if I synthese the followng code (substract using two's-complement and adder) using xst and then have a look to the RTL schematic, the co output is on ground and a 16bit adder is infered. The carry out (co) is the interesting signal for me. Did I wrote wrong code (TB not yet)? How to correct it? BTW, is there a way to 'tune' this entities especially to avoid such castings and conversations?