4-bit Ripple-Carry Adder from Full Adders

A ripple-carry adder builds an N-bit adder out of N single-bit full adders, chained so that each stage’s carry-out feeds the next stage’s carry-in. This is a classic example of building a larger circuit purely through structural instantiation of a smaller building block.

You are given a full_adder submodule (identical to the one you built earlier):

module full_adder (
    input a,
    input b,
    input cin,
    output sum,
    output cout
);
    assign sum = a ^ b ^ cin;
    assign cout = (a & b) | (a & cin) | (b & cin);
endmodule

Build top_module with 4-bit inputs a and b, a carry-in cin, a 4-bit output sum, and a carry-out cout, implementing a 4-bit ripple-carry adder using four instances of full_adder.

Interface

Notes

• You will need three internal wires to carry the carry-out of stage 0 into stage 1, stage 1 into stage 2, and stage 2 into stage 3.
• The carry-in of the least-significant full adder is the module’s cin; the carry-out of the most-significant full adder is the module’s cout.