Route two 4-bit inputs to two outputs. With sel = 0 the connection is straight (out0 = in0, out1 = in1); with sel = 1 it is crossed (out0 = in1, out1 = in0). Inputs: in0, in1 (4 bits each); sel. Outputs: out0, out1 (4 bits each). The testbench applies all 512 combinations.