In MS-DOS, it contains the core operating system code, the DOS kernel.
Since MS-DOS 7.0, the DOS kernel code has been combined into IO.SYS (aka WINBOOT.SYS), and MSDOS.SYS has become an ASCII 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 pre-DOS 7 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 MS-DOS 7.0.
MSDOS.SYS in Windows 9x (95/98/ME):
- Microsoft Knowledge Base (MSKB): List of MSDOS.SYS articles
- MSKB: Contents of the Windows Msdos.sys File
- MSKB: How to create an Msdos.sys file larger than 1024 bytes
- 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