Hierarchy:

CPU

The CPU is at the top of the PCIe hierarchy.
PCIe uses a tree topology (inverted tree), meaning:

• No loops
• No mesh or arbitrary networks
• Just branches, like a family tree

This is done so that configuration software (BIOS, OS, drivers) can still work the same way as it did with PCI — it expects to find devices by walking a simple tree structure.

Root Complex (RC)

Think of the Root Complex as the big translator between the CPU and the PCIe world.

• It’s not a single chip — it may include:
  • CPU interface
  • Memory/DRAM controller interface
  • Other host logic
• It exposes Root Ports (logical connection points for PCIe Links).
  Essentially:
• CPU → Root Complex → PCIe Links → Rest of the PCIe devices
  📌 Key point: The Root Complex is the top of the PCIe hierarchy and the “brain” that starts configuration cycles.

Switches

• Purpose: Fan-out capability (like a network switch).
• A Switch has:
  • One Upstream Port (facing toward the Root Complex)
  • Multiple Downstream Ports (facing toward Endpoints)
    It acts as a packet router — looks at packet headers, forwards them to the correct port.

Bridges

Bridges are translators between PCIe and other bus types:

• Forward Bridge: PCIe → legacy PCI/PCI-X bus (so you can plug old PCI cards into a new PCIe system)
• Reverse Bridge: Legacy PCI system → PCIe card (so you can use PCIe devices in an old system)

Endpoints (Devices)

Endpoints are the actual devices (network cards, GPUs, SSDs, etc.) that do useful work.

• Native PCIe Endpoint: Designed from scratch for PCIe
  • Uses Memory-Mapped I/O (MMIO)
  • Doesn’t rely on old PCI-specific features
• Legacy PCIe Endpoint: Old PCI/PCI-X device with a PCIe wrapper
  • May use IO space or support old PCI operations (like locked transactions)
    Endpoints always have only one Upstream Port (they are the leaves of the tree).

🔑 Summary Table