Internal Directory Structures

Every file on the system is stored in a directory. A directory is nothing more than a file itself, except that it is specially structured and marked on the disk so that it has special meaning. A directory is a table that contains information about files (and subdirectories) that it contains, and links to where the file (or subdirectory) data begins on the disk. The paper analogy would be a table of contents to a book, except that directories of course use a hierarchical tree structure and books do not.

Each entry in a directory is 32 bytes in length, and stores the following information:

• File Name and Extension: This is the 11-character name of the file using the conventional 8.3 DOS file naming standard, for example, COMMAND.COM. Note that the "dot" in "COMMAND.COM" is implied and not actually stored on the disk. See here for more on file naming and also on VFAT long file names, which use a special structure. The file name field is also used to indicate directory entries that have been deleted.
• File Attribute Byte: There are several different attributes which the operating system uses to give special treatment to certain files; these are stored in a single byte in each directory entry. These attributes are discussed in detail here. Note that it is one of these file attributes that indicates whether an entry in the directory represents a "real" file, or a subdirectory.
• Last Change Date/Time: There is a space for each file to indicate the date and time that it was created or modified. You should know that these fields can be arbitrarily modified by any program to be whatever they want, so this date/time shouldn't be taken too religiously. I occasionally am asked if the date/time on a file can be used to prove when someone did something or not on their PC. It cannot, because it's too easy to change this information.
• File Size: The size of the file in bytes.
• Link to Start Cluster: The number of the cluster that starts the file (or subdirectory) is stored in the directory. This is what allows the operating system to find a file when it is needed, and how all the different files and directories are linked together on the disk. See here for more on cluster chaining.

Every regular directory on the disk has two special entries, that refer to the directory itself and to the parent directory. These are named "." (single dot) and ".." (double dot) respectively. These entries are used for navigation purposes; if you type "chdir .." then DOS will change your current directory to the parent of the one you were in.

Next: Root Directory and Regular Directories