IO.SYS

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IO.SYS is an essential part of MS-DOS and Windows 9x. It contains the default MS-DOS device drivers^[1] (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 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, etc..). At this point, the default devices are available.
3. Loads the DOS kernel and calls its initialization routine. The kernel is stored in MSDOS.SYS with MS-DOS and in IO.SYS with Windows 9x. At this point, "normal" file access is available.
4. Processes the MSDOS.SYS file with Windows 9x.
5. Processes the CONFIG.SYS file, in MS-DOS 2.0 and higher and Windows 9x.
6. Loads
   operating system shell
   if specified).
7. Displays the bootsplash in Windows 9x. If LOGO.SYS is present, it is used as the bootsplash. Otherwise, the bootsplash in IO.SYS is used.

The IO.SYS filename was also used by

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

Disk layout requirements

• The two first entries of the root directory must be allocated by IO.SYS and MSDOS.SYS, in that order.^[5][6]
• IO.SYS must be the first file stored in the
  FAT directory table for files.^[5][6]
• The files IO.SYS and MSDOS.SYS must be contiguous.^[5][6]

However, MS-DOS version 3.3 allows sector 4 and higher to be fragmented; version 5.0 allows the first 3 sectors of IO.SYS to be allocated anywhere (as long as they are contiguous).^{[nb 1][7][5][8]}

COMMAND.COM 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

Notes

References

1. ^ "MS-DOS Device Driver Names Cannot be Used As File Names". Revision 2.0. Microsoft. 2003-05-12. KB74496. Q74496. Archived from the original on 2012-07-21.
2. VEB Robotron-Projekt Dresden. 1988-01-29. C3013-0000-0 M3030, Textdiskette DCP1700 T1/2 C3010-9991-2M3020, Disk-Nr.: 5962 /88 ROBOTRON PROJEKT DRESDEN KA2 DD-DS 80*16*256. Archived
   (PDF) from the original on 2021-08-29. Retrieved 2021-12-03.
3. ^ Kurth, Rüdiger; Groß, Martin; Hunger, Henry (2016-11-29) [2007]. "Betriebssystem DCP". www.robotrontechnik.de (in German). Archived from the original on 2019-04-03. Retrieved 2019-04-28.
4. ^ Paul, Matthias R. (1997-10-02) [1997-09-29]. "Caldera OpenDOS 7.01/7.02 Update Alpha 3 IBMBIO.COM - README.TXT and BOOT.TXT - A short description of how OpenDOS is booted". Archived from the original on 2003-10-04. Retrieved 2009-03-29. [1] (NB. Also has a few bits on how other DOSes are booted.)
5. ^ ^{a b c d} "SYS.COM Requirements in MS-DOS Versions 2.0–6.0". Revision 2.0. Microsoft. 2003-05-23. KB66530. Q66530. Archived from the original on 2012-07-17.
6. ^
   OCLC 16581341. (xix+1570 pages; 26 cm) (NB. This edition was published in 1988 after extensive rework of the withdrawn 1986 first edition by a different team of authors. [2]
   )
7. ISBN 978-0-201-60835-9. (xxvi+738+iv pages, 3.5"-floppy [3][4]) Errata: [5][6][7]
8. ^ Kim, Mark K. (2004-09-24). "Re: [vox-tech] rescuing winxp?". mail-archive.com. Archived from the original on 2019-04-06. Retrieved 2019-04-06.

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