module mux_df(
    input  a,b,s,
    output y
    );
wire sbar;
assign y = (a&sbar)|(s&b);
assign sbar = ~s;
endmodule




module mux_bh(
    input  a,b,s,
    output y
    );
reg y;
wire s;
always @( s or a or b )
begin
   if( s == 0)
      y = a;
	else
		y = b;  
end
endmodule



module mux_gl(
    input  a,b,s,
    output y
    );
wire  q1,q2,sbar;
not n1( sbar,s);
and a1( q1, sbar, a);
and a2( q2, s, b);
or o1( y, q1, q2);
endmodule



module mux_4_1(
    input a1,a2,a3,a4,
	 input [1:0]s,
	 output y
    );
wire t1,t2;	 
mux_df m1(a1,a2,s[0],t1);
mux_df m2(a3,a4,s[0],t2);
mux_df m3(t1,t2,s[1],y);
endmodule



module mux_4_1_mix(
    input a1,a2,a3,a4,
	 input [1:0] s,
	 output y
    );
reg y;
wire t1,t2;	 
mux_df m1(a1,a2,s[0],t1);
mux_gl m2(a3,a4,s[0],t2);
always @( s[1] or t1 or t2 )
begin
   if( s[1] == 0)
      y = t1;
	else
		y = t2;  
end
endmodule



module mux_case(
    input  a,b,s,
    output y
    );
reg y;
always @(s or a or b)
begin
   case(s)
       0 : y = a;
       1 : y = b;
   endcase
end
endmodule



module mux_cs(
    input  a,b,s,
    output y
    );
wire s;
assign y = (s == 0)? a : b;
endmodule




//////Test benches/////
module mux_tb;

	// Inputs
	reg a;
	reg b;
	reg s;

	// Outputs
	wire y;

	// Instantiate the Unit Under Test (UUT)
	mux_gl uut (
		.a(a), 
		.b(b), 
		.s(s), 
		.y(y)
	);
	initial begin
		// Initialize Inputs
		a = 0;
		b = 0;
		s = 0;

		// Wait 100 ns for global reset to finish
		#100;
      a = 1;
		b = 0;
		s = 0; 
		#20;
		a = 1;
		b = 0;
		s = 1;
		#20;
		a = 1;
		b = 1;
		s = 0;
		// Add stimulus here
	end     
endmodule
/////////////////////////////////////
module mux_4_1_tb;

	// Inputs
	reg a1;
	reg a2;
	reg a3;
	reg a4;
	reg [1:0] s;

	// Outputs
	wire y;

	// Instantiate the Unit Under Test (UUT)
	mux_4_1 uut (
		.a1(a1), 
		.a2(a2), 
		.a3(a3), 
		.a4(a4), 
		.s(s), 
		.y(y)
	);

	initial begin
		// Initialize Inputs
		a1 = 1;
		a2 = 1;
		a3 = 1;
		a4 = 0;
		s = 2'b00;

		// Wait 100 ns for global reset to finish
		#100;
      a1 = 1;
		a2 = 0;
		a3 = 0;
		a4 = 0;
		s = 2'b01; 
		#20;
		a1 = 1;
		a2 = 0;
		a3 = 0;
		a4 = 1;
		s = 2'b10;
		#20;
		a1 = 0;
		a2 = 0;
		a3 = 0;
		a4 = 1;
		s = 2'b11;
		// Add stimulus here

	end
      
endmodule