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In a PCI Express (PCIe) system, a root complex device connects the processor and memory subsystem to the PCI Express switch fabric composed of one or more switch devices.
Similar to a host bridge in a PCI system, the root complex generates transaction requests on behalf of the processor, which is interconnected through a local bus. Root complex functionality may be implemented as a discrete device, or may be integrated with the processor. A root complex may contain more than one PCI Express port and multiple switch devices can be connected to ports on the root complex or cascaded.
References
Root complex Wikipedia(Text) CC BY-SA