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

ENTITY e_mips_decoder IS
    PORT (
        i_instr:  IN  std_logic_vector(31 DOWNTO 0);
        o_src_s:  OUT std_logic_vector( 4 DOWNTO 0);
        o_src_t:  OUT std_logic_vector( 4 DOWNTO 0);
        o_dest:   OUT std_logic_vector( 4 DOWNTO 0);
        o_imm_a:  OUT std_logic_vector( 4 DOWNTO 0);
        o_imm_16: OUT std_logic_vector(15 DOWNTO 0);
        o_imm_26: OUT std_logic_vector(25 DOWNTO 0);
        o_op:     OUT t_op;
        o_link:   OUT t_link;
        o_cmp:    OUT t_cmp;
        o_alu:    OUT t_alu;
        o_imm:    OUT t_imm
    );
END ENTITY e_mips_decoder;

ARCHITECTURE a_mips_decoder OF e_mips_decoder IS

    TYPE t_enc_type IS (enc_reg, enc_imm, enc_jmp);

    SIGNAL s_opcode:   std_logic_vector(5 DOWNTO 0);
    SIGNAL s_enc_type: t_enc_type;
    SIGNAL s_func:     std_logic_vector(5 DOWNTO 0);

BEGIN

    s_opcode <= i_instr(31 DOWNTO 26);

    p_enc_type: PROCESS(s_opcode)
    BEGIN
        CASE s_opcode IS
            WHEN "000000" => s_enc_type <= enc_reg;
            WHEN "000010" => s_enc_type <= enc_jmp;
            WHEN "011010" => s_enc_type <= enc_jmp;
            WHEN OTHERS   => s_enc_type <= enc_imm;
        END CASE;
    END PROCESS p_enc_type;

    p_src_s: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_src_s <= i_instr(25 DOWNTO 21);
            WHEN enc_imm => o_src_s <= i_instr(25 DOWNTO 21);
            WHEN OTHERS  => o_src_s <= "00000";
        END CASE;
    END PROCESS p_src_s;

    p_src_t: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_src_t <= i_instr(20 DOWNTO 16);
            WHEN enc_imm => o_src_t <= i_instr(20 DOWNTO 16);
            WHEN OTHERS  => o_src_t <= "00000";
        END CASE;
    END PROCESS p_src_t;

    p_dest: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_dest <= i_instr(15 DOWNTO 11);
            WHEN OTHERS  => o_dest <= "00000";
        END CASE;
    END PROCESS p_dest;

    p_imm_a: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_imm_a <= i_instr(10 DOWNTO 6);
            WHEN OTHERS  => o_imm_a <= "00000";
        END CASE;
    END PROCESS p_imm_a;

    p_func: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => s_func <= i_instr(5 DOWNTO 0);
            WHEN OTHERS  => s_func <= "000000";
        END CASE;
    END PROCESS p_func;

    p_imm_16: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_imm_16 <= i_instr(15 DOWNTO 0);
            WHEN OTHERS  => o_imm_16 <= X"0000";
        END CASE;
    END PROCESS p_imm_16;

    p_imm_26: PROCESS(i_instr, s_enc_type)
    BEGIN
        CASE s_enc_type IS
            WHEN enc_reg => o_imm_26 <= i_instr(25 DOWNTO 0);
            WHEN OTHERS  => o_imm_26 <= X"000000" & "00";
        END CASE;
    END PROCESS p_imm_26;

    p_op: PROCESS(s_opcode, s_func)
    BEGIN
        o_op   <= op_none;
        o_link <= link_none;
        o_cmp  <= cmp_none;
        o_alu  <= alu_none;
        o_imm  <= imm_none;
        CASE s_opcode IS
            WHEN "000000" =>
                CASE s_func IS
                    WHEN "000000" => o_op <= op_alu; o_alu <= alu_sll; o_imm <= imm_a;
                    WHEN "000010" => o_op <= op_alu; o_alu <= alu_srl; o_imm <= imm_a;
                    WHEN "000011" => o_op <= op_alu; o_alu <= alu_sra; o_imm <= imm_a;
                    WHEN "000100" => o_op <= op_alu; o_alu <= alu_sll;
                    WHEN "000110" => o_op <= op_alu; o_alu <= alu_srl;
                    WHEN "000111" => o_op <= op_alu; o_alu <= alu_sra;
                    WHEN "001000" => o_op <= op_j;
                    WHEN "001001" => o_op <= op_j; o_link <= link_link;
                    -- TODO: 010xxx, 011xxx missing
                    WHEN "100000" => o_op <= op_alu; o_alu <= alu_add;
                    WHEN "100001" => o_op <= op_alu; o_alu <= alu_add;
                    WHEN "100010" => o_op <= op_alu; o_alu <= alu_sub;
                    WHEN "100011" => o_op <= op_alu; o_alu <= alu_sub;
                    WHEN "100100" => o_op <= op_alu; o_alu <= alu_and;
                    WHEN "100101" => o_op <= op_alu; o_alu <= alu_or;
                    WHEN "100110" => o_op <= op_alu; o_alu <= alu_xor;
                    WHEN "100111" => o_op <= op_alu; o_alu <= alu_nor;
                    WHEN "101010" => o_op <= op_alu; o_alu <= alu_slt;
                    WHEN "101011" => o_op <= op_alu; o_alu <= alu_slt;
                    WHEN OTHERS => NULL;
                END CASE;
            WHEN "000010" => o_op <= op_j;
            WHEN "000011" => o_op <= op_j; o_link <= link_link; o_imm <= imm_26;
            WHEN "000100" => o_op <= op_b; o_cmp <= cmp_eq;
            WHEN "000101" => o_op <= op_b; o_cmp <= cmp_ne;
            WHEN "000110" => o_op <= op_b; o_cmp <= cmp_lez;
            WHEN "000111" => o_op <= op_b; o_cmp <= cmp_gtz;
            WHEN "001000" => o_op <= op_alu; o_alu <= alu_add; o_imm <= imm_16se;
            WHEN "001001" => o_op <= op_alu; o_alu <= alu_add; o_imm <= imm_16se;
            WHEN "001010" => o_op <= op_alu; o_alu <= alu_slt; o_imm <= imm_16se;
            WHEN "001011" => o_op <= op_alu; o_alu <= alu_slt; o_imm <= imm_16se;
            WHEN "001100" => o_op <= op_alu; o_alu <= alu_and; o_imm <= imm_16ze;
            WHEN "001101" => o_op <= op_alu; o_alu <= alu_or; o_imm <= imm_16ze;
            WHEN "001110" => o_op <= op_alu; o_alu <= alu_xor; o_imm <= imm_16ze;
            -- TODO: 011xxx, 100xxx, 101xxx missing
            WHEN OTHERS => NULL;
        END CASE;
    END PROCESS p_op;

END ARCHITECTURE a_mips_decoder;