what is an operating system explain its functions
Operating system An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs. All computer programs, excluding firmware, require an operating system to function.
Single user mode Unix-like operating systems provide single user mode functionality either through the System V-style runlevels, BSD-style boot-loader options, or other boot-time options.
iOS iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that presently powers many of the company's mobile devices, including the iPhone, iPad, and iPod Touch. It is the second most popular mobile operating system globally after Android.
Denial-of-service attack A wide array of programs are used to launch DoS-attacks.