`timescale 1ns/1ns module mux4_1( input [1:0]d1,d2,d3,d0, input [1:0]sel, output[1:0]mux_out ); //*************code***********// assign mux_out = (sel == 0) ? d3 : (sel == 1) ? d2 : (sel == 2) ? d1 : (sel == 3) ? d0 : d0; //*************code***********// endmodule wire线网类型不能在always块中进行赋值