MS-DOS and in Microsoft Windows ,

399-421: A file system can be managed by the user via various utility programs. Windows 4.1 Nashville (previously Cleveland ), popularly known as Windows 96 by contemporary press, was the codename for a cancelled release of Microsoft Windows scheduled to be released in 1996, between "Chicago" ( Windows 95 ) and "Memphis" ( Windows 98 , at the time scheduled for release in 1996, later 1997). Nashville

456-443: A file system creates a file, it allocates space for data. Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows. To delete a file, the file system records that the file's space is free; available to use for another file. A local file system manages storage space to provide a level of reliability and efficiency. Generally, it allocates storage device space in

513-422: A file system implementation. The physical file system layer provides relatively low-level access to a storage device (e.g. disk). It reads and writes data blocks , provides buffering and other memory management and controls placement of blocks in specific locations on the storage medium. This layer uses device drivers or channel I/O to drive the storage device. A file name , or filename , identifies

570-417: A file to consuming applications and in some cases users. A file name is unique so that an application can refer to exactly one file for a particular name. If the file system supports directories, then generally file name uniqueness is enforced within the context of each directory. In other words, a storage can contain multiple files with the same name, but not in the same directory. Most file systems restrict

627-706: A forked file system on the Macintosh, and Microsoft supports streams in NTFS. Some file systems maintain multiple past revisions of a file under a single file name; the file name by itself retrieves the most recent version, while prior saved version can be accessed using a special naming convention such as "filename;4" or "filename(-4)" to access the version four saves ago. See comparison of file systems#Metadata for details on which file systems support which kinds of metadata. A local file system tracks which areas of storage belong to which file and which are not being used. When

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684-477: A granular manner, usually multiple physical units (i.e. bytes ). For example, in Apple DOS of the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used a track/sector map . The granular nature results in unused space, sometimes called slack space , for each file except for those that have the rare size that is a multiple of the granular allocation. For a 512-byte allocation, the average unused space

741-692: A maximum of 255 heads only. File system A file system provides a data storage service that allows applications to share mass storage . Without a file system, applications could access the storage in incompatible ways that lead to resource contention , data corruption and data loss . There are many file system designs and implementations – with various structure and features and various resulting characteristics such as speed, flexibility, security, size and more. Files systems have been developed for many types of storage devices , including hard disk drives (HDDs), solid-state drives (SSDs), magnetic tapes and optical discs . A portion of

798-550: A multiple of 16. The maximum value supported on floppy disks is 240, the maximum value supported by MS-DOS/PC DOS on hard disks is 512. DR-DOS supports booting off FAT12/FAT16 volumes, if the boot file is located in the first 2048 root directory entries. This value must reflect the media descriptor stored (in the entry for cluster 0 ) in the first byte of each copy of the FAT. Certain operating systems before DOS 3.2 ( 86-DOS , MS-DOS / PC DOS 1.x and MSX-DOS version 1.0) ignore

855-524: A near jump ( 0xE9 0x?? 0x?? as seen on most ( Compaq , TeleVideo ) DOS 2.x formatted disks as well as on some ( Epson , Olivetti ) DOS 3.1 disks). For backward compatibility MS-DOS, PC DOS and DR-DOS also accept a jump ( 0x69 0x?? 0x?? ) on removable disks. On hard disks, DR DOS additionally accepts the swapped JMPS sequence starting with a NOP ( 0x90 0xEB 0x?? ), whereas MS-DOS/PC DOS do not. (See below for Atari ST compatibility.) The presence of one of these opstring patterns (in combination with

912-510: A recognized BPB format and a media descriptor of either 0xF8 or 0xFA to 0xFF MS-DOS/PC DOS treats bit 1 as a flag to choose a 9-sectors per track format rather than an 8-sectors format, and bit 0 as a flag to indicate double-sided media. Values 0x00 to 0xEF and 0xF1 to 0xF7 are reserved and must not be used. 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

