The commands that are a part of resident DOS are known as internal commands. Because they are always in internal memory, DOS always knows where to find them. Also, it does not have to load them from disk storage into internal memory before it can execute them. To use one of these commands, all you ever have to do is to enter the command name at the command prompt.
If a command isn't in resident DOS, it's called an external command. Each of these commands is stored in a disk file called a command file. On a MS¬DOS system, the names of command files always have an extension of COM or EXE. Usually, these command files are stored in the DOS directory on one of the drives of your Pc.
Before DOS can execute an external command, it must find the command file for the command and load the command into internal memory. But not all systems are set up so DOS is able to find its external commands. Table 1 on next page shows some of the internal and external command and its meanings.
Subdirectory Shortcuts -The dot-dot ( .. ) file in every subdirectory is used by DOS to support linkage to the subdirectory's parent directory. That is why the root directory has no dot-dot file; the root has no parent. You can use the dot-dot file as a shortcut to change to the parent of the subdirectory on which you are currently logged. By changing to the dot-dot subdirectory, you automatically change to the parent of the current subdirectory without remembering its name.