INPUT/OUTPUT SYSTEM

　　Processors need a source of input data and a destination for output data．Processors also require storage space for large volumes of intermediate data．Thus the I/O system provides these storage facilities by connecting the processor’ s memory to the I/O devices．[1] These devices vary widely in latency and bandwidth；[2] thus，a major function of the I/O system is to match their latencies and bandwidths to that of the memory system．I/O devices consist of such diverse items as keyboards，modems，and hard disks．Each of these devices has its own latency and bandwidth requirements that must be satisfied for a properly functioning system．
　　The major tasks of the I/O system are the following：
　（1）to establish the connection between the memory and the I/O devices；
　（2）to synchronize and manage the control of the data transfer；
　（3）to provide buffering when the source and sink bandwidths and latencies differ；
　（4）to perform code conversions if required；
　（5）to interrupt the processor when required；
　（6）to terminate the operation when it is completed．
　　These tasks are accomplished by hardware and software，in varying amounts of each．The more hardware-oriented，the higher the performance and the higher the hardware cost．The more software-oriented，the lower the performance and the lower the hardware cost．Control can be classified into two broad categories：grogrammed I/O and coprocessor I/O．

　　Programmed 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．With programmed I/O，shown in Fig. 1-12，there are several architectural registers（minimally an input register and an output register）that are addressed by special move instructions．[3]

　　These move instructions move information from the accurnulator（or general-purpose register）to/from the I/O registers．The peripherals are connected directly to the bits of these registers. [4]

 　　输入输出（I/O）系统
   
　　处理器需要有输入数据的源设备和用于数据输出的目的设备。它还要求有存储大量中间数据的存储空间，I/O系统就是通过把处理器的存储器与I/O设备连接起来的存储设施。这些设备在等待时间和带宽上是大不相同的；因此I/O系统的主要功能是使其在执行时间和带宽上与存储系统协调起来。I/O设备包括键盘、调制解调器和硬盘等多种设备。对每种设备自身的等待时间和带宽，必须满足系统正确操作的要求。
   　 I/O系统的主要任务是：
    　（1）建立存储器和I/O设备之间的连接；
    　（2）对数据传输的控制进行同步化与管理；
    　（3）当信源和信宿的带宽和等待时间不同时，提供缓存功能；
　　（4）需要时完成代码的转换；
    　（5）需要时中断处理器的工作；
   　 （6）任务完成时终止操作。
    　这些任务都是通过硬件和软件完成的，不过对硬件或软件的依赖程度各不相同。越是依靠硬件，则性能越高，硬件成本越大；越是依靠软件，则性能越差，硬件费用越低。控制方法可以分为两大类：程序控制I/O和协处理器I/O。

　　程序控制I/O

　　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。如图1-12所示的程序控制I/O，有几个体系结构寄存器（至少1个输入寄存器和1个输出寄存器），它们由专用的传送指令寻址。
    这些传送指令在累加器（或通用寄存器）和I/O寄存器之间传送信息，而外围设备直接与每个寄存器相连。