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--         ELETTRONICA DEI SISTEMI DIGITALI         --
--                                                  --
--        Soluzione Esercizio del 31/01/2001        --
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-- NOTA: Soluzione non verificata, potrebbe esistere un errore su qualche indice,
-- si prega di verificare!

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.all;

entity SHIFTER is
  port ( a    : in  unsigned(7 downto 0);
         s    : in  unsigned(3 downto 0);
         aol  : in  std_logic;
         b    : out unsigned(7 downto 0)   );
end SHIFTER;

architecture SHIFT_P of SHIFTER is
signal sh_left,sh_right   : unsigned(7 downto 0);
signal sh_count : unsigned(2 downto 0);
signal sn : unsigned(3 downto 0);

begin  -- SHIFT_P
  process(a,s,aol)
  variable index,count : integer range 0 to 7;

  begin
  for i in 0 to 3 loop
      NG: sn(i) <= not s(i);
  end loop;
    -- CALCOLO DELLO SHIFT COUNT        --  --  --  --  --  --  --  --  --  --
    
    if s(3)='0' then sh_count <= s(2 downto 0);       
    else             sh_count <= sn(2 downto 0) + "001";
    end if;    

    count := Conv_integer(sh_count);
    
    SHIFT_LEFT: for i in 7 downto 0 loop
                  if i>=count then
                       index := i-count;
                       sh_left(i) <= a(index);
                  else
                       sh_left(i) <= '0';
                  end if;
                end loop;  -- i            

    SHIFT_RIGHT: for i in 0 to 7 loop
                   if i<=7-count then
                      index:=i+count;
                      sh_right(i) <= a(index);
                   else
                      sh_right(i) <= aol and a(7);
                   end if;
            end loop;  -- i    
end process; 
   

-- SHIFTOUT VALUE SELECTION MULTIPLEXER
process(sh_left,sh_right,s)
begin
    if s(3)='0' then
                 b <= sh_left;
    else
                 b <= sh_right;
    end if;           
end process;  

end SHIFT_P;