<operating system> (DOS) The name of a number of operating systems which include facilities for storing files on disk. Such a system must handle physical disk I/O, the mapping of file names to disk addresses and protection of files from unauthorised access (in a multi-user system). Ideally, it will present a uniform interface to different kinds of storage device such as floppy disks, hard disks and magnetic tape drives. It may also provide some kind of locking to prevent unintentional simultaneous access by two processes to the same file (or record).