An operating system (OS) is a suite of programs that manages the hardware resources of a computer and provides services, such as processor scheduling and memory management, required by other programs. Operating systems are present on all types of computing device, from smartphones and video games consoles through to mainframes and servers. Operating systems for multi-user computers, such as mainframes, may offer accounting functions for processor time, disk storage, printing and other resources. The operating system is the key component of the system software in a computer system and almost all other programs need it to be present in order to function.
As we have already noted, the operating system is responsible for hardware functions, such as input and output and memory allocation, and sits between programs and the computer hardware. The programs themselves are normally executed directly by the hardware, but they make system calls to OS functions and may be interrupted by the OS.
Examples of popular modern operating systems include Android, BSD, iOS, Linux, Mac OS X, Microsoft Windows and IBM z/OS. All these, except Windows and z/OS, are based on UNIX.
You can read more in this Wikipedia article on Operating Systems.
Reviews of many current operating systems can be found at:
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