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Root Directory and Regular Directories
A hierarchical directory structure is used to organize the files that exist on any hard disk volume. This "logical tree" is used on almost every file system, because of the intuitive way it arranges files, and the power it gives the user to create meaningful organization schemes for files. Much as a real tree has all its branches and roots come together in one spot, so too does a file system directory structure. The directory at the "base" of the logical tree is called, appropriately enough, the root directory. The root directory is special because it follows rules that do not apply to the other, "regular" directories on the hard disk.
There can only be one root directory for any disk volume; obviously, having more than one would result in confusion, and there isn't any need to have more than one anyway. In the conventional FAT file system, the root directory is fixed in place at the start of the DOS volume; it "anchors" the directory tree. The root directory located on the disk volume directly after the two copies of the FAT, which are themselves directly below the other key disk structures. This contrasts with regular directories, which can be located anywhere on the disk.
In addition to being fixed in location, the root directory is also fixed in size (under FAT12/FAT16/VFAT). Regular directories can have an arbitrary size; they use space on the disk much the way files do, and when more space is needed to hold more entries, the directory can be expanded the same way a file can. The root directory is limited to a specific number of entries because of its special status. The number of entries that the root directory can hold depends on the type of volume:
One of the improvements introduced in the newer FAT32 version of the FAT file system was to remove these restrictions on the root directory. Under FAT32, the root directory is treated much more like a regular directory, and can be relocated and expanded in size like any other. (It's still a good idea not to load up the root directory with too many files.)
There are a couple of other special things you should know about the root directory. One is that it cannot be deleted; the reason for this I would think to be obvious. :^) Also, the root directory has no parent, since it is at the top of the tree structure. The root directory still contains a ".." entry, but instead of pointing to the cluster number of the parent directory like a regular directory's parent entry, it contains a null value (zero).