Configuration Space Compatibility

• Endpoints and Bridges still have PCI-like headers.
  This means that:
  • Device drivers, BIOS, and OS code written for PCI can still recognize and configure PCIe devices.
  • No need to rewrite software just because you switch to PCIe.

📌 Key point:
Even though PCIe uses packets instead of shared parallel buses, the configuration mechanism looks exactly the same to software.

Bridges, Switches, and Root Complex Look “PCI-like”

• In PCIe, switches are essentially a collection of virtual PCI-to-PCI bridges.
• The Root Complex also exposes Root Ports that look like bridges.

To legacy software:

• The Root Complex appears as bus number 0 (just like in PCI).
• Each Root Port appears as a PCI-to-PCI bridge leading to another bus number.
• Each Switch appears as multiple bridges on a shared internal bus.

📌 Key point:
Internally, these may not be “real” PCI buses (they are packet routers), but the software still sees them as if they were traditional PCI buses.

Enumeration Works the Same Way

Enumeration = The process where configuration software:

1. Starts from bus 0 (inside the Root Complex).
2. Walks each bridge, assigns bus numbers.
3. Discovers all devices and allocates system resources (I/O, memory space, interrupts).

Because PCIe preserves the same logical view:

• The enumeration process is unchanged.
• Old BIOS/OS code can discover PCIe devices without modification.