Computer systems consist of hardware and software.  Hardware is the physical part of the system. Once designed，hardware is difficult and expensive to change.  Software is the set of programs that instruct the hardware and is easier to modify than hardware.
    计算机系统由硬件和软件组成。硬件是系统的物理部分。硬件一旦设计完毕，要修改是困难的，并且花费也大。软件是指挥硬件的程序集合，比硬件容易修改。

  Every computer has four basic hardware components：
    •  Input devices．
    •  Output devices.
    •  Main memory.
    •  Central processing unit（CPU）．
  每台计算机都有如下4种基本硬件部件：
 输入设备；
 输出设备；
 主存储器；
 中央处理器。

　　A processor is composed of two functional units—a control unit and an arithmetic/logic unit—and a set of special workspaces called registers．
    处理器由两个功能部件（控制部件和算术逻辑部件）与一组称为寄存器的特殊工作空间组成。

　　The Control Unit　　控制部件
　　The Arithmetic and Logic Unit　　算术逻辑部件
　　Registers　　寄存器
　　Memory Devices　　存储器
　　RANDOM-ACCESS MEMORY　　随机存储器
　　READ-ONLY MEMORY　　只读存储器
　　MAGNETIC DISKS　　磁盘
　　CD-ROMS　　只读光盘
　　MAGNETIC TAPE　　磁带

Programmed I/O　　程序控制I/O
　　Programmed I/O，also known as direct I/O，is accomplished by a program executing on the processor itself to control the I/O operations and transfer the data．
　　程序控制I/O，又称直接I/O，由在处理器上执行的程序去控制I/O操作和传送数据。

Memory-Mapped I/O　　存储器映射I/O
　　Memory-Mapped I/O is another form of programmed I/O that maps the device connections to bits in memory address space，
    存储器映射I/O是另一种程序控制I/O，它把I/O设备连接映射成存储器地址空间中的位组，

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