module tb3; // random inputs, decimal values including a check

reg  [3:0] a, b;  // 4-bit inputs (to be chosen randomly)
reg  c;           // carry input  (to be chosen randomly)
wire [4:0] s;     // 5-bit output of the RCA circuit
reg  [4:0] check; // 5-bit sum value used to check correctess

// instantiate the 4-bit ripple-carry adder
RCA rca1(a, b, c, s);

// simulation of 20 random addition operations
initial repeat (20) begin
    // get new operand values and compute a check value
    a = $random; b = $random; c = $random;
    check = a + b + c;

    // compute and display the sum every 10 time units
    #10 $display($time, "  %d + %d + %d = %d (%d)", a, b, c, s, check);
end

endmodule