d2.txt

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Hardware:
Lowest level of the computer
The physical components from which the computer system is constructed
BIOS  （ Basic Input/Output System）
Most primitive level of software
Deals directly with the signals that control each hardware component
Most of its work is done when the system first boots up
Device drivers:
Helper programs used by Operating systems to control a device
In order to add new hardware, the right device driver must be present.
Provided by the device manufacturer in floppies or other storage media
Instructions are to be followed to install the device and its drivers.
Windows OS may sometimes have such drivers stored.

kernel:
Performs functions that are critical to maintaining the operating system
The kernel is responsible for 
Memory management
Process and task management
Disk management. 
Interrupt Handling
Resident in RAM at all times
Because space is a factor, the kernel is kept small
handles the various types of interrupts

Operating System (OS):
Implements all other functions the OS is to perform
File system management
Communicates with the kernel when basic actions such as data transfer operation to a peripheral
Master controller of all the activities of the computer
Sets standards for all the application software used in a computer system
Different operating systems have different user interfaces and run same applications	
Examples:  Microsoft Windows, UNIX, LINUX, Mac OS，iSeries

Operating System (OS):
Implements all other functions the OS is to perform
File system management
Communicates with the kernel when basic actions such as data transfer operation to a peripheral
Master controller of all the activities of the computer
Sets standards for all the application software used in a computer system
Different operating systems have different user interfaces and run same applications	
Examples:  Microsoft Windows, UNIX, LINUX, Mac OS，iSeries
  
External Services of OS: 
Help users start programs
Manage stored data
Maintain security of the system
Provides ways to select an application program, find, rename and delete documents and other data stored on disk
Internal services of OS:
controls input and output
Allocates system resources (e.g. memory, disk drive capacity, processor time)
Manages storage for programs and data
Detects equipment failures

Operating systems can be classified into two categories.
A server operating system is designed for computers that provide centralized storage facilities and communications capabilities for networks and Web sites.
A desktop operating system is designed for a single-user microcomputer.

操作系统（OS）：是一种管理计算机硬件和软件等计算机系统资源，控制系统工作流程，方便用户使用计算机的一系列程序组成的系统软件。操作系统是直接运行在裸机上的最基本的系统软件，是系统软件的核心，其它的所有软件都必须在操作系统的支持下才能运行。 
  操作系统的地位：紧贴系统硬件之上，所有其他软件之下，是其他软件的共同环境。
   操作系统的作用：在整个计算机系统中，操作系统充当“总管家”和“服务生”的角色，即操作系统是管理硬件资源协调后台工作的“总管家”，同时又是提供用户与计算机交互接口的“服务生”。   


操作系统的五大功能:
处理器管理——包括作业和进程调度，进程控制和进程通信管理。 
存储管理——包括内存分配，地址映射，内存保护和内存扩充管理。 
设备管理——包括缓冲区管理，设备分配，设备驱动和设备无关性管理 
文件管理——包括文件存储空间的管理，文件操作的一般管理，目录管理，文件的读写管理和存取控制。 
    注意：文件是管理的基本单位
用户接口管理——包括命令界面，程序界面，图形界面管理。



Application Program Interface (API):
A set of routines, protocols, and tools for building software applications
Abstraction defined by OS for applications
Application communicates with OS through the API.
Run time Libraries:
A library of routines that are bound to the program during execution
Collection of Software routines which application programs rely on
Functions that make appropriate API calls needed to enable the OS to perform accordingly
Application:
Layer where the routines perform tasks the application is designed to do.
 


User interface:
Responsible for the communication between the application and the user
Typically it is a GUI, composed of buttons and pull-down menus
The GUI passes the information  to the application
Scripts or macros:
Routines that many applications use to allow users to create documents using the application’s  built-in commands.
Allow users to automate sequences of actions they perform frequently
Can perform any function that does not require additional user input


The computer industry relies on specialists in each layer of software. 
Some people make their living writing BIOS software.  Others concentrate on improving GUI technology.
The greatest number of programmers are found at the application level.

The BIOS performs three major functions
initializes the hardware when the computer is first turned on
loads the O/S
provides basic support for devices such as the keyboard, mouse, and serial ports
The BIOS resides on a ROM chip
Non-volatile
Slower than DRAM
Loads itself into DRAM
Uses CMOS
CMOS is powered by battery, therefore, it is volatile
Contains BIOS parameter settings for hardware and memory speed 


Boot disk is a floppy/CD containing bootable copy of OS.
Used in the case of hard drive corruption when the system crashes.
Computer that can be booted by a floppy is not fully secure.
Anyone can boot their version of OS, which ignores the file protections and can access any file on the system.




Process is an instance of  a running program
Process includes a set of memory pages, a set of open file descriptors (if the process has any I/O), a process ID, etc. 
Keeps track of all the processes running
Kernel maintains a queue of processes
On a UNIX machine, the ps and top commands display process information.

进程是程序及其数据在计算机上的一次运行活动，它属于一种动态的概念(Active entity)。进程的运行实体是程序，离开程序进程没有存在的意义。从静态角度看，进程是由程序、数据和进程控制块PCB三部分组成的。而程序是一组有序的指令集合，属于一种静态的概念(Passive entity)。
进程是程序的一次执行过程，它是动态地创建和消亡的，具有一定的生命期，是暂时存在的；而程序则是永久存在的，可长期保存。
一个进程可以执行一个或几个程序，一个程序也

】可以构成多个进程。