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.
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.
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.
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 uses 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+ operating systems (Vista, 2008, 7, 8, 8.1, and 10) use bootmgr instead, as they have a different boot sequence.
MSDOS.SYS in Windows 9x (95/98/ME):
- Microsoft Knowledge Base (MSKB): List of MSDOS.SYS articles
- MDGx: Windows 95/98/ME Complete MSDOS.SYS Reference
- UKT Support: Contents of the MSDOS.SYS File
- Computer Hope: Information about Window MSDOS.SYS file
- MDGx: WINBOOT.INI
- Robbins, Judd (1991). Mastering DOS 5 (3rd ed., pp. 68-69). San Francisco, California: Sybex.
- "Windows Does Not Start Normally With Bootgui=1 in the Msdos.sys". 2.0. Microsoft. 2013-10-24. KB136416, Q136416. Retrieved 2015-08-20.
- The dummy comment in unmodified copies of MSDOS.SYS explains these reasons explicitly.
- "Contents of the Windows MSDOS.SYS file". 1.4. Microsoft. 2007-01-19. KB118579, Q118579. Retrieved 2015-08-20.
- "How to create an MSDOS.SYS file larger than 1024 bytes". 1.3. Microsoft. 2004-12-21. KB129998, D36528. Retrieved 2014-11-22.