Important Files of MS-DOS

The MS-DOS operating system consists of the following system files (or programs).
(i) IO.SYS
(ii)MSDOS.SYS
(iii)COMMAND.COM

(i ) IO.SYS
The management of input and output (IO) operations are done by IO.SYS system file.This file can not be seen using DIR command.It is a hidden file.

(ii) MSDOS.SYS
The system file MSDOS.SYS is concerned with the system kernel. It is also a hidden file so it cannot be seen using DIR command.
Note : The hidden files IO.SYS and MSDOS.SYS are registered, when the CHKDSK command is used.

(iii) COMMAND.COM
The third essential system program or file is the COMMAND.COM. It is a command interpreter. This file can be seen using DIR command. This file interprets the command whenever we give it to the computer. It examines all input commands and compares it to an internal list of reference words. If the word is same on the list, the corresponding command is executed otherwise an error message is displayed on the screen.
Those three programs remains in the memory as long as the computer is switched On. Therefore they are called resident program.