IO.SYS is an essential part of DOS and Windows 9x. The boot sector of the boot drive's main purpose is to load IO.SYS into memory and transfer control to it. IO.SYS then loads the hardware interfacing routines that non-NT based Windows and DOS operating systems rely on, and then loads MSDOS.SYS. For other uses, see DOS (disambiguation). ... Microsoft Windows is a range of commercial operating environments for personal computers. ... The boot sector is a region of a hard disk, floppy disk, or similar data storage device. ... MSDOS.SYS is an important system file on MS-DOS and Windows 9x systems. ...

As defined by the A+ Training Guide, 5th edition, copywrite 2004, by: Charles J. Brooks; IO.SYS is a special hidden, read-only boot up file that the Bootstrap Loader finds and moves into RAM to manage the boot up process. After the boot up is complete, this file manages the basic I/O routines of the system. This includes communication between the system and I/O devices such as hard disks, printers, FDD, as well as others.