Sum-

chapter1:the definition of operating system is a program that acts as an intermediary between a user of a computer and the computer hardware.
three main purposes of an operating system：To provide an environment for a computer user to execute programs；To allocate the separate resources of the computer as needed to solve the problem given；supervision of thexecution of user programs to prevent errors and improper
use of the computer, and management of the operation and control of I/