Root complex
Beginner
The part of the processor where all its PCIe wires start.
Novice
The logic in the CPU that connects the PCIe links to the processor and its memory. Each link starts at one of its root ports.
Expert
The top of the PCIe hierarchy: root ports originate links, translate device DMA into memory accesses, and enforce ordering and isolation. Peer-to-peer traffic that must cross between root ports or sockets goes through it, often at reduced performance.