Five fields in the predefined header deal with device identification.... Generic configuration software will be able to easily determine what devices are available on the system's PCI bus(es). All of these registers are read-only.
Vendor ID
This field identifies the manufacturer of the device. Valid vendor identifiers are allocated by the PCI SIG to ensure uniqueness. 0FFFFh is an invalid value for Vendor ID.
Device ID This field identifies the particular device. This identifier is allocated by the vendor.
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И ещё: Chapter 6.1
System software may need to scan the PCI bus to determine what devices are actually
present. To do this, the configuration software must read the Vendor ID in each possible
PCI "slot." The host bus to PCI bridge must unambiguously report attempts to read the
Vendor ID of non-existent devices. Since 0FFFFh is an invalid Vendor ID, it is adequate
for the host bus to PCI bridge to return a value of all 1’s on read accesses to
Configuration Space registers of non-existent devices. (Note that these accesses will be
terminated with a Master-Abort.)