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---------------------------------------------------------------------

-- TITLE: Cache Controller

-- AUTHOR: Steve Rhoads (rhoadss@yahoo.com)

-- DATE CREATED: 12/22/08

-- FILENAME: cache.vhd

-- PROJECT: Plasma CPU core

-- COPYRIGHT: Software placed into the public domain by the author.

--    Software 'as is' without warranty.  Author liable for nothing.

-- DESCRIPTION:

--    Control 4KB unified cache that uses the upper 4KB of the 8KB

--    internal RAM.  Only lowest 2MB of DDR is cached.

---------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use work.mlite_pack.all;

use ieee.std_logic_unsigned.all;

entity cache is

   generic(memory_type : string := "DEFAULT");

   port(clk             : in std_logic;

        reset           : in std_logic;

        address_next    : in std_logic_vector(31 downto 2);

        byte_we_next    : in std_logic_vector(3 downto 0);

        cpu_address     : in std_logic_vector(31 downto 2);

        mem_busy        : in std_logic;

        cache_check     : out std_logic;   --Stage1: address_next in first 2MB DDR

        cache_checking  : out std_logic;   --Stage2: comparing tags

        cache_miss      : out std_logic);  --Stage2-3: cache miss

end; --cache

architecture logic of cache is

   subtype state_type is std_logic_vector(1 downto 0);

   constant STATE_CHECK    : state_type := "00";

   constant STATE_CHECKING : state_type := "01";

   constant STATE_MISSED   : state_type := "10";

   constant STATE_WRITING  : state_type := "11";

   signal state_reg        : state_type;

   signal state            : state_type;

   signal state_next       : state_type;

   signal cache_address    : std_logic_vector(10 downto 0);

   signal cache_tag_in     : std_logic_vector(8 downto 0);

   signal cache_tag_reg    : std_logic_vector(8 downto 0);

   signal cache_tag_out    : std_logic_vector(8 downto 0);

   signal cache_we         : std_logic;

begin

   cache_proc: process(clk, mem_busy, cache_address, cache_we,

      state_reg, state, state_next,

      address_next, byte_we_next, cache_tag_in, --Stage1

      cache_tag_reg, cache_tag_out,             --Stage2

      cpu_address) --Stage3

   begin

      case state_reg is

      when STATE_CHECK =>

         cache_checking <= '0';

         cache_miss <= '0';

         state <= STATE_CHECK;

      when STATE_CHECKING =>

         cache_checking <= '1';

         if cache_tag_out /= cache_tag_reg or cache_tag_out = ONES(8 downto 0) then

            cache_miss <= '1';

            cache_we <= '0';

            state <= STATE_MISSED;

         else

            cache_miss <= '0';

            state <= STATE_CHECK;

         end if;

      when STATE_MISSED =>

         cache_checking <= '0';

         cache_miss <= '1';

         cache_we <= '1';

         if mem_busy = '1' then

            state <= STATE_MISSED;

         else

            state <= STATE_CHECK;

         end if;

      when STATE_WRITING =>

         cache_checking <= '0';

         cache_miss <= '0';

         if mem_busy = '1' then

            cache_we <= '0';

            state <= STATE_WRITING;

         else

            state <= STATE_CHECK;

         end if;

      when others =>

         state <= STATE_CHECK;

      end case; --state

      if state = STATE_CHECK and state_reg /= STATE_MISSED then

         cache_address <= '0' & address_next(11 downto 2);

         if address_next(30 downto 21) = "0010000000" then  --first 2MB of DDR

            cache_check <= '1';

            if byte_we_next = "0000" then

               cache_we <= '0';

               state_next <= STATE_CHECKING;

            else

               cache_we <= '1';

               state_next <= STATE_WRITING;

            end if;

         else

            cache_check <= '0';

            cache_we <= '0';

            state_next <= STATE_CHECK;

         end if;

      else

         cache_address <= '0' & cpu_address(11 downto 2);

         cache_check <= '0';

         state_next <= state;

      end if;

      if byte_we_next = "0000" or byte_we_next = "1111" then

         cache_tag_in <= address_next(20 downto 12);

      else

         cache_tag_in <= ONES(8 downto 0);  --invalid tag

      end if;

      if reset = '1' then

         state_reg <= STATE_CHECK;

         cache_tag_reg <= ZERO(8 downto 0);

      elsif rising_edge(clk) then

         state_reg <= state_next;

         if state = STATE_CHECK then

            cache_tag_reg <= cache_tag_in;

         end if;

      end if;

   end process;

   cache_xilinx: if memory_type = "XILINX_16X" generate

   begin

      cache_tag: RAMB16_S9  --Xilinx specific

      port map (

         DO   => cache_tag_out(7 downto 0),

         DOP  => cache_tag_out(8),

         ADDR => cache_address,             --registered

         CLK  => clk,

         DI   => cache_tag_in(7 downto 0),  --registered

         DIP  => cache_tag_in(8),

         EN   => '1',

         SSR  => ZERO(0),

         WE   => cache_we);

   end generate; --cache_xilinx

   cache_generic: if memory_type /= "XILINX_16X" generate

   begin

      cache_tag: process(clk, cache_address, cache_tag_in, cache_we)

         constant ADDRESS_WIDTH : natural := 10;

         type storage_array is

            array(natural range 0 to 2 ** ADDRESS_WIDTH - 1) of

            std_logic_vector(8 downto 0);

         variable storage : storage_array;

         variable index   : natural := 0;

      begin

         if rising_edge(clk) then

            index := conv_integer(cache_address(ADDRESS_WIDTH-1 downto 0));

            if cache_we = '1' then

               storage(index) := cache_tag_in;

            end if;

            cache_tag_out <= storage(index);

         end if;

      end process; --cache_tag

   end generate; --cache_generic

end; --logic