Synopsis Operating system is the heart of any computer. This field is changing field at a breathtakingly rapid rate, as computers are now prevalent in virtually every application. Yet the fundamental concepts remain the same and it is clear in all books of operating systems. This work provides a clear description on the concepts that are required for understanding the proper functioning of any operating system. The hardware text required for an understanding of operating system are included in this work for coding examples we use predominantly 'C' as well as Java, but the reader can still understand the algorithms without a through knowledge of these languages. Apart from these some case studies on Unix and on Windows are also included in this work.