### From HDLBits

In this exercise, you will create a circuit with two levels of hierarchy. Your `top_module` will instantiate two copies of `add16` (provided), each of which will instantiate 16 copies of `add1` (which you must write). Thus, you must write two modules: `top_module` and `add1`.

Like module_add, you are given a module `add16` that performs a 16-bit addition. You must instantiate two of them to create a 32-bit adder. One `add16` module computes the lower 16 bits of the addition result, while the second `add16` module computes the upper 16 bits of the result. Your 32-bit adder does not need to handle carry-in (assume 0) or carry-out (ignored).

Connect the `add16` modules together as shown in the diagram below. The provided module `add16` has the following declaration:

`module add16 ( input[15:0] a, input[15:0] b, input cin, output[15:0] sum, output cout );`

Within each `add16`, 16 full adders (module `add1`, not provided) are instantiated to actually perform the addition. You must write the full adder module that has the following declaration:

`module add1 ( input a, input b, input cin, output sum, output cout ); `

Recall that a full adder computes the sum and carry-out of a+b+cin.

In summary, there are three modules in this design:

• `top_module` — Your top-level module that contains two of...
• `add16`, provided — A 16-bit adder module that is composed of 16 of...
• `add1` — A 1-bit full adder module.

If your submission is missing a `module add1`, you will get an error message that says `Error (12006): Node instance "user_fadd[0].a1" instantiates undefined entity "add1"`.

### Module Declaration

```module top_module (
input [31:0] a,
input [31:0] b,
output [31:0] sum
);```