Editing IO.SYS (section)

== Boot sequence ==
In the [[IBM PC compatible|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 {{Mono|IO.SYS}} into memory and transfers control to it. {{Mono|IO.SYS}} then:

# Loads the rest of itself into memory.
# Initializes each default [[device driver]] in turn ([[System console|console]], disk, [[serial port]], etc..). At this point, the default devices are available.
# Loads the DOS kernel and calls its initialization routine. The kernel is stored in {{Mono|[[MSDOS.SYS]]}} with MS-DOS and in {{Mono|IO.SYS}} with Windows 9x. At this point, "normal" file access is available.
# Processes the {{Mono|MSDOS.SYS}} file with Windows 9x.
# Processes the {{Mono|[[CONFIG.SYS]]}} file, in MS-DOS 2.0 and higher and Windows 9x.
# Loads {{Mono|[[COMMAND.COM]]}} (or other [[operating system shell]] if specified).
# Displays the [[bootsplash]] in Windows 9x. If {{Mono|[[LOGO.SYS]]}} is present, it is used as the bootsplash. Otherwise, the bootsplash in {{Mono|IO.SYS}} is used.

The {{Mono|IO.SYS}} filename was also used by {{ill|Disk Control Program|de}} (DCP), an MS-DOS derivative by the former East-German [[VEB Robotron]].<ref name="Robotron_1988_DCP1700"/><ref name="DCP_2016"/>

[[IBM PC&nbsp;DOS]] and [[DR&nbsp;DOS]] use the file {{Mono|[[IBMBIO.COM]]}} for the same purpose; it in turn loads {{Mono|[[IBMDOS.COM]]}}.

In Windows 9x, the {{mono|IO.SYS}} not only contains the [[DOS BIOS]], but also holds the DOS kernel, which previously resided in {{mono|MSDOS.SYS}}. Under some conditions, Windows 9x uses the alternative filenames {{Mono|WINBOOT.SYS}}<ref name="Paul_1997_OD-A3"/> or {{Mono|JO.SYS}} instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named {{Mono|IO.W40}} for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the {{Mono|IO.SYS}} of the older system is named {{Mono|IO.DOS}} for as long as Windows 9x is active.

[[DR-DOS 7.06]] (only this version) also follows this scheme and the {{mono|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 {{Mono|KERNEL.SYS}}.