// 4-to-1 multiplexer. Port list is taken exactly from
// the I/O diagram.
module mux4_to_1 (out, i0, i1, i2, i3, s1, s0);

// Port declarations from the I/O diagram
output out;
input i0, i1, i2, i3;
input s1, s0;
//output declared as register
reg out;

//recompute the signal out if any input signal changes.
//All input signals that cause a recomputation of out to
//occur must go into the always @(...) 
always @(s1 or s0 or i0 or i1 or i2 or i3)
begin
	case ({s1, s0})
	2'b00: out = i0;
	2'b01: out = i1;
	2'b10: out = i2;
	2'b11: out = i3;
	default: out = 1'bx;
	endcase
end

endmodule

// Define the stimulus module (no ports)
module stimulus;

// Declare variables to be connected
// to inputs
reg IN0, IN1, IN2, IN3;
reg S1, S0;

// Declare output wire
wire OUTPUT;

// Instantiate the multiplexer
mux4_to_1 mymux(OUTPUT, IN0, IN1, IN2, IN3, S1, S0);

// Stimulate the inputs
initial 
begin
	// set input lines
	IN0 = 1; IN1 = 0; IN2 = 1; IN3 = 0;
	#1 $display("IN0= %b, IN1= %b, IN2= %b, IN3= %b\n",IN0,IN1,IN2,IN3);
			

	// choose IN0
	S1 = 0; S0 = 0;
	#1 $display("S1 = %b, S0 = %b, OUTPUT = %b \n", S1, S0, OUTPUT);

	// choose IN1
	S1 = 0; S0 = 1;
	#1 $display("S1 = %b, S0 = %b, OUTPUT = %b \n", S1, S0, OUTPUT);

	// choose IN2
	S1 = 1; S0 = 0;
	#1 $display("S1 = %b, S0 = %b, OUTPUT = %b \n", S1, S0, OUTPUT);

	// choose IN3
	S1 = 1; S0 = 1;
	#1 $display("S1 = %b, S0 = %b, OUTPUT = %b \n", S1, S0, OUTPUT);
end

endmodule