IO.SYS

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IO.SYS is an essential part of DOS. 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:

1. Loads the rest of itself into memory.
2. Initializes each default device driver in turn (console, disk, serial port, et cetera). At this point, the default devices are available.
3. Loads MSDOS.SYS and calls its initialization routine. At this point, "normal" file access is available.
4. Processes the CONFIG.SYS file.
5. Loads command.com (or other operating system shell if specified).

[edit] External Links

• MS-DOS Device Driver Names Cannot be Used As File Names — includes list of default device drivers.
• SYS.COM Requirements in MS-DOS Versions 2.0-6.0 — includes requirements for IO.SYS to be loaded.