How a Function Is Discovered
The configuration software executing on the processor typically discovers the existence of a function by performing a read from its PCI-compatible Vendor ID register. A unique 16-bit value is assigned to each vendor by the PCI-SIG and is hardwired into the Vendor ID register of each function designed by that vendor. The Vendor ID of FFFFh is reserved and will never be assigned to any vendor.
A function is considered present if the value read from its Vendor ID register is a value other than FFFFh. In a system that depends on a return of all ones for a configuration read from a non-existent register, the Root Complex must be designed to return all ones for a configuration read request that results in a UR (Unsupported ...
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