969-520: A reversed signature 0xAA 0x55 has been observed on disk images. This can be the result of a faulty implementation in the formatting tool based on faulty documentation, but it may also indicate a swapped byte order of the disk image, which might have occurred in transfer between platforms using a different endianness . BPB values and FAT12, FAT16 and FAT32 file systems are meant to use little-endian representation only and there are no known implementations of variants using big-endian values instead. If

1026-578: A seemingly read-only disk access (such as a DIR A: ) if the medium is not write-protected. Given the dependency on certain values explained above, this may, depending on the actual BPB format and contents, cause MS-DOS/PC DOS and OS/2 to no longer recognize a medium and throw error messages despite the fact that the medium is not defective and can still be read without problems under other operating systems. Windows 9x reads that self-marked disks without any problems but giving some strange values for non-meaning parameters which not exist or are not used when

1083-488: A single-sector setup (logical sector 0 only) for the bootstrap loader, Microsoft's boot sector code has grown to span over logical sectors 0 and 2 since the introduction of FAT32, with logical sector 0 depending on sub-routines in logical sector 2. The Backup Boot Sector area consists of three logical sectors 6, 7, and 8 as well. In some cases, Microsoft also uses sector 12 of the reserved sectors area for an extended boot loader. FAT uses little-endian format for all entries in

1140-431: A stream of bytes . Typically, to read file data, a program provides a memory buffer and the file system retrieves data from the medium and then writes the data to the buffer. A write involves the program providing a buffer of bytes that the file system reads and then stores to the medium. Some file systems, or layers on top of a file system, allow a program to define a record so that a program can read and write data as

1197-585: A structure; not an unorganized sequence of bytes. If a fixed length record definition is used, then locating the n record can be calculated mathematically, which is relatively fast compared to parsing the data for record separators. An identification for each record, also known as a key, allows a program to read, write and update records without regard to their location in storage. Such storage requires managing blocks of media, usually separating key blocks and data blocks. Efficient algorithms can be developed with pyramid structures for locating records. Typically,

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1254-506: A test for a valid media descriptor value at offset 0x015 ) serves as indicator to DOS 3.3 and higher that some kind of BPB is present (although the exact size should not be determined from the jump target since some boot sectors contain private boot loader data following the BPB), while for DOS 1.x (and some DOS 3.0) volumes, they will have to fall back to the DOS 1.x method to detect the format via

1311-432: A user's use of storage space. A file system typically ensures that stored data remains consistent in both normal operations as well as exceptional situations like: Recovery from exceptional situations may include updating metadata, directory entries and handling data that was buffered but not written to storage media. A file system might record events to allow analysis of issues such as: Many file systems access data as

1368-464: A value of 4 here. Volumes declaring 2 FATs in this entry will never be treated as TFAT volumes. If the value differs from 2, some Microsoft operating systems may attempt to mount the volume as a TFAT volume and use the second cluster ( cluster 1 ) of the first FAT to determine the TFAT status. A value of 0 without a FAT32 EBPB (no signature 0x29 or 0x28 at offset 0x042 ) may also indicate

1425-568: A variable-sized root directory in some non-standard FAT12 and FAT16 implementations, which store the root directory start cluster in the cluster 1 entry in the FAT. This extension, however, is not supported by mainstream operating systems, as it can conflict with other uses of the cluster 1 entry for maintenance flags, the current end-of-chain-marker, or TFAT extensions. This value must be adjusted so that directory entries always consume full logical sectors, given that each directory entry takes up 32 bytes. MS-DOS/PC DOS require this value to be

1482-405: Is 256 bytes. For 64 KB clusters, the average unused space is 32 KB. Generally, the allocation unit size is set when the storage is configured. Choosing a relatively small size compared to the files stored, results in excessive access overhead. Choosing a relatively large size results in excessive unused space. Choosing an allocation size based on the average size of files expected to be in

1539-596: Is 512 bytes, which can be reduced downto 128 bytes without support for the FS Information Sector . Floppy drives and controllers use physical sector sizes of 128, 256, 512 and 1024 bytes (e.g., PC/AX). The Atari Portfolio supports a sector size of 512 for volumes larger than 64 KB, 256 bytes for volumes larger 32 KB and 128 bytes for smaller volumes. Magneto-optical drives used sector sizes of 512, 1024 and 2048 bytes. In 2005 some Seagate custom hard disks used sector sizes of 1024 bytes instead of

1596-522: Is accessible through an operating system structure called the Drive Parameter Block ( DPB ) in DOS and OS/2. The total count of reserved sectors is indicated by a field inside the Boot Sector, and is usually 32 on FAT32 file systems. For FAT32 file systems, the reserved sectors include a File System Information Sector at logical sector 1 and a Backup Boot Sector at logical sector 6. While many other vendors have continued to utilize

1653-534: Is invisible to the end user and the system still works correctly. However this can degrade performance on some storage hardware that work better with contiguous blocks such as hard disk drives . Other hardware such as solid-state drives are not affected by fragmentation. A file system often supports access control of data that it manages. The intent of access control is often to prevent certain users from reading or modifying certain files. Access control can also restrict access by program in order to ensure that data

1710-510: Is modified in a controlled way. Examples include passwords stored in the metadata of the file or elsewhere and file permissions in the form of permission bits, access control lists , or capabilities . The need for file system utilities to be able to access the data at the media level to reorganize the structures and provide efficient backup usually means that these are only effective for polite users but are not effective against intruders. Methods for encrypting file data are sometimes included in

1767-550: Is stored inside the allocation group itself. Additional attributes can be associated on file systems, such as NTFS , XFS , ext2 , ext3 , some versions of UFS , and HFS+ , using extended file attributes . Some file systems provide for user defined attributes such as the author of the document, the character encoding of a document or the size of an image. Some file systems allow for different data collections to be associated with one file name. These separate collections may be referred to as streams or forks . Apple has long used

Design of the FAT file system - Misplaced Pages Continue

1824-425: 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. Since DOS 2.0, valid x86-bootable disks must start with either a short jump followed by a NOP ( opstring sequence 0xEB 0x?? 0x90 as seen since DOS 3.0—and on DOS 1.1) or

1881-680: The NTFS file system in the Windows NT family of operating systems, and the ODS-2 (On-Disk Structure-2) and higher levels of the Files-11 file system in OpenVMS . In addition to data, the file content, a file system also manages associated metadata which may include but is not limited to: A file system stores associated metadata separate from the content of the file. Most file systems store

1938-522: The DL value provided at boot time or the value stored in the partition table. This enables booting off alternative drives, even when the VBR code ignores the DL value. This signature must be located at fixed sector offset 0x1FE for sector sizes 512 or higher. If the physical sector size is larger, it may be repeated at the end of the physical sector. Atari STs will assume a disk to be Atari 68000 bootable if

1995-423: The DOS 2.0 format, but could use the DOS 3.0 format as well. A zero entry indicates that this entry is reserved, but not used. A bug in all versions of MS-DOS/PC DOS up to including 7.10 causes these operating systems to crash for CHS geometries with 256 heads, therefore almost all BIOSes choose a maximum of 255 heads only. A zero entry indicates that this entry is reserved, but not used. It must not be used if

2052-811: The boot code area or the serial number in order to ensure that the checksum 0x1234 is not matched (unless the shared fat code overlay would be both IBM PC and Atari ST executable at the same time). The minimum allowed value for non-bootable FAT12/FAT16 volumes with up to 65,535 logical sectors is 32 bytes, or 64 bytes for more than 65,535 logical sectors. The minimum practical value is 128. Some pre-DOS 3.31 OEM versions of DOS used logical sector sizes up to 8192 bytes for logical sectored FATs . Atari ST GEMDOS supports logical sector sizes between 512 and 4096. DR-DOS supports booting off FAT12/FAT16 volumes with logical sector sizes up to 32 KB and INT 13h implementations supporting physical sectors up to 1024 bytes/sector. The minimum logical sector size for standard FAT32 volumes

2109-498: The boot sector parameters altogether and use the media descriptor value from the first byte of the FAT to choose among internally pre-defined parameter templates. Must be greater or equal to 0xF0 since DOS 4.0. On removable drives, DR-DOS will assume the presence of a BPB if this value is greater or equal to 0xF0 , whereas for fixed disks, it must be 0xF8 to assume the presence of a BPB. Initially, these values were meant to be used as bit flags; for any removable media without

2166-401: The checksum over the 256 big-endian words of the boot sector equals 0x1234 . If the boot loader code is IBM compatible, it is important to ensure that the checksum over the boot sector does not match this checksum by accident. If this would happen to be the case, changing an unused bit (e.g., before or after the boot code area) can be used to ensure this condition is not met. In rare cases,

2223-405: The cluster size can reach 256 KB. Some versions of DR-DOS provide limited support for 128 KB clusters with 512 bytes/sector using a sectors/cluster value of 0. MS-DOS/PC DOS will hang on startup if this value is erroneously specified as 0. Since DR-DOS 7.0x FAT32 formatted volumes use a single-sector boot sector, FS info sector and backup sector, some volumes formatted under DR-DOS use

2280-421: The computer main memory can be set up as a RAM disk that serves as a storage device for a file system. File systems such as tmpfs can store files in virtual memory . A virtual file system provides access to files that are either computed on request, called virtual files (see procfs and sysfs ), or are mapping into another, backing storage. From c.  1900 and before the advent of computers

2337-519: The concepts. The logical file system layer provides relatively high-level access via an application programming interface (API) for file operations including open, close, read and write – delegating operations to lower layers. This layer manages open file table entries and per-process file descriptors. It provides file access, directory operations, security and protection. The virtual file system , an optional layer, supports multiple concurrent instances of physical file systems, each of which called

Design of the FAT file system - Misplaced Pages Continue

2394-666: The default 512 bytes. Advanced Format hard disks use 4096 bytes per sector ( 4Kn ) since 2010, but will also be able to emulate 512 byte sectors ( 512e ) for a transitional period. Linux, and by extension Android, supports a logical sector size far larger, officially documented in the Man page for the filesystem utilities as up to 32KB. For most DOS-based operating systems, the maximum cluster size remains at 32 KB (or 64 KB) even for sector sizes larger than 512 bytes. For logical sector sizes of 1 KB, 2 KB and 4 KB, Windows NT 4.0 supports cluster sizes of 128 KB, while for 2 KB and 4 KB sectors

2451-496: The disk was formatted with older BPB specification, e.g. disk serial number (which exists only for disks formatted on DOS 5.0 or later, and in Windows 9x after overwriting OEM label with ?????IHC will report it as 0000-0000 or any other value stored in disk serial number field when using disk formatted on other system). This applies only to removable disk drives. Some boot loaders make adjustments or refuse to pass control to

2508-416: The file system. This is very effective since there is no need for file system utilities to know the encryption seed to effectively manage the data. The risks of relying on encryption include the fact that an attacker can copy the data and use brute force to decrypt the data. Additionally, losing the seed means losing the data. Some operating systems allow a system administrator to enable disk quotas to limit

2565-530: The header (except for, where explicitly mentioned, some entries on Atari ST boot sectors) and the FAT(s). It is possible to allocate more FAT sectors than necessary for the number of clusters. The end of the last sector of each FAT copy can be unused if there are no corresponding clusters. The total number of sectors (as noted in the boot record) can be larger than the number of sectors used by data (clusters × sectors per cluster), FATs (number of FATs × sectors per FAT),

2622-500: The length of a file name. Some file systems match file names as case sensitive and others as case insensitive. For example, the names MYFILE and myfile match the same file for case insensitive, but different files for case sensitive. Most modern file systems allow a file name to contain a wide range of characters from the Unicode character set. Some restrict characters such as those used to indicate special attributes such as

2679-409: The logical sector size is larger than 512 bytes, the remainder is not included in the checksum and is typically zero-filled. Since some PC operating systems erroneously do not accept FAT formatted floppies if the 0x55 0xAA signature is not present here, it is advisable to place the 0x55 0xAA in this place (and add an IBM compatible boot loader or stub) and use an unused word in the private data or

2736-557: The logical sectors entry at offset 0x013 is zero. DOS 3.2 BPB: Officially, MS-DOS 3.20 still used the DOS 3.0 format, but SYS and FORMAT were adapted to support a 6 bytes longer format already (of which not all entries were used). It must not be used if the logical sectors entry at offset 0x013 is zero. 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

2793-508: The logical sectors entry at offset 0x013 is zero. A zero entry indicates that this entry is reserved, but not used. A value of 0 may indicate LBA-only access, but may cause a divide-by-zero exception in some boot loaders, which can be avoided by storing a neutral value of 1 here, if no CHS geometry can be reasonably emulated. A bug in all versions of MS-DOS/PC DOS up to including 7.10 causes these operating systems to crash for CHS geometries with 256 heads, therefore almost all BIOSes choose

2850-401: The media byte in the FAT (in logical sector 1 ). Although officially documented as free for OEM use, MS-DOS/PC DOS (since 3.1), Windows 95/98/SE/ME and OS/2 check this field to determine which other parts of the boot record can be relied upon and how to interpret them. Therefore, setting the OEM label to arbitrary or bogus values may cause MS-DOS, PC DOS and OS/2 to not recognize

2907-595: The names of all the files in one directory in one place—the directory table for that directory—which is often stored like any other file. Many file systems put only some of the metadata for a file in the directory table, and the rest of the metadata for that file in a completely separate structure, such as the inode . Most file systems also store metadata not associated with any one particular file. Such metadata includes information about unused regions— free space bitmap , block availability map —and information about bad sectors . Often such information about an allocation group

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2964-477: The root directory (n/a for FAT32), and hidden sectors including the boot sector: this would result in unused sectors at the end of the volume. If a partition contains more sectors than the total number of sectors occupied by the file system it would also result in unused sectors, at the end of the partition, after the volume. On non-partitioned storage devices , such as floppy disks , the Boot Sector ( VBR )

3021-537: The signature at sector offset 0x042 is 0x29 and both total sector entries are 0, the file system entry may serve as a 64-bit total sector count entry and the OEM label entry may be used as alternative file system type instead of the normal entry at offset 0x052 . In a similar fashion, if this entry is set to " EXFAT␠␠␠ ", it indicates the usage of an exFAT BPB located at sector offset 0x040 to 0x077 , whereas NTFS volumes use " NTFS␠␠␠␠ " to indicate an NTFS BPB . (In conjunction with at least

3078-429: The storage tends to minimize unusable space. As a file system creates, modifies and deletes files, the underlying storage representation may become fragmented . Files and the unused space between files will occupy allocation blocks that are not contiguous. A file becomes fragmented if space needed to store its content cannot be allocated in contiguous blocks. Free space becomes fragmented when files are deleted. This

3135-442: The terms file system , filing system and system for filing were used to describe methods of organizing, storing and retrieving paper documents. By 1961, the term file system was being applied to computerized filing alongside the original meaning. By 1964, it was in general use. A local file system's architecture can be described as layers of abstraction even though a particular file system design may not actually separate

3192-469: The volume properly and cause data corruption on writes. Common examples are " IBM␠␠3.3 ", " MSDOS5.0 ", " MSWIN4.1 ", " IBM␠␠7.1 ", " mkdosfs␠ ", and " FreeDOS␠ ". Some vendors store licensing info or access keys in this entry. The Volume Tracker in Windows 95/98/SE/ME will overwrite the OEM label with " ?????IHC " signatures (a left-over from " ␠OGACIHC " for " Chicago ") even on

3249-498: Was intended to be a minor release focusing on a tighter integration between Windows and Internet Explorer , in order to better compete with Netscape Navigator . Microsoft claimed that Nashville would add Internet integration features to the Windows 95 and NT 4.0 desktop , building on the new features in the Internet Explorer 3.0 web browser (due for release a few months before Nashville). Touted features included

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