(DOS) The original disk operating system from IBM.

DOS was the low-end OS of choice on the IBM 360, the high-end system was called just "OS". DOS had a smaller kernel and less functionality than OS and could run on the typical 32K 360/30 and 64K 360/40 class machines.

DOS was a successor to TOS.

(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).

A DOS should present a uniform interface to different 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).