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System file (DOS BIOS in MS-DOS, combined with DOS kernel in Windows 9x)
IO.SYS is an essential part of MS-DOS and Windows 9x. It contains the default MS-DOS device drivers (hardware interfacing routines) and the DOS initialization program.
In the PC bootup sequence, 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:
In Windows 9x, the IO.SYS not only contains the DOS BIOS, but also holds the DOS kernel, which previously resided in MSDOS.SYS. Under some conditions, Windows 9x uses the alternative filenames WINBOOT.SYS or JO.SYS instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named IO.W40 for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the IO.SYS of the older system is named IO.DOS for as long as Windows 9x is active.
DR-DOS 7.06 (only this version) also follows this scheme and the IO.SYS filename in order to become bootable via MS-DOS boot sectors.
Similarly, FreeDOS uses a combined system file as well, but names it KERNEL.SYS.
Disk layout requirements
The two first entries of the root directory must be allocated by IO.SYS and MSDOS.SYS, in that order.
^The MS-DOS 5.0 manual incorrectly states that the system files no longer need to be contiguous. However, for the boot process to work the system files still need to occupy the first two directory entries and the first three sectors of IO.SYS still need to be stored contiguously. SYS continues to take care of these requirements.