The warning below occurs when you run the PCI core through Implementation.
WARNING:MapLib:708 - The bidirectional IOB AD<9> uses IOSTANDARD PCI33_3. To
achieve a PCI compliant implementation, please add a BYPASS property to the
pad signal. The mapper will create logic to implement the IOB in a compliant
manner. Please note that the new logic might affect interfaces specified by
KEEP_HIERARCHY. The new logic might also fail to merge with logic inside a
closed area group.
This warning can be safely ignored. This warning will be removed in ISE 11.4.