4-digit BCD adder

题目如下：

You are provided with a BCD (binary-coded decimal) one-digit adder named bcd_fadd that adds two BCD digits and carry-in, and produces a sum and carry-out.

代码如下：

module top_module ( 
    input [15:0] a, b,
    input cin,
    output cout,
    output [15:0] sum );
	wire [2:0] cout_temp;
    
    bcd_fadd bcd_1(a[3:0],b[3:0],cin,cout_temp[0],sum[3:0]);
    bcd_fadd bcd_2(a[7:4],b[7:4],cout_temp[0],cout_temp[1],sum[7:4]);
    bcd_fadd bcd_3(a[11:8],b[11:8],cout_temp[1],cout_temp[2],sum[11:8]);
    bcd_fadd bcd_4(a[15:12],b[15:12],cout_temp[2],cout,sum[15:12]);

endmodule