IO.SYS

IO.SYS is an essential part of MS-DOS. It contains the default MS-DOS device drivers  and the DOS initialization program.

In the PC booting process, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then:

1. Loads the rest of itself into memory.
2. Initializes each default device driver in turn.
3. Loads the DOS kernel and calls its initialization routine. The kernel is stored in MSDOS.SYS with MS-DOS and in IO.SYS with Windows 9x.
4. Processes the MSDOS.SYS file with Windows 9x.
5. Processes the CONFIG.SYS file.
6. Loads command.com
7. Displays the bootsplash in Windows 9x.