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Operating System (OS)

An operating system (OS) is the program that, after being initially loaded into the computer by a boot program, manages all the other programs in a computer. The other programs are called applications or application programs. The application programs make use of the operating system by making requests for services through a defined application program interface (API). In addition, users can interact directly with the operating system through a user interface such as a command line or a graphical user interface (GUI).

An operating system performs these services for applications:

• In a multitasking operating system where multiple programs can be running at the same time, the operating system determines which applications should run in what order and how much time should be allowed for each application before giving another application a turn.
• It manages the sharing of internal memory among multiple applications.
• It handles input and output to and from attached hardware devices, such as hard disks, printers, and dial-up ports.
• It sends messages to each application or interactive user (or to a system operator) about the status of operation and any errors that may have occurred.
• It can offload the management of what are called batch jobs (for example, printing) so that the initiating application is freed from this work.
• On computers that can provide parallel processing, an operating system can manage how to divide the program so that it runs on more than one processor at a time.

All major computer platforms (hardware and software) require and sometimes include an operating system, and operating systems must be developed with different features to meet the specific needs of various form factors.

Common desktop operating systems:

Windows is Microsoft’s flagship operating system, the de facto standard for home and business computers. Introduced in 1985, the GUI-based OS has been released in many versions since then. The user-friendly Windows 95 was largely responsible for the rapid development of personal computing.

Mac OS is the operating system for Apple's Macintosh line of personal computers and workstations.

Linux is a Unix-like operating system that was designed to provide personal computer users a free or very low-cost alternative. Linux has a reputation as a very efficient and fast-performing system. 

Windows operating systems have long dominated the market and continue to do so. As of August 2016, Windows systems had a market share of over 85 percent. In contrast, Mac OS was at a little over 6 percent and Linux was just over 2 percent. Nevertheless, Windows is losing market share from a long-held 90 percent and higher.

A mobile OS allows smartphones, tablet PCs and other mobile devices to run applications and programs. Mobile operating systems include Apple iOS, Google Android, BlackBerry OS and Windows 10 Mobile. 

An embedded operating system is specialized for use in the computers built into larger systems, such as cars, traffic lights, digital televisions, ATMs, airplane controls, point of sale (POS) terminals, digital cameras, GPS navigation systems, elevators, digital media receivers and smart meters.