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An operating system (OS) is a software program that manages the hardware and software resources of a computer. A key component of system software, the OS performs basic tasks, such as controlling and allocating memory, prioritizing the processing of instructions, controlling input and output devices, facilitating networking, and managing files.
Modern general-purpose computers, including personal computers and mainframes, have an operating system (a general purpose operating system) to run other programs, such as application software. Examples of operating systems for personal computers include Microsoft Windows, Linux, Mac OS (Darwin), and Unix.
The lowest level of any operating system is its kernel. This is the first layer of software loaded into memory when a system boots or starts up. The kernel provides access to various common core services to all other system and application programs. These services include, but are not limited to: task scheduling, memory management, disk access, and access to hardware devices.
Apart from the kernel, an operating system is often distributed with system software that manages a graphical user interface (although Windows and Macintosh have integrated these programs into the operating system), as well as utility programs for tasks such as managing files and configuring the operating system. Oftentimes distributed with operating systems are application software that does not directly relate to the operating system's core function, but which the operating system distributor finds advantageous to supply with the operating system.
The delineation between the operating system and application software is not precise, and is occasionally subject to controversy. From commercial or legal points of view, the delineation can depend on the contexts of the interests involved. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's web browser was part of its operating system, or whether it was a separable piece of application software.
Like the term "operating system" itself, the question of what exactly the "kernel" should manage is subject to some controversy, with debates over whether things like file systems should be included in the kernel. Various camps advocate microkernels, monolithic kernels, and so on.
Operating systems are used on most, but not all, computer systems. The simplest computers, including the smallest embedded systems and many of the first computers did not have operating systems. Instead, they relied on the application programs to manage the minimal hardware themselves, perhaps with the aid of libraries developed for the purpose. Commercially-supplied operating systems are present on virtually all modern devices described as computers, from personal computers to mainframes, as well as mobile computers such as PDAs and mobile phones.