Abstract view of the components of a computer system.
1.1 What Operating Systems Do?
1.1.1 User View
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The Operating System is designed to maxmize resource utilization to ensure that all available CPU time , memory and I/O are used efficently and no users takes more than her fair share.
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The Operating System is designed to compromise between individual usablity and resource utilization.
1.1.2 System View
An operating system is a control program.
The Operating System is the program most intimately invllved with the hardware . we can view
an operating system as a resource allocator(资源分配器）.
For CPU time ,memory space（ 内存空间）,file-storage space（文件储存空间) ,I/O devices, System Operating acts as the manger of these resources.
1.1.3 Defining Operating Systems
During the computer’s development ,computers are present wthin cars,ships,homes and so on . The early computers are mostly used for military uses such as codes breaking and calculation . As time goes by these computers evolved into general-purpose（多方面的）, multifunction mainframes（大型功能机）, and that’s when operating systems were born .
Operating systems exist because they offer a reasonable way to solve the problem of creating a usable computing system. The fundamental goal of computer systems is to execute user programs and to make solving user problems easier. Since bare hardware alone is not particularly easy to use, application programs are developed. These programs require certain common operations, such as those controlling the I/O devices. The common functions of controlling and allocating resources are then brought together into one piece of software: the operating system.
Mobile operating systems often include not only a core kernel9（内核） but also middleware—a set of software frameworks that provide additional services to application developers.
1.2 Computer-System Organization
This section is concerned whih Computer-System Organization
1.2.1 Computer-System Organization
A modern general-purpose computer system consists of one or more CPUs and a number of device controllers connected through a common bus that provides access to shared memory （such as this picture）
1.2.2 Storage Structure
The wide variety of storage systems can be organized in a hierarchy according to speed and cost.
1.2.3 I/O Structure
Storage is only one of many types of I/O devices within a computer. A large portion of operating system code is dedicated to managing I/O , both because of its importance to the reliability and performance of a system and because of the varying nature of the devices.