The Operating System (OS) is a special computer program that is essential to the efficient running of all the other programs within the computer. The operating system carries out tasks, key press instructions and controls which operations within the computer are carried out and in which order. Some common operating systems are Windows, Linux and Macintosh When the computer is switched on it carries out a Power On Self Test (POST) and boot-up entirely on its own because the necessary software is in ROM memory. This process ensures that all the hardware components are running and that the CPU and memory are functioning correctly. A series of beeps denotes any detected errors. The second function of the boot-up operation is to search drives for an operating system and then launch it, by reading the operating system files and copying them to the RAM. The operating system is not part of the PC and needs to be located at each boot-up, this makes upgrading or fixing errors in the OS much simpler and allows a choice of operating systems. A Graphical User Interface (GUI) is a way of showing the computer's facilities using icons (pictures) and menus (names). Instead of typing a technical instruction the mouse is used to click on an icon (picture) or menu (names) to select or perform an action.