A number of mechanisms, including as system calls, built-in commands, and user-level programming support, can be used to provide most operating system functions. Give three instances of useful...

A number of mechanisms, including as system calls, built-in commands, and user-level programming support, can be used to provide most operating system functions.
Give three instances of useful operating system functionality from either Linux, MacOS, or Windows, indicating the technique by which the service is given.
(Related to the concept of an operating system)