-- This procedure adds two n-bit bit_vectors and a carry and 
-- returns an n-bit sum and a carry.  Add1 and Add2 are assumed 
-- to be of the same length and dimensioned n-1 downto 0.
procedure Addvec
   (Add1,Add2: in bit_vector;
    Cin: in bit;
    signal Sum: out bit_vector;
    signal Cout: out bit;
    n:in positive) is
    variable C: bit; 
begin
   C := Cin;  
   for i in 0 to n-1 loop    
      Sum(i) <= Add1(i) xor Add2(i) xor C;
      C := (Add1(i) and Add2(i)) or (Add1(i) and C) or (Add2(i) and C);
   end loop;
   Cout <= C;  
end Addvec;