-- Entity declaration for a 4-bit binary adder. --
--------------------------------------------------
entity ADD4 is
  port (A,B: in BIT_VECTOR (3 downto 0);
        CIN: in BIT;
        S: out BIT_VECTOR (3 downto 0);
        COUT: out BIT);
end ADD4;
-------------------------------------------------------
-- Typical definition of adder using a program loop. --
-------------------------------------------------------
architecture LOOP_ADDER of ADD4 is
begin
  process (A, B, CIN)
    variable CARRY: BIT := '0';
    variable SUM: BIT_VECTOR (3 downto 0);
  begin
    CARRY := CIN;
    for I in 0 to 3 loop
      SUM(I) := A(I) xor B(I) xor CARRY;
      CARRY := (A(I) and B(I)) or (A(I) and CARRY) or
               (B(I) and CARRY);
    end loop;
    S <= SUM;
    COUT <= CARRY;
  end process;
end LOOP_ADDER;
---Figure 12.61 VHDL program for a ripple carry adder.