module adder_16bit (
    input wire [15:0] a,
    input wire [15:0] b,
    input wire Cin,
    output wire [15:0] y,
    output wire Co
);

    wire Co_temp;

    add8 add8_inst1 (
        .a(a[15:8]),
        .b(b[15:8]),
        .Cin(Co_temp), 
        .y(y[15:8]),
        .Co(Co)
    );

    add8 add8_inst2 (
        .a(a[7:0]),
        .b(b[7:0]),
        .Cin(Cin),
        .y(y[7:0]),
        .Co(Co_temp)
    ); 

endmodule

module add8 (
    input wire [7:0] a,
    input wire [7:0] b,
    input wire Cin,
    output wire [7:0] y,
    output wire Co
);

    wire Co_temp;

    add4 add4_inst1 (
        .a(a[7:4]),
        .b(b[7:4]),
        .Cin(Co_temp),
        .y(y[7:4]),
        .Co(Co)
    );

    add4 add4_inst2 (
        .a(a[3:0]),
        .b(b[3:0]),
        .Cin(Cin),
        .y(y[3:0]),
        .Co(Co_temp)
    );

endmodule

module add4 (
    input wire [3:0] a,
    input wire [3:0] b,
    input wire Cin,
    output wire [3:0] y,
    output wire Co
);

    wire Co_temp;

    add2 add2_inst1 (
        .a(a[3:2]),
        .b(b[3:2]),
        .Cin(Co_temp),
        .y(y[3:2]),
        .Co(Co)
    );

    add2 add2_inst2 (
        .a(a[1:0]),
        .b(b[1:0]),
        .Cin(Cin),
        .y(y[1:0]),
        .Co(Co_temp)
    ); 

endmodule

module add2 (
    input wire [1:0] a,
    input wire [1:0] b,
    input wire Cin,
    output wire [1:0] y,
    output wire Co
);

    wire Co_temp;

    add1 add1_inst1 (
        .a(a[1]),
        .b(b[1]),
        .Cin(Co_temp),
        .y(y[1]),
        .Co(Co)
    );

    add1 add1_inst2 (
        .a(a[0]),
        .b(b[0]),
        .Cin(Cin),
        .y(y[0]),
        .Co(Co_temp)
    );

endmodule

module add1 (
    input wire a,
    input wire b,
    input wire Cin,
    output wire y,
    output wire Co
);
    assign y = ((~a) & (~b) & Cin | (~a) & b & (~Cin) | a & (~b) & (~Cin) | (a & b & Cin));
    assign Co = ((~a & b & Cin) | (a & ~b & Cin) | (a & b & ~Cin) | (a & b & Cin));

endmodule