`timescale 1ns/1ns module mux4_1( input [1:0]d1,d2,d3,d0, input [1:0]sel, output reg [1:0]mux_out//记得定义为reg形 ); //*************code***********// always @(*)begin case(sel) 0: mux_out = d3; 1: mux_out = d2; 2: mux_out = d1; 3: mux_out = d0; default: mux_out = 2'bxx; endcase end //*************code***...