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An Endpoint Checks an ID-Routed TLP

If the Type field in a received TLP indicates ID routing is to be used, then an endpoint device simply checks the ID field in the packet header against its own Bus Number, Device Number, and Function Number(s). In PCI Express, each device “captures” (and remembers) its own Bus Number and Device Number contained in TLP header bytes 8-9 each time a configuration write (Type 0) is detected on its primary link. At reset, all bus and device numbers in the system revert to 0, so a device will not respond to transactions other than configuration cycles until at least one configuration write cycle (Type 0) has been ...

From

Cover of PCI Express System Architecture

Note

This is how the PCIe Device number is implicitly derived.