module FA(co, s, a, b, ci);
output co, s;
input a, b, ci;

assign co = a&b | a&ci  | b&ci;
assign s = a^b^ci;

endmodule


module Adder4(s, a, b);
output [4:0] s;
input [3:0] a, b;

wire [3:1] c;

FA fa0(c[1], s[0], a[0], b[0], 1'b0);
FA fa1(c[2], s[1], a[1], b[1], c[1]);
FA fa2(c[3], s[2], a[2], b[2], c[2]);
FA fa3(s[4], s[3], a[3], b[3], c[3]);
endmodule

module Mux4x8(z, s, i0, i1, i2, i3);
output [7:0] z;
input [1:0] s;
input [7:0] i0, i1, i2, i3;

assign z =	s == 2'b00 ? i0 : 
			s == 2'b01 ? i1 :
			s == 2'b10 ? i2 : i3;
endmodule

module Stimulus;

reg [3:0] a, b;
wire [4:0] s;

Adder4 my_adder(s, a, b);

initial begin
	a = 4'd0;
	b = 4'd0;
end

always #31 a = a+1;
always #53 b = b+1;

endmodule


module Stimulus2;
wire [7:0] z;
reg [1:0] sel;
reg [7:0] i [0:3];

Mux4x8 mux(z, sel, i[0], i[1], i[2], i[3]);

initial begin
	sel = 0;
	i[0] = 8'd13;
	i[1] = 8'd23;
	i[2] = 8'd37;
	i[3] = 8'd53;
end

always #20 sel = sel +1;


endmodule