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IO.SYS is a hidden system file in the root directory of the primary partition of the primary drive and is an essential part of DOS and Windows 9x. 
It defines the basic input / output (I / O) capabilities for the system and helps in communicating with different peripherals. The boot sector loads the IO.SYS, which in turn loads the next system file, the MSDOS.SYS.
In Windows 9x and DOS operating systems, which rely on the hardware interfacing routines. IO.SYS loads these routines before loading MSDOS.SYS.
IO.SYS manages the overall loading of the Operating System.
