Table 1-3 provides examples of the four general-purpose bus types from the taxonomy：centralized and decentralized control and synchronous and asynchronous communications．
PCl
The peripheral component interconnect（PCI）is a recent high-bandwidth，processor-independent bus that can function as a mezzanine or peripheral bus．Compared with other common bus specifications，PCI delivers better system performance for high-speed I/O subsystems（e．g．，graphic display adapters，network interface controllers，disk controllers，and so on）．The current standard allows the use of up to 64 data 1ines at 33 MHz，for a raw，tansfer rate of 264 MB/s，or 2．112 Gb/s．[3] But it is not just a high speed that makes PCI attractive．Economically，PCI is specifically designed to meet the I/O requirement of modern systems；it requires very few chips to implement and supports other buses attached to the PCI bus．
PCI is designed to support a variety of microprocessor-based configurations，including both single - and multiple -processor systems．Accordingly，it provides a general-purpose set of functions．It makes use of synchronous timing and a centralized arbitration scheme．
Fig. 1-21（a）shows a typical use of PCI in a single-processor system．A combined DRAM controller and bridge to the PCI bus provides tight coupling with the processor and the ability to deliver data at high speeds．[4] The bridge acts as a data buffer so that the speed of the PCI bus may differ from that of the processor’ s I/O capability．In a multiprocessor system（Fig. 1-21（b）），one or more PCI configurations may be connected by bridges to the processor’s system bus．The system bus supports only the processor / cache units，main memory，and the PCI bridges．Again，the use of bridges keeps the PCI independent of the processor speed yet provides the ability to receive and deliver data rapidly．[5]
4．总线设计举例
表1-3从分类学角度给出了4种通用总线类型：集中式和分散式控制及同步和异步通信。
PCI
外设部件互连（PCI）是一种近来使用的大带宽、独立于处理器的总线，可以作为底层或外围总线。与其他通用总线规范比较，PCI为高速I/O子系统（如图形显示适配器、网络接口控制器、磁盘控制器）提供更好的系统性能。现在的标准在33MHz主频时，可使用多达64条数据线，原数据传输速率可达264Mb/s，或2. 112Gb/s。PCI不仅因高传输速率而具有吸引力，它还是为满足低价位的现代系统而专门设计的；要求很少的芯片即可实现并支持与PCI总线连接的其他类型总线。
PCI支持各种基于微处理器的配置，包括单处理器系统和多处理器系统。相应地，它提供一套通用功能，并采用同步定时和集中仲裁机制。
图1-21（a）展示了PCI在单处理器系统中的典型应用。连向PCI总线的DRAM控制器和桥接器的组合与处理器紧密耦合，具有高速数据传输能力。桥接器的作用相当于数据缓冲器，用于解决PCI总线速度与处理器I/O能力不匹配的问题。在多处理器系统中（如图1-21（b）所示）可通过桥接器把1个或多个PCI配置与处理器系统总线相连。系统总线只支持处理器-高速缓存部件、主存储器和PCI桥接器。再者，桥接器的使用让 PCI与处理器速度无关，却带来了数据接收和发送的高速度。