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--! @file

--! @brief Arithmetic logic unit http://en.wikipedia.org/wiki/Arithmetic_logic_unit

--! Use standard library and import the packages (std_logic_1164,std_logic_unsigned,std_logic_arith)

library IEEE;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;

--! Use CPU Definitions package

use work.pkgOpenCPU32.all;

--! ALU is a digital circuit that performs arithmetic and logical operations.

--! ALU is a digital circuit that performs arithmetic and logical operations. It's the fundamental part of the CPU

entity Alu is

    generic (n : integer := nBits - 1);                                 --! Generic value (Used to easily change the size of the Alu on the package)

         Port ( A : in  STD_LOGIC_VECTOR (n downto 0);           --! Alu Operand 1

           B : in  STD_LOGIC_VECTOR (n downto 0);                --! Alu Operand 2

           S : out  STD_LOGIC_VECTOR (n downto 0);               --! Alu Output

           sel : in  aluOps);                                                                   --! Select operation

end Alu;

--! @brief Arithmetic logic unit, refer to this page for more information http://en.wikipedia.org/wiki/Arithmetic_logic_unit

--! @details This circuit will be excited by the control unit to perfom some arithimetic, or logic operation (Depending on the opcode selected)

--! \n You can see some samples on the Internet: http://www.vlsibank.com/sessionspage.asp?titl_id=12222

architecture Behavioral of Alu is

begin

        --! Behavior description of combinational circuit (Can not infer any FF(Flip flop)) of the Alu

        process (A,B,sel) is

        variable mulResult : std_logic_vector(((nBits*2) - 1)downto 0);

        begin

                case sel is

                        when alu_pass =>

                                --Pass operation

                                S <= A;

                        when alu_passB =>

                                --Pass operation

                                S <= B;

                        when alu_sum =>

                                --Sum operation

                                S <= A + B;

                        when alu_sub =>

                                --Subtraction operation

                                S <= A - B;

                        when alu_inc =>

                                --Increment operation

                                S <= A + conv_std_logic_vector(1, n+1);

                        when alu_dec =>

                                --Decrement operation

                                S <= A - conv_std_logic_vector(1, n+1);

                        when alu_mul =>

                                --Multiplication operation

                                mulResult := A * B;

                                S <= mulResult((nBits - 1) downto 0);

                        when alu_and =>

                                --And operation

                                S <= A and B;

                        when alu_or =>

                                --Or operation

                                S <= A or B;

                        when alu_xor =>

                                --Xor operation

                                S <= A xor B;

                        when alu_not =>

                                --Not operation

                                S <= not A;

                        when others =>

                                S <= (others => 'Z');

                end case;

        end process;

end Behavioral;