File Allocation Table
FAT, the MS-DOS file system is supported by most of today's OSes. Today FAT comes in three different flavours – FAT12, FAT16 and FAT32. The names refer to the number of bits used by the entries in table that gave the file system its name! The "File Allocation Table" itself is actually one of the structures inside the FAT file system as seen on-disk. The purpose of this table is to keep track of which areas of the disk are available and which areas are in use. Another important part about FAT is the "Long File Name" extension to FAT sometimes referred to as VFAT. The terms LFN and VFAT are closely related, but VFAT really means just Virtual FAT. FAT Overview It’s time to get slightly technical. I’ll first just mention all the structures almost in the order in which they usually appear inside the partition. When talking about the order of things I’m referring to order as seen through the Logical Block Address of a particular structure. Cluster This term is very fundamental for FAT. A cluster is a group of sectors on the FAT media. Only the part of the partition called the "data area" is divided into clusters. The rest of the partition is simply sectors. Files and directories store their data in these clusters. The size of one cluster is specified in a structure called the Boot Record and can range from a single sector to 128 sector(s). Boot Record All the three flavours of FAT have a Boot Record, which is located within an area of reserved sectors. The DOS format program reserves 1 sector for FAT12 and FAT16 and usually 32 sectors for FAT32. File Allocation Table The actual "File Allocation Table" structure is a relatively simple structure, as are all of the FAT structures really. The FAT is a simple array of 12-bit, 16-bit or 32-bit data elements. Usually there will be two identical copies of the FAT. There is a field in the Boot Record that specifies the number of FAT copies. With FAT12 and FAT16, MS-DOS uses only the first copy, but the other copies are kept in sync. FAT32 was enhanced to specify which FAT copy is the active one in a 4-bit value part of a "Flags" field. It’s quite common to think of the FAT as a singly linked list. Each of the chains in the FAT specify which parts of the disk belong to a given file or directory. Root Directory The Root Directory is formatted like any other directory except it does not contain the "dot" and "dot-dot" entries. See the details section for more information. The root directory can always be found immediately following the file allocation table(s) for FAT12 and FAT16 volumes. Data Area Time has come to describe the user data area. What is there to say really? The user data area (or just data area if you like) is where the contents of files and directories are stored. Simple as that… See the formulas above for how to calculate the size of the data area. And yes, the data area is divided into sector groups called clusters. All the clusters in a single FAT volume have the same size. To further educate you, the term slack space refers to any unused space at the end of a cluster and cannot be used by any other file or directory. Note that directories are not known to suffer from slack space problems. This is simply because the exact size in bytes of a directory is not recorded as with files and generally no one seem to care anyway. The data area section will not be explained in detail. There is simply nothing more to say about it. Information on how to access files and directories is the closest we get to data area details. Wasted Sectors If the number of data sectors is not evenly divisible by the cluster size you end up with a few wasted data sectors. Also if the partition as declared in the partition table is larger than what is claimed in the Boot Record the volume can be said to have wasted sectors. If you are not familiar with the term partition table, I suggest that you go to Hale Landis’ web site and look for the How It Works series of documents at - http://www.ata-atapi.com.