One of the system files in the MS-DOS Operating System. A hidden file in the root directory, sometimes called the kernal for DOS. When an application needs to access a device or peripheral, this file translates the request into actions that IO.SYS can perform.

n. One of two hidden system files installed on an MS-DOS startup disk. MSDOS.SYS, called IBMDOS.SYS in IBM releases of MS-DOS, contains the software that makes up the heart (kernel) of the operating system. See also IO.SYS.

MSDOS.SYS is an important system file on MS-DOS and Windows 9x systems. It is run after IO.SYS and contains the core operating system code for DOS (the DOS kernel). On Windows 9x systems the DOS kernel code was merged into IO.SYS and MSDOS.SYS is simply an ASCII file containing configuration directives.