Configuration space is defined geographically; in other words, the location of a peripheral device is determined by its physical location within an interconnected tree of PCI bus bridges. A device is usually located by its bus number and device (slot) number. Each peripheral device contains a set of well-defined configuration registers in its PCI configuration space. The registers are used not only to identify devices but also to supply device configuration information to the configuration framework. For example, base address registers in the device configuration space must be mapped before a device can respond to data access. Figure 2-3 illustrates the configuration space registers.
The method for generating configuration cycles is host dependent. In x86 machines, special I/O ports are used. On other platforms, the PCI configuration space may be memory-mapped to certain address locations corresponding to the PCI host bridge in the host address domain. When a device configuration register is accessed by the processor, the request will be routed to the PCI host bridge. The bridge then translates the access into proper configuration cycles on the bus.