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Root complex

The meaning of «root complex»

In a PCI Express (PCIe) system, a root complex device connects the CPU 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,[2] the root complex generates transaction requests on behalf of the CPU, which is interconnected through a local bus. Root complex functionality may be implemented as a discrete device (northbridge chip), or may be integrated in the CPU. 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.[3]

The PCIE Root Complex holds a master copy of a 'Type 1 Configuration Table' that defines the host memory space that is accessible from each Endpoint device. In addition, each PCIE Endpoint device holds a master copy of their own memory space map in the host system memory as a 'Type 0 Configuration Table', this configuration table in each device allows the host to access the local memory of a PCIe device. Both the Type 1 and Type 0 configuration tables are set up by the Host Operating System that controls the Root Complex by a process known as enumeration and which acts to build a device memory map for the system by querying each bridge, and endpoint device connected on the bus network. Similarly, a PCIE Bridge acts a tiered rootcomplex with a "Type 0 Configuration Table"

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