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.
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.
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 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.