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Io.sys is an old, hidden system file from MS-DOS and early Windows that was essential for booting the operating system. It contained the low-level, hardware-dependent routines needed to initialize the computer and load the rest of the OS, including the kernel (msdos.sys). This file is not used in modern operating systems.

Key functions and characteristics

Bootstrapping: Io.sys’s primary role was to execute when a computer was turned on, loading the MS-DOS or Windows kernel and preparing the basic hardware for use.

Hardware Interface: It included the default MS-DOS device drivers for handling basic hardware like the keyboard and serial ports.

Evolution over time: In MS-DOS, it was paired with msdos.sys. In Windows 9x (95, 98), the functions of msdos.sys were merged into io.sys, though msdos.sys still existed as a configuration file. In later versions of Windows, this file was removed entirely.