library IEEE;
use IEEE.std_logic_1164.all;

entity rol16 is
    port (
        DIN: in STD_LOGIC_VECTOR(15 downto 0);  -- Data inputs
        S: in STD_LOGIC_VECTOR (3 downto 0);    -- Shift amount, 0-15
        DOUT: out STD_LOGIC_VECTOR(15 downto 0) -- Data bus output
    );
end rol16;

architecture rol16_arch of rol16 is
begin
process(DIN, S)
  variable X, Y, Z: STD_LOGIC_VECTOR(15 downto 0);
  begin
    if S(0)='1' then X := DIN(14 downto 0) & DIN(15); else X := DIN; end if;
    if S(1)='1' then Y := X(13 downto 0) & X(15 downto 14); else Y := X; end if;
    if S(2)='1' then Z := Y(11 downto 0) & Y(15 downto 12); else Z := Y; end if;
    if S(3)='1' then DOUT <= Z(7 downto 0) & Z(15 downto 8); else DOUT <= Z; end if;
  end process;
end rol16_arch;