摘要： 在一个典型的具有３级总线结构的ＰＣ机里，一方面各级总线上的主设备都可面对自己的地址空间启动各自总线上的访问周期，另一方面系统中各类地址空间实体分布在各级总线上，使得各级总线主设备的访问地址与各处实际空间的映射关系千头万绪．该文把物理上分布于各处的地址空间组织成一个整体，并视之为一个独立于各级总线主设备地址空间之外的、又为各级主设备所共有的空间（系统空间），然后在此基础上着重分析了两级总线桥如何共同把各级总线主设备对自己地址空间的访问映射于这个系统空间中，站在系统的高度理清这千头万绪的映射关系．

关键词: 总线, Host桥, PCI-ISA桥, 存储器, 地址空间映射

Abstract: On typical PC with 3-level bus structure, all masters on buses of each level can start access period on buses of their own in address spaces of themselves. Since all kinds of address space realities are distributed on buses of each level, mapping address spaces accessed by each mastcr into physical space realities becomes complex. In this paper. the address spaces which are physically present everywhere are organized as a whole(so-called system space in this article). The system space is considered as a part independent of address spaces of masters on buses of cath level. and is shared by all masters. The article also analyzes how the 2-level bus bridges together map the address space of eath master into the system space.