<b>Operating System a Case Study</b> An Operating System or OS is the most important software and program that can run all types of Computer, Mobile phone, Hand Held Device, etc. It helps to connect and interface computer hardware to program and application software and other common essential services. The OS is initially loaded on at boot time. Like a host, the OS acts for running and handling of application programs and operation of the hardware on the machine. The working principle of OS is first to configure then check ability to perform and then allow the program to run. Its main purpose is to handle the activities based on computer hardware. The OS also provides management of input, output and storage devices. Mostly in a system, some different computer programs run at the same time and they need to access CPU, memory, and storage. In an operating system, the user can communicate with the Command Line Interface by typing some command, and the user receives the response back from the system. But Modern Time Operating system uses a graphical user interface or GUI for easy to use. Now user can communicate with a system using a mouse to click button, menus, and everything is displayed on the screen using Text, Graphics or image. It is like a bank manager it makes sure that the working of application software must not be interfered by any other activity. Introduction, Type of operating system, operating system service, security, function of operating system, Example of some operating system 166-175 Issue-3 Volume-2 Sumanta Kabiraj | Anjli Gupta | Prof. Subhashis Kumar Chandra