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

-- TITLE: Memory Controller

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

-- DATE CREATED: 1/31/01

-- FILENAME: mem_ctrl.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:

--    Memory controller for the Plasma CPU.

--    Supports Big or Little Endian mode.

--    Four cycles for a write unless a(31)='1' then two cycles.

--    This entity could implement interfaces to:

--       Data cache

--       Address cache

--       Memory management unit (MMU)

--       DRAM controller

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

