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IO.SYS is a hidden executable binary file or hidden system file which processes instructions when the operating system is booted. It was an essential part of MS-DOS and Windows 9x systems. The instructions tell the operating system how the computer is set up. Together with the MSDOS.SYS system file, they made up Microsoft’s MS-DOS and were loaded into the memory of the computer.
IO.SYS is an important part of MS-DOS since it contains its default drivers and the DOS initialization program. After the introduction of Microsoft’s Windows 95, the MSDOS.SYS file was merged with IO.SYS, but it still exists in computers as a text file which determines whether the computer booted into DOS or Windows. However, recent versions of Windows no longer require the IO.SYS file for booting.
Unlike the MSDOS.SYS file which turned into a text file after the Windows 9x release, the IO.SYS file cannot be edited by a standard text editor. In the event that the user needs to edit the system file, it is handled through the CONFIG.SYS file.