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In general, the operating system is the first layer of software loaded into computer memory when it starts up. As the first software layer, all other software that gets loaded after it depends on this software to provide them with various common core services. These common core services include, but are not limited to: disk access, memory management, task scheduling, and user interfacing. Since these basic common services are assumed to be provided by the OS, there is no need to re-implement those same functions over and over again in every other piece of software that you may use. The portion of code that performs these core services is called the " kernel" of the operating system. Operating system kernels had been evolved from libraries that provided the core services into unending programs that control system resources because of the early needs of accounting for computer usage and then protecting those records.
Colloquially, the term is most often used to mean all the software which "comes with" a computer system before any applications are installed.
The operating system ensures that other applications are able to use memory, input and output devices and have access to the file system. If multiple applications are running, the operating system schedules these such that all processes have sufficient processor time where possible and do not interfere with each other.
As operating systems evolve, ever more services are expected to be common core. These days, an OS may be required to provide network and Internet connectivity. They may be required to protect the computer's other software from damage by malicious programs, such as viruses. The list of common core services is ever expanding.
Programs communicate with each other through Application Programming Interfaces, or API's, similar to how humans interact with programs through User interfaces. This is especially true between application programs and the OS. The OS's common core services are accessed by application programs through the OS's API's.
The Microsoft WindowsImage use policy. Microsoft Windows is a range of commercial operating environments for personal computers. The range was first introduced by Microsoft in 1985 and eventually has come to dominate the world personal computer market. All recent versions of family of operating systems originated as a graphical layer on top of the older MS-DOSMicrosoft's disk operating system, MS-DOS was the first popular operating system for the IBM PC. It was originally released with the PC in 1981 and had seven major versions before Microsoft stopped development in 1995. It was the key product in Microsoft' environment for the IBM PC. Modern versions are based on the newer Windows NT core that first took shape in OS/2. Windows runs on 32- and 64-bit Intel and AMD computers, although earlier versions also ran on the DEC Alpha and PowerPC architectures. Today, Windows is the most popular desktop operating system, enjoying a near- monopoly of around 90% of the worldwide desktop market share. It is also widely used on low-end and mid-range servers, supporting applications such as Web servers and database servers.
The Unix-like family is a more diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "Unix" is a trademark of The Open Group which licenses it for use to any operating system that has been shown to conform to the definitions that they have cooperatively developed. The name is commonly used to refer to the large set of operating systems which resemble the original Unix. Unix systems run on a wide variety of machine architectures. Unix systems are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD are increasingly popular, and have made inroads on the desktop market as well. Apple's Mac OS X, a BSD variant, has replaced Apple's earlier (non-Unix) Mac OS in a small but dedicated market, becoming one of the most popular Unix systems in the process.
Mainframe operating systems, such as IBM's z/OS, and embedded operating systems such as eCos, are usually unrelated to Unix and Windows.
Older operating systems which are still used in niche markets include the Windows-like OS/2 from IBM; VMS from Hewlett-Packard (formerly DEC); and Mac OS, the non-Unix precursor to Apple's Mac OS X.
Research and development of new kinds of operating systems is an active subfield of computer science.