Assigning Pins Manually
Preassigning pins can restrict the fitting and pinlocking capability of the software. It is usually best to allow the fitter to automatically assign pins for best fit, then maintain the pinout by using the Lock Pins command after successfully running the fitter. If you must preassign pins, minimize the number of preassigned signals.
If you have specific pinout requirements, you can add pin assignments to your design file, as shown in the following examples.
The PROHIBIT property allows you to reserve device pins for later use, or simply to prevent them from being used at all. For instance, if you anticipate design changes in the future and want to set traces on your printed circuit board now, you can use PROHIBIT to prevent the fitter from using pins associated with those traces. Then, when you decide to use the traces, you can use the LOC attribute to assign those pin locations to new input/output pins you place in your design.
In the Timing Constraints Editor, PROHIBIT constraints can be entered using a dialog box provided in the Ports tab.
Assigning Pins in VHDL and Verilog
Prohibiting Pin assignment in UCF file