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 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.
The size of files and subdirectories can be increased arbitrarily (as long as there are free clusters) by simply adding more links to the file's chain in the FAT. Files are allocated in units of clusters, so if a 1 KB file resides in a 32 KB cluster, 31 KB are wasted.
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 0x030 (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 0x041 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.
The FAT file system itself does not impose any limits on the depth of a subdirectory tree for as long as there are free clusters available to allocate the subdirectories, however, the internal Current Directory Structure (CDS) under MS-DOS/PC DOS limits the absolute path of a directory to 66 characters (including the drive letter, but excluding the NUL byte delimiter), thereby limiting the maximum supported depth of subdirectories to 32, whatever occurs earlier. Concurrent DOS, Multiuser DOS and DR DOS 3.31 to 6.0 (up to including the 1992-11 updates) do not store absolute paths to working directories internally and therefore do not show this limitation. The same applies to Atari GEMDOS, but the Atari Desktop does not support more than 8 sub-directory levels. Most applications aware of this extension support paths up to at least 127 bytes. FlexOS, 4680 OS and 4690 OS support a length of up to 127 bytes as well, allowing depths down to 60 levels. PalmDOS, DR DOS 6.0 (since BDOS 7.1) and higher, Novell DOS, and OpenDOS sport a MS-DOS-compatible CDS and therefore have the same length limits as MS-DOS/PC DOS.
Character 229 (0xE5) 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.
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 exFAT, use free space bitmaps that indicate used and available clusters, which could then be quickly looked up in order to find free contiguous areas. Another solution is the linkage of all free clusters into one or more lists (as is done in Unix file systems). Instead, the FAT has to be scanned as an array to find free clusters, which can lead to performance penalties with large disks.
In fact, seeking for files in large subdirectories or computing the free disk space on FAT volumes is one of the most resource intensive operations, as it requires reading the directory tables or even the entire FAT linearly. Since the total amount of clusters and the size of their entries in the FAT was still small on FAT12 and FAT16 volumes, this could still be tolerated on FAT12 and FAT16 volumes most of the time, considering that the introduction of more sophisticated disk structures would have also increased the complexity and memory footprint of real-mode operating systems with their minimum total memory requirements of 128 KB or less (such as with DOS) for which FAT has been designed and optimized originally.
With the introduction of FAT32, long seek and scan times became more apparent, particularly on very large volumes. A possible justification suggested by Microsoft's Raymond Chen for limiting the maximum size of FAT32 partitions created on Windows was the time required to perform a "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-ATX-style PCs, on plain DOS systems and some battery-powered consumer products.
DOS 3.0 and higher will not immediately reuse disk space of deleted files for new allocations but instead seek for previously unused space before starting to use disk space of previously deleted files as well. This not only helps to maintain the integrity of deleted files for as long as possible but also speeds up file allocations and avoids fragmentation, since never before allocated disk space is always unfragmented.DOS accomplishes this by keeping a pointer to the last allocated cluster on each mounted volume in memory and starts searching for free space from this location upwards instead of at the beginning of the FAT, as it was still done by DOS 2.x. If the end of the FAT is reached, it would wrap around to continue the search at the beginning of the FAT until either free space has been found or the original position has been reached again without having found free space. These pointers are initialized to point to the start of the FATs after bootup, but on FAT32 volumes, DOS 7.1 and higher will attempt to retrieve the last position from the FS Information Sector.This mechanism is defeated, however, if an application often deletes and recreates temporary files as the operating system would then try to maintain the integrity of void data effectively causing more fragmentation in the end. In some DOS versions, the usage of a special API function to create temporary files can be used to avoid this problem.
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.
All operations that involve moving data around are called reshapes, and require a temporary region (window) that can be written to without corrupting the array. One of the reasons for the v1 superblock and the data offset is to provide spare space for this. The preferred mechanism is to relocate the data up or down a few stripes, reading data from the live area and writing it into the window. Every few stripes, the superblock is updated, making the window "live" and freeing up the space that has just been relocated as the new window. This results in the data offset being shifted a few megabytes in either direction (which is one reason why you can NOT safely recreate a live array). If the system crashes or is halted during a reshape, it is assumed the window is corrupt, and the reshape can continue.
Data can be entered into a computer in a variety of different ways.Users might designate position or direction by pointing at a display.Users might enter numbers, letters, or more extended textual material bykeyed inputs, or in some applications by spoken inputs. Data might bekeyed into displayed forms or tables, into constrained message formats,or as free text. In graphic interaction users might draw pictures ormanipulate displayed graphic elements. These different types of dataentry all merit consideration here.
For unformatted ("free") text, natural units will be characters, words,phrases, sentences, paragraphs, and pages; for specially formatted text,such as computer program listings, allow specification of other logicalunits, including lines, subsections, sections, etc. 2b1af7f3a8