[HDLBits] Bcdadd4

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.

module bcd_fadd (
    input [3:0] a,
    input [3:0] b,
    input     cin,
    output   cout,
    output [3:0] sum );

Instantiate 4 copies of bcd_fadd to create a 4-digit BCD ripple-carry adder. Your adder should add two 4-digit BCD numbers (packed into 16-bit vectors) and a carry-in to produce a 4-digit sum and carry out.

module top_module ( 
    input [15:0] a, b,
    input cin,
    output cout,
    output [15:0] sum );
    wire out[3:0];
    bcd_fadd add1(a[3:0],b[3:0],cin,out[0],sum[3:0]);
    bcd_fadd add2(a[7:4],b[7:4],out[0],out[1],sum[7:4]);
    bcd_fadd add3(a[11:8],b[11:8],out[1],out[2],sum[11:8]);
    bcd_fadd add4(a[15:12],b[15:12],out[2],cout,sum[15:12]);
endmodule