Operating system (OS) is a collection of programs that manages the resources in the computer system. It is a software foundation on which all other software rests. This allows the users to write programs in order to interact with the machine. It is the core of the system software.
Operating system is also called resource manager. It manages resources such as memory and input/output devices. It provides an interface between human being and computer system. That means a human being can interact with computer through through the operating system. The OS allows application software to access system resources.
Functions of OS include memory management, processor management, device management, file management, security, job accounting and corordination between other software and users.
Components
There two major components: Kernel and User Interface.
Kernel is the core of the operating system. It is a collection of programs getting loaded into the memory on booting. It does resource management. There are two types of kernels: monolithic kernel and microkernel.
User Interface (UI) allows user to enter and receive information. It can be text based or a graphical user interface. Later operating systems use graphical user interface.
Dual Mode Operation
An OS can be in one of two modes: kernel mode and user mode. A special bit is added to hardware to indicate the current mode. In user mode (bit 1), the computer system is executing on behalf of user application. The OS switches to kernel mode (bit 0) when user application requests a service. That is, the computer system executes on behalf of the operating system.
Dual boot system
A computer can have more than one operating system but only one will be active at any given time. The user has to choose the OS that is to be active once the computer is turned on. This configuration is called dual boot or multi-boot system.
Examples: DOS, Windows, Mac OS, Unix, Linux.
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