The directory at the "base" of the directory structure that defines the
logical tree that organizes files on a hard disk is the root directory. The root
directory is special because it follows special 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 chaos, and there isn't any need to have more than one anyway. In order
to "anchor" the directory tree, the root directory is fixed in place at the
start of the DOS volume. It is located directly below the two copies of the FAT, which is itself directly below the other key
disk structures. This contrasts with regular (sub) directories, which can be located
anywhere on the disk.
In addition to being fixed in location, the root directory is also fixed in size.
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:
Volume Type |
Maximum Number of
Root Directory Entries |
360KB 5.25"
Floppy Disk |
112 |
720KB 3.5"
Floppy Disk |
112 |
1.2MB 5.25"
Floppy Disk |
224 |
1.44MB 3.5"
Floppy Disk |
224 |
2.88MB 3.5"
Floppy Disk |
448 |
Hard Disk |
512 |
Note that the newer FAT32 version of the FAT file system does not have the
restriction on placement and size of the root directory. In this enhancement the root
directory is treated like a regular directory and can be relocated and expanded in size
like any other.
There are a couple of other special things 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).
Next: File Names and Extensions
