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Besides defining cycles for PCI memory space, which is directly addressable by the PowerPC processor, the PCI specification supports four other types of cycles--I/O space, configuration space, interrupt acknowledge, and special--which are not directly supported by the PowerPC architecture. To provide a PCI-compliant interface, Macintosh bridges create these additional address spaces and cycle types by accessing memory-mapped regions of the bridge control space shown in Table 2-1. Because the additional spaces and cycle types are manufactured by the bridge, they are abstracted from driver code and expansion card firmware by the interface routines defined in Expansion Bus Manager Using these routines, you can create all types of data transactions on Macintosh PCI buses in a hardware-independent way.