-- MIPS I system
-- Copyright 2011-2012 Stefan Schuermans <stefan@schuermans.info>
-- 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_mul IS
    PORT (
        rst:      IN  std_logic;
        clk:      IN  std_logic;
        i_a:      IN  std_logic_vector(31 DOWNTO 0);
        i_b:      IN  std_logic_vector(31 DOWNTO 0);
        i_signed: IN  std_logic;
        i_start:  IN  std_logic;
        o_busy:   OUT std_logic;
        o_res:    OUT std_logic_vector(63 DOWNTO 0)
    );
END ENTITY e_mips_mul;

ARCHITECTURE a_mips_mul OF e_mips_mul IS

    TYPE t_state IS (idle, state_1, state_2, state_3, state_4, post);
    SIGNAL n_state: t_state;
    SIGNAL r_state: t_state := idle;

    SIGNAL n_a:   unsigned(31 DOWNTO 0);
    SIGNAL n_b:   unsigned(31 DOWNTO 0);
    SIGNAL n_neg: boolean;
    SIGNAL n_res: unsigned(63 DOWNTO 0);
    SIGNAL r_a:   unsigned(31 DOWNTO 0) := (OTHERS => '0');
    SIGNAL r_b:   unsigned(31 DOWNTO 0) := (OTHERS => '0');
    SIGNAL r_neg: boolean               := false;
    SIGNAL r_res: unsigned(63 DOWNTO 0) := (OTHERS => '0');

BEGIN

    p_mul: PROCESS(r_state, r_a, r_b, r_neg, r_res,
                   i_a, i_b, i_signed, i_start)
        VARIABLE v_a:   unsigned(15 DOWNTO 0);
        VARIABLE v_b:   unsigned(15 DOWNTO 0);
        VARIABLE v_res: unsigned(31 DOWNTO 0);
        VARIABLE v_add: unsigned(63 DOWNTO 0);
        VARIABLE v_sum: unsigned(63 DOWNTO 0);
    BEGIN
        o_busy  <= '0';
        n_state <= idle;
        n_a     <= r_a;
        n_b     <= r_b;
        n_neg   <= r_neg;
        n_res   <= r_res;
        v_a     := (OTHERS => '0');
        v_b     := (OTHERS => '0');
        v_add   := (OTHERS => '0');
        CASE r_state IS
            WHEN idle =>
                IF i_start = '1' THEN
                    o_busy  <= '1';
                    n_state <= state_1;
                    IF i_signed = '1' THEN
                        IF i_a(31) = '1' THEN
                            n_a <= unsigned(-signed(i_a));
                        ELSE
                            n_a <= unsigned(i_a);
                        END IF;
                        IF i_b(31) = '1' THEN
                            n_b <= unsigned(-signed(i_b));
                        ELSE
                            n_b <= unsigned(i_b);
                        END IF;
                        n_neg <= i_a(31) = '1' XOR i_b(31) = '1';
                    ELSE
                        n_a   <= unsigned(i_a);
                        n_b   <= unsigned(i_b);
                        n_neg <= false;
                    END IF;
                END IF;
            WHEN state_1 =>
                o_busy  <= '1';
                n_state <= state_2;
                v_a     := r_a(15 DOWNTO  0);
                v_b     := r_b(15 DOWNTO  0);
            WHEN state_2 =>
                o_busy  <= '1';
                n_state <= state_3;
                v_a     := r_a(31 DOWNTO 16);
                v_b     := r_b(15 DOWNTO  0);
            WHEN state_3 =>
                o_busy  <= '1';
                n_state <= state_4;
                v_a     := r_a(15 DOWNTO  0);
                v_b     := r_b(31 DOWNTO 16);
            WHEN state_4 =>
                o_busy  <= '1';
                n_state <= post;
                v_a     := r_a(31 DOWNTO 16);
                v_b     := r_b(31 DOWNTO 16);
            WHEN OTHERS => NULL;
        END CASE;
        v_res := v_a * v_b;
        CASE r_state IS
            WHEN state_1 => v_add := X"00000000" & v_res;
            WHEN state_2 => v_add := X"0000" & v_res & X"0000";
            WHEN state_3 => v_add := X"0000" & v_res & X"0000";
            WHEN state_4 => v_add := v_res & X"00000000";
            WHEN OTHERS => NULL;
        END CASE;
        v_sum := r_res + v_add;
        CASE r_state IS
            WHEN idle =>
                n_res <= (OTHERS => '0');
            WHEN state_1 | state_2 | state_3 | state_4 =>
                n_res <= v_sum;
            WHEN post =>
                IF r_neg THEN
                    n_res <= unsigned(-signed(r_res));
                ELSE
                    n_res <= r_res;
                END IF;
            WHEN OTHERS => NULL;
        END CASE;
    END PROCESS p_mul;

    p_sync: PROCESS(rst, clk)
    BEGIN
        IF rst = '1' THEN
            r_state <= idle;
            r_a     <= (OTHERS => '0');
            r_b     <= (OTHERS => '0');
            r_neg   <= false;
            r_res   <= (OTHERS => '0');
        ELSIF rising_edge(clk) THEN
            r_state <= n_state;
            r_a     <= n_a;
            r_b     <= n_b;
            r_neg   <= n_neg;
            r_res   <= n_res;
        END IF;
    END PROCESS p_sync;

    o_res <= std_logic_vector(n_res);

END ARCHITECTURE a_mips_mul;