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MSDOS.SYS is a vital system file on MS-DOS and Windows 9x operating systems.

In versions of MS-DOS from 1.1x through 6.22, the file comprised the MS-DOS kernel and is responsible for file access and program management. MSDOS.SYS is loaded by the DOS BIOS IO.SYS as part of the boot procedure.[1]

In Windows 95 (MS-DOS 7.0) through Windows ME (MS-DOS 8.0), the DOS kernel has been combined with the DOS BIOS into a single file, IO.SYS (aka WINBOOT.SYS), while MSDOS.SYS became a plain text file containing boot configuration directives instead. If a WINBOOT.INI file exists, the system will retrieve these configuration directives from WINBOOT.INI rather than from MSDOS.SYS.[2]

Some DOS utilities expect the MSDOS.SYS file to have a minimal file size of at least 1 KB. This is the reason why a large dummy comment is typically found in the MSDOS.SYS configuration file since Windows 95.[3][4][5]

By default, the file is located in the root directory of the bootable drive/partition (normally C:\ for hard disks) and has the hidden, read-only, and system file attributes set.

IBM PC DOS as well as DR DOS since 5.0 (with the exception of DR-DOS 7.06) used the file IBMDOS.COM for the same purpose, whereas DR DOS 3.31 to 3.41 used DRBDOS.SYS instead. FreeDOS used the file KERNEL.SYS for the same purpose.

Windows NT-based operating systems (NT 3.1-4, 2000, XP, and 2003) use the NTLDR file and NT 6+ (Vista, 2008, and 7) uses bootmgr instead, as they have a different boot sequence.

See also[edit]

External links[edit]

MSDOS.SYS in Windows 9x (95/98/ME):


  1. ^ Robbins, Judd (1991). Mastering DOS 5 (3rd ed., pp. 68-69). San Francisco, California: Sybex.
  2. ^ "Windows Does Not Start Normally With Bootgui=1 in the Msdos.sys". 2.0. Microsoft. 2013-10-24. KB136416, Q136416. Retrieved 2015-08-20. 
  3. ^ The dummy comment in unmodified copies of MSDOS.SYS explains these reasons explicitly.
  4. ^ "Contents of the Windows MSDOS.SYS file". 1.4. Microsoft. 2007-01-19. KB118579, Q118579. Retrieved 2015-08-20. 
  5. ^ "How to create an MSDOS.SYS file larger than 1024 bytes". 1.3. Microsoft. 2004-12-21. KB129998, D36528. Retrieved 2014-11-22.