An operating system is difficult to define,[7] but has been called "the layer of software that manages a computer's resources for its users and their applications".[8] Operating systems include the software that is always running, called a kernel—but can include other software as well.[7][9] The two other types of programs that can run on a computer are system programs—which are associated with the operating system, but may not be part of the kernel—and applications—all other software.[9] There are three main purposes that an operating system fulfills:[10] Operating systems allocate resources between different applications, deciding when they will receive central processing unit (CPU) time or space in memory.[10] On modern personal computers, users often want to run several applications at once. In order to ensure that one program cannot monopolize the computer's limited hardware resources, the operating system gives each application a share of the resource, either in time (CPU) or space (memory).[11][12] The operating system also must isolate applications from each other to protect them from errors and security vulnerability is another application's code, but enable communications between different applications.[13] Operating systems provide an interface that abstracts the details of accessing hardware details (such as physical memory) to make things easier for programmers.[10][14] Virtualization also enables the operating system to mask limited hardware resources; for example, virtual memory can provide a program with the illusion of nearly unlimited memory that exceeds the computer's actual memory.[15] Operating systems provide common services, such as an interface for accessing network and disk devices. This enables an application to be run on different hardware without needing to be rewritten.[16] Which services to include in an operating system varies greatly, and this functionality makes up the great majority of code for most operating systems.[17]
https://en.m.wikipedia.org/wiki/Operating_system
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