RootComplex

A root complex (RC) is the PCI Express host interface that connects a computer’s central processing unit or chipset to the PCIe fabric. It serves as the top level of the PCIe hierarchy, providing the mechanism through which the CPU and memory subsystem access PCIe endpoints, switches, and other PCIe devices.

In typical systems, the root complex may be integrated into the processor package, in a motherboard chipset, or in a motherboard’s root complex specifically designated for PCIe. The RC presents one or more root ports, each of which forms a PCIe link to a downstream device or switch. These links can operate at various lane widths (for example x1, x4, x8, or x16) and generations (such as Gen3, Gen4, or Gen5), with the RC handling the negotiation of speed and width during link training.

Primary responsibilities of the RC include managing configuration space access, initializing and enumerating PCIe devices, and routing transactions between the CPU/memory system and the PCIe fabric. It handles addressing, memory and I/O space mapping, and interrupt delivery (including MSI and MSI-X). The RC also manages power states and other PCIe features such as hot-plug support and quality of service where applicable.

In the PCI Express topology, the root complex is the root of the hierarchy, with endpoints occupying leaf positions and switches providing additional branches. The RC's design and capabilities influence overall system bandwidth, device topology, and performance.