The command processor or command interpreter is contained in the command.com program. This essential program displays our system prompt and the command line interface. The entire set of commands available in DOS is divided into two types
Internal commands
Internal commands are the simplest memory resident commands, most commonly used commands and execute immediately because they are a part of a file named COMMAND.COM. When we list the files of our directory the internal commands are not seen. Once DOS is loaded, there is no need for the DOS Diskette in a drive to use these commands. E.g. DIR, COPY, MOVE etc. If we execute internal command, the command processor executes the command itself.
External commands
They are separate utility programs or files stored in DOS directory. Any filename with an extension of .COM, .EXE or .BAT is considered as an external command. They are faster than internal commands because they do not look for the COMMAND.COM file and are executed directly. E.g. CHKDSK, FORMAT, SCANDISK etc. It does not execute the command itself but transfers the control to the appropriate program. The programs are loaded into the memory when they are specifically asked for.
