4:1 MUX

Step 1: 2:1 MUX (Basic Building Block)

module mux2to1(

    input I0, I1,

    input S,

    output Y

);

    assign Y = (S == 1'b0) ? I0 : I1;

endmodule

Step 2: 4:1 MUX (Top-Down Structural Design)

module mux4to1(

    input S1, S0,

    input I3, I2, I1, I0,

    output Y

);

wire Y0, Y1;

mux2to1 M1 (I0, I1, S0, Y0);

mux2to1 M2 (I2, I3, S0, Y1);

mux2to1 M3 (Y0, Y1, S1, Y);

endmodule

TOP TO DOWN APPROACH:--

// 2 : 1 Multiplexer

module mux2_1(

    input I0, I1,

    input S,

    output Y

);

    assign Y = (~S & I0) | (S & I1);

endmodule

// 4 : 1 Multiplexer (Top-Down Structural)

module mux4_1(

    input I0, I1, I2, I3,

    input S1, S0,

    output Y

);

    wire w1, w2;

    // First stage

    mux2_1 M1 (I0, I1, S0, w1);

    mux2_1 M2 (I2, I3, S0, w2);

    // Second stage

    mux2_1 M3 (w1, w2, S1, Y);

endmodule