// 1-bit Full Adder
module full_adder_1bit (
input a, b, cin,
output sum, cout
);
// dataflow (fully combinational)
assign sum = a ^ b ^ cin;
assign cout = (a & b) | (b & cin) | (a & cin);
endmodule
// 4-bit Ripple Carry Adder – chain 4 full adders
module rca4_chain (
input [3:0] a,
input [3:0] b,
input cin,
output [3:0] sum,
output cout
);
wire c0, c1, c2, c3, c4;
assign c0 = cin;
full_adder_1bit fa0 (.a(a[0]), .b(b[0]), .cin(c0), .sum(sum[0]), .cout(c1));
full_adder_1bit fa1 (.a(a[1]), .b(b[1]), .cin(c1), .sum(sum[1]), .cout(c2));
full_adder_1bit fa2 (.a(a[2]), .b(b[2]), .cin(c2), .sum(sum[2]), .cout(c3));
full_adder_1bit fa3 (.a(a[3]), .b(b[3]), .cin(c3), .sum(sum[3]), .cout(c4));
assign cout = c4;
endmodule
