IO.SYS

Template:Distinguish Template:More citations needed Template:Short description Template:Use dmy dates Template:Use list-defined references Template:Mono is an essential part of MS-DOS and Windows 9x. It contains the default MS-DOS device drivers<ref name="Microsoft_Built-in_devices"/> (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 Template:Mono into memory and transfers control to it. Template:Mono 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 in Template:Mono with MS-DOS and in Template:Mono with Windows 9x. At this point, "normal" file access is available.
4. Processes the Template:Mono file with Windows 9x.
5. Processes the Template:Mono file, in MS-DOS 2.0 and higher and Windows 9x.
6. Loads Template:Mono (or other operating system shell if specified).
7. Displays the bootsplash in Windows 9x. If Template:Mono is present, it is used as the bootsplash. Otherwise, the bootsplash in Template:Mono is used.

The Template:Mono filename was also used by Template:Ill (DCP), an MS-DOS derivative by the former East-German VEB Robotron.<ref name="Robotron_1988_DCP1700"/><ref name="DCP_2016"/>

IBM PC DOS and DR DOS use the file Template:Mono for the same purpose; it in turn loads Template:Mono.

In Windows 9x, the Template:Mono not only contains the DOS BIOS, but also holds the DOS kernel, which previously resided in Template:Mono. Under some conditions, Windows 9x uses the alternative filenames Template:Mono<ref name="Paul_1997_OD-A3"/> or Template:Mono instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named Template:Mono for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the Template:Mono of the older system is named Template:Mono for as long as Windows 9x is active.

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

Similarly, FreeDOS uses a combined system file as well, but names it Template:Mono.

Disk layout requirements

• The two first entries of the root directory must be allocated by Template:Mono and Template:Mono, in that order.<ref name="msrq"/><ref name="Duncan_1988_MS-DOS_Encyclopedia"/>
• Template:Mono must be the first file stored in the FAT directory table for files.<ref name="msrq"/><ref name="Duncan_1988_MS-DOS_Encyclopedia"/>
• The files Template:Mono and Template:Mono must be contiguous.<ref name="msrq"/><ref name="Duncan_1988_MS-DOS_Encyclopedia"/>

However, MS-DOS version 3.3 allows sector 4 and higher to be fragmented; version 5.0 allows the first 3 sectors of Template:Mono to be allocated anywhere (as long as they are contiguous).<ref group="nb" name="NB_PC_DOS_Contiguous_files"/><ref name="Chappell_1994_DOS-Internals"/><ref name="msrq"/><ref name="rxp"/>

Template:Mono can be treated like any ordinary file.

See also

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