When I synthesize a design with XST, the following warning appears:
"WARNING:Xst:647 - Input <name> is never used."
"WARNING:Xst:648 - Output <name> is never used."
This particular port has been declared in your HDL description, but does not drive or is not driven by any internal logic.
Unused input ports will remain in the design, but they will be completely unconnected. If the port is not intended to be used, this message can be safely ignored. To avoid this message, remove any loadless or sourceless elements from your HDL description.
Output ports will remain in the final netlist and will be driven by a logic 0. To avoid the message and to save the port resource, remove the unused output port from your HDL description.