These files are created for special purpose like booting, creating and coordinating with devices for input, output  and saving internal commands of DOS in memory.
In other words these are program files which are created which creating DOS. You will get brief introduction to these files on next page.
IO.SYS & MSDOS.SYS:These both files are hidden files. These are system files and are present on every disc. At the time of booting both of these Files are automatically get saved in Temporary Memory. IO.sys stores the standard operation related to input and output devices. And MSDOS.sys file stores command that contact with operating system.
COMMAND.COM: This is a command interpreter file in MS-DOS which is a group of IO.sys, MSDOS.sys and COMMAND.COM. At the time of booting of computer, all these three files are automatically saved in the memory. COMMAND.COM file contains all the definitions of Internal command and programs of these commands. This file is mainly responsible for executing command which are typed by user in command prompt.
AUTOEXE.BAT: After booting Operating System it looks for AUTOEXE.BAT file in the disc and executes it. This process is repeated when computer is switched on. It is a batch file which contains MS-DOS command like a program file and it automatically executes the command written in it. Therefore it is named AUTOEXE.BAT which means self executable batch file.
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