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 used the file KERNEL.SYS for the same purpose.
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.
- 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 2014-11-22.
- "How to create an MSDOS.SYS file larger than 1024 bytes". 1.3. Microsoft. 2004-12-21. KB129998, D36528. Retrieved 2014-11-22.