DOS:- Disk operating system 

Operating system is a set of control programs which coordinates between user and hardware and forms an interface between them.

As stated in earlier chapter that Operating System (OS) is a system software which communicates with the hardware of computer.It accepts command in simple English language and converts it to signals or machine language which computer hardware can understand, Computer hardware are alactronic componants which communicates among themselves in electronic signals for performing their task. Users of computer does not understand electronic signals so it is next to impossible for a person to operate computer without OS.

Operating System performs lots of jobs. The jobs are defined in small computer programs. Thase programs are collectively known as OS. OS resides in main memory and monitors the activities of the computer. Operating System ensures the smooth functioning and continues operation of the computer.