During the enumeration process the system software discovers all of the switch and endpoint devices that are connected to the system, determines the memory requirements and then configures the PCIe devices. ... The switch in this example has no internal memory so as to simplify the configuration process.

What is PCI enumeration?

In particular PCI provides a mechanism where each card or device specifies both its identity and the resources needed (memory-mapped or I/O space, interrupts) and the BIOS and/or OS can discover all devices in the system and allocate interrupts, physical memory space, and I/O space to each device.

How are PCI slots numbered?

The types of PCIe slots available in your PC will depend on the motherboard you buy. PCIe slots come in different physical configurations: x1, x4, x8, x16, x32. The number after the x tells you how many lanes (how data travels to and from the PCIe card) that PCIe slot has.