Low-Cost PCIe System (Consumer Desktop)

• Looks very similar to an old PCI system — just with PCIe Links instead of a shared PCI bus.
• Typical features:
  • A few PCIe Root Ports directly from the CPU or chipset.
  • A couple of add-in card slots (x16 for GPU, maybe x1 for Wi-Fi or sound card).
  • Basic I/O controllers (USB, SATA, Ethernet) connected via PCIe or chipset fabric.
• The goal here is low cost, not maximum expandability.

📌 Key idea:
Even though it’s PCIe, it still feels like the old architecture — CPU at the top, a few slots and devices hanging off the “bus.”

High-End PCIe System (Server)

• Includes many more PCIe ports, often with:
  • Multiple Root Ports for different peripherals.
  • Onboard high-speed network interfaces (Ethernet, Fibre Channel, etc.).
  • Storage controllers (NVMe, RAID) integrated into the system.
• Early on, there was an idea to make PCIe replace traditional networks (because PCIe is packet-based and full-duplex), but this never really took off.
  • Servers still use Ethernet/InfiniBand for network traffic, not PCIe over cables.

📌 Key idea:
PCIe is still mainly for inside-the-box communication (CPU ⇆ GPU ⇆ storage ⇆ NIC), not for external networking.

Root Complex Details (Important!)

• The Root Complex (RC) is not always one chip.
  In modern CPUs:
  • Part of the RC is inside the CPU package (the PCIe controller, memory controller, routing logic).
  • Other parts may be in a separate chipset (PCH, southbridge).
• Collectively, these parts form the Root Complex and appear as bus 0 to software.
• The text uses the term Uncore to refer to everything in the CPU package that is not part of the CPU cores (e.g., PCIe controller, memory controller).

📌 Key idea:
The Root Complex is a logical concept, not necessarily a single chip — it’s whatever provides the interface between CPU cores and PCIe devices.