IO.SYS

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 ofIO.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 (console, disk, serial port, etc..). At this point, the default devices are available.
3. Loads the DOS kernel and calls its initialization routine. The kernel is stored inMSDOS.SYS with MS-DOS and inIO.SYS with Windows 9x. At this point, "normal" file access is available.
4. Processes theMSDOS.SYS file with Windows 9x.
5. Processes theCONFIG.SYS file, in MS-DOS 2.0 and higher and Windows 9x.
6. LoadsCOMMAND.COM (or other operating system shell if specified).
7. Displays the bootsplash in Windows 9x. IfLOGO.SYS is present, it is used as the bootsplash. Otherwise, the bootsplash inIO.SYS is used.

TheIO.SYS filename was also used by Disk Control Program [de] (DCP), an MS-DOS derivative by the former East-German VEB Robotron.

IBM PC DOS and DR DOS use the fileIBMBIO.COM for the same purpose; it in turn loadsIBMDOS.COM.

In Windows 9x, theIO.SYS not only contains the DOS BIOS, but also holds the DOS kernel, which previously resided inMSDOS.SYS. Under some conditions, Windows 9x uses the alternative filenamesWINBOOT.SYS orJO.SYS instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily namedIO.W40 for as long as Windows' dual-boot feature has booted the previous OS. Likewise, theIO.SYS of the older system is namedIO.DOS for as long as Windows 9x is active.

DR-DOS 7.06 (only this version) also follows this scheme and theIO.SYS filename in order to become bootable via MS-DOS boot sectors.

Similarly, FreeDOS uses a combined system file as well, but names itKERNEL.SYS.

• The two first entries of the root directory must be allocated byIO.SYS andMSDOS.SYS, in that order.
• IO.SYS must be the first file stored in the FAT directory table for files.
• The filesIO.SYS andMSDOS.SYS must be contiguous.

However, MS-DOS version 3.3 allows sector 4 and higher to be fragmented; version 5.0 allows the first 3 sectors ofIO.SYS to be allocated anywhere (as long as they are contiguous).

COMMAND.COM can be treated like any ordinary file.

• MSDOS.SYS
• IBMBIO.COM
• DRBIOS.SYS
• COMMAND.COM
• List of DOS system files
• Hardware abstraction layer (HAL)
• Remote Program Load
• Architecture of Windows 9x