An operating system is the program that manages all the application programs in a computer system. This also includes managing the input and output devices, and assigning system resources. Operating systems evolved as the solution to the problems that were evident in early computer systems, and coincide with the changing computer systems. Three cycles are clear in the evolution of computers, the mainframe computers, minicomputers and microcomputers, and each of these stages influenced the development of operating systems. Now, advances in software and hardware technologies have resulted in an increased demand for more sophist