Internal commands.

• DOS stays in the internal memory of your systems unit whenever your PC is turned on. This portion of DOS is called resident DOS. It includes the command processor and also includes many of the DOS commands.
• 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 enter the command name at the command prompt.

Example of internal commands are:

External commands.

• If a command is not 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.

Examples of external commands are: