-- MIPS I system
-- Copyright 2011-2012 Stefan Schuermans <stefan@blinkenarea.org>
-- Copyleft GNU public license V2 or later
--          http://www.gnu.org/copyleft/gpl.html

LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.numeric_std.all;
USE work.mips_types.all;

ENTITY e_mips_alu IS
    PORT (
        i_alu: IN  t_alu;
        i_op1: IN  std_logic_vector(31 DOWNTO 0);
        i_op2: IN  std_logic_vector(31 DOWNTO 0);
        o_res: OUT std_logic_vector(31 DOWNTO 0)
    );
END ENTITY e_mips_alu;

ARCHITECTURE a_mips_alu OF e_mips_alu IS

    SIGNAL s_shift_arith: std_logic;
    SIGNAL s_shift_left:  std_logic;
    SIGNAL s_shift_out:   std_logic_vector(31 DOWNTO 0);

    COMPONENT e_mips_shifter IS
        PORT (
            i_arith: IN  std_logic;
            i_left:  IN  std_logic;
            i_val:   IN  std_logic_vector(31 DOWNTO 0);
            i_num:   IN  std_logic_vector(31 DOWNTO 0);
            o_val:   OUT std_logic_vector(31 DOWNTO 0)
        );
    END COMPONENT e_mips_shifter;

BEGIN

    shifter: e_mips_shifter
        PORT MAP (
            i_arith => s_shift_arith,
            i_left  => s_shift_left,
            i_val   => i_op2,
            i_num   => i_op1,
            o_val   => s_shift_out
        );

    p_alu: PROCESS(i_alu, i_op1, i_op2, s_shift_out)
        VARIABLE v_op1_s: signed(31 DOWNTO 0);
        VARIABLE v_op2_s: signed(31 DOWNTO 0);
        VARIABLE v_op1_u: unsigned(31 DOWNTO 0);
        VARIABLE v_op2_u: unsigned(31 DOWNTO 0);
    BEGIN
        v_op1_s := signed(i_op1);
        v_op2_s := signed(i_op2);
        v_op1_u := unsigned(i_op1);
        v_op2_u := unsigned(i_op2);
        s_shift_arith <= '0';
        s_shift_left <= '0';
        CASE i_alu IS
            WHEN alu_add => o_res <= std_logic_vector(v_op1_s + v_op2_s);
            WHEN alu_and => o_res <= i_op1 AND i_op2;
            WHEN alu_nor => o_res <= i_op1 NOR i_op2;
            WHEN alu_or  => o_res <= i_op1 OR i_op2;
            WHEN alu_sub => o_res <= std_logic_vector(v_op1_s - v_op2_s);
            WHEN alu_sll => s_shift_arith <= '0'; s_shift_left <= '1'; o_res <= s_shift_out;
            WHEN alu_sra => s_shift_arith <= '1'; s_shift_left <= '0'; o_res <= s_shift_out;
            WHEN alu_srl => s_shift_arith <= '0'; s_shift_left <= '0'; o_res <= s_shift_out;
            WHEN alu_slt => IF v_op1_s < v_op2_s THEN
                                o_res <= X"00000001";
                            ELSE
                                o_res <= X"00000000";
                            END IF;
            WHEN alu_sltu => IF v_op1_u < v_op2_u THEN
                                 o_res <= X"00000001";
                             ELSE
                                 o_res <= X"00000000";
                             END IF;
            WHEN alu_up => o_res <= i_op2(15 DOWNTO 0) & X"0000";
            WHEN alu_xor => o_res <= i_op1 XOR i_op2;
            WHEN OTHERS => o_res <= X"00000000";
        END CASE;
    END PROCESS p_alu;

END ARCHITECTURE a_mips_alu;