--         Description: This unit performs simple logical operations.
--
--
--
--
-------------------------------------------------------------------------------
library IEEE; 
use IEEE.std_logic_1164.all; 
use IEEE.std_logic_arith.all; 
library work;
use work.mc8051_p.all;
  
-----------------------------ENTITY DECLARATION--------------------------------

entity alucore is

  generic (DWIDTH : integer := 8);      -- Data width of the ALU

  port (op_a_i    : in std_logic_vector(DWIDTH-1 downto 0);
        op_b_i    : in std_logic_vector(DWIDTH-1 downto 0);
        alu_cmd_i : in std_logic_vector(3 downto 0);
        cy_i      : in std_logic_vector((DWIDTH-1)/4 downto 0);
        cy_o      : out std_logic_vector((DWIDTH-1)/4 downto 0);
        result_o  : out std_logic_vector(DWIDTH-1 downto 0));

end alucore;

-- op_a_i....... operand A
-- op_b_i....... operand B
-- alu_cmd_i.... command for the ALU core
-- cy_i......... carry flags (MSB is CY, rest is AC)

-- cy_o......... resulting carry out (MSB is CY, rest is AC)
-- result_o..... result
-------------------------------------------------------------------------------
architecture rtl of alucore is

  constant LAND : std_logic_vector(3 downto 0) := "0011";
  constant LOR  : std_logic_vector(3 downto 0) := "0101";
  constant LXOR : std_logic_vector(3 downto 0) := "0110";
  constant RL   : std_logic_vector(3 downto 0) := "0111";
  constant RLC  : std_logic_vector(3 downto 0) := "1000";
  constant RR   : std_logic_vector(3 downto 0) := "1001";
  constant RRC  : std_logic_vector(3 downto 0) := "1010";
  constant COMP : std_logic_vector(3 downto 0) := "1011";
  constant INV  : std_logic_vector(3 downto 0) := "1100";

begin                 -- architecture structural

  p_alu: process (alu_cmd_i, op_a_i, op_b_i, cy_i)

  begin
  
  case alu_cmd_i is
-------------------------------------------------------------------------------
    when LAND =>  -- op_a_i and op_b_i
      result_o <= op_a_i and op_b_i;
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when LOR =>  -- op_a_i or op_b_i
      result_o <= op_a_i or op_b_i;
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when LXOR =>  -- op_a_i xor op_b_i
      result_o <= op_a_i xor op_b_i;
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when RL =>  -- rotate left op_a_i
      if DWIDTH > 1 then
        result_o(DWIDTH-1 downto 1) <= op_a_i(DWIDTH-2 downto 0);
	result_o(0) <= op_a_i(DWIDTH-1);
      else
        result_o <= op_a_i;
      end if;
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when RLC =>  -- rotate left op_a_i with CY
      if DWIDTH > 1 then
        result_o(DWIDTH-1 downto 1) <= op_a_i(DWIDTH-2 downto 0);
	result_o(0) <= cy_i((DWIDTH-1)/4);
      else
        result_o(0) <= cy_i((DWIDTH-1)/4);
      end if;
      cy_o <= cy_i;      
      cy_o((DWIDTH-1)/4) <= op_a_i(DWIDTH-1);      
-------------------------------------------------------------------------------
    when RR =>  -- rotate right op_a_i
      if DWIDTH > 1 then
        result_o(DWIDTH-2 downto 0) <= op_a_i(DWIDTH-1 downto 1);
	result_o(DWIDTH-1) <= op_a_i(0);
      else
        result_o <= op_a_i;
      end if;
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when RRC =>  -- rotate right op_a_i with CY
      if DWIDTH > 1 then
        result_o(DWIDTH-2 downto 0) <= op_a_i(DWIDTH-1 downto 1);
	result_o(DWIDTH-1) <= cy_i((DWIDTH-1)/4);
      else
        result_o(0) <= cy_i((DWIDTH-1)/4);
      end if;
      cy_o <= cy_i;      
      cy_o((DWIDTH-1)/4) <= op_a_i(0);      
-------------------------------------------------------------------------------
    when COMP =>  -- Compare op_a_i with op_b_i
      if op_a_i = op_b_i then
        result_o <= (others => '0');
      else
        result_o <= (others => '1');
      end if;
      cy_o <= cy_i;         
      if op_a_i < op_b_i then
        cy_o((DWIDTH-1)/4) <= '1';
      else
        cy_o((DWIDTH-1)/4) <= '0';          
      end if;
-------------------------------------------------------------------------------
    when INV =>  -- invert op_a_i
      result_o <= not(op_a_i);
      cy_o <= cy_i;      
-------------------------------------------------------------------------------
    when others =>  -- turn unit off
      result_o <= (others => '0');      
      cy_o <= (others => '0');
-------------------------------------------------------------------------------
  end case;
  
 end process p_alu;

end rtl;
-------------------------------------------------------------------------------