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.
Boot sequence.
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 into memory and transfers control to it. then:
The filename was also used by (DCP), an MS-DOS derivative by the former East-German VEB Robotron.
IBM PC DOS and DR DOS use the file for the same purpose; it in turn loads .
In Windows 9x, the not only contains the DOS BIOS, but also holds the DOS kernel, which previously resided in . Under some conditions, Windows 9x uses the alternative filenames or instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the of the older system is named for as long as Windows 9x is active.
DR-DOS 7.06 (only this version) also follows this scheme and the filename in order to become bootable via MS-DOS boot sectors.
Similarly, FreeDOS uses a combined system file as well, but names it .
Disk layout requirements.
However, MS-DOS version 3.3 allows sector 4 and higher to be fragmented; version 5.0 allows the first 3 sectors of to be allocated anywhere (as long as they are contiguous).
 can be treated like any ordinary file.