The first two I/O tasks noted in Section 1-5 concern the establishment of connections between the memory and the I/O devices and the control of data transfers over this connection．Most modern computers use a bus，or buses，for interconnecting the major components of the system．A bus serves as a pathway between components for passing address，instructions，and data．A simple high-level view of a bus is shown in Fig. 1-17．This is the bus shown in Fig. 1-16 that is used with the coprocessor I/O．This bus interconnects the processor，main memory，and I/O devices by means of their controllers．
Buses can be classified in a number of ways：by purpose，control，and communication technique．
1．Dedicated or general purpose
The major difference between dedicated and general-purpose buses is that a dedicated bus is point-to-point between two physical devices，whereas a general-purpose bus interconnects more than two physical devices．Fig. 1-18 illustrates the two types．Dedicated buses are used in cases in which the latency and the bandwidth requirements are such that sharing the bus with another user can result in unacceptable system performance．[1] Note that a dedicated bus does not need address lines because the source and destination are addressed by implication．That is，device 1 always sends to device 2 or device 2 to device 1．Some dedicated buses are unidirectional with information flow in only one direction．For example，the bus that connects a memory to a graphics controller may be unidirectional。
Because dedicated buses are used internal to the processor or for special high-bandwidth applications without general-purpose capabilities，these buses are not considered further．Instead，the following paragraphs discuss the control and communications design techniques found with general-purpose buses．
Also，with a general-purpose bus，a number of users share the same bus and simultaneous requests for the bus are resolved by one of a number of resolution techniques．Some of the devices on a general-purpose bus are both senders and receivers or only senders or only receivers．For example，a printer controller’s primary function is to receive data and send some status information．A disk controller，on the other hand，sends and receives data and sends status information．
总线和控制器
第1. 5节提到I/O的前两个任务是在存储器和I/O设备之间建立连接并在这种连接上控制数据的传送。大多数现代计算机用一条或多条总线连接系统的各主要部件。总线的作用是为各个部件之间传送地址、指令和数据提供通路。图1-17是总线的高层简单视图。这是图1-16中用于协处理器I/O的总线。这一总线由控制器将处理器、主存储器和I/O设备连在一起。
总线可以按多种方式分类，即按目的，按控制方法和按通信技术分类。
1．专用或通用
专用与通用总线的主要区别为：专用总线是两个物理设备之间的点对点连接，而通用总线连接多于两个物理设备。图1-18展示了这两种连接类型。当与另一个用户共享总线时，如果由于等待时间和带宽要求得不到满足使系统性能下降得不能忍受时，须采用专用总线。注意，由于在专用总线中，源和目的地址是隐含的，所以不需要地址线。也就是设备1总是向设备2发送信息或相反。某些专用总线是单向的，信息流只有一个方向。例如，连接存储器和图形控制器的总线就可以是单向的。
由于专用总线用于处理器内部，或用于不需要通用功能的大带宽专用系统，因而我们对这些总线不做进一步讨论。相反，下面将讨论通用总线中的控制和通信设计技术。
同样，对于通用总线，多个用户共享同一条总线，当他们同时对总线提出请求时，可采用多种技术中的一种加以解决。通用总线上的某些设备可以既是发送器又是接收器，也可以只是发送器，或只是接收器。例如，打印机控制器的主要功能是接收数据并发送某些状态信息。而磁盘控制器既要发送和接收数据，又要发送状态信息。