-- This function adds 2 4-bit vectors and a carry.
-- It returns a 5-bit sum

function add4 (A,B: bit_vector(3 downto 0); carry: bit) 
  return bit_vector is

variable cout: bit;
variable cin: bit := carry;
variable Sum: bit_vector(4 downto 0):="00000";
begin
loop1: for i in 0 to 3 loop
  cout := (A(i) and B(i)) or (A(i) and cin) or (B(i) and cin);
  Sum(i) := A(i) xor B(i) xor cin;
  cin := cout; 
end loop loop1;
Sum(4):= cout;
return Sum;
end add4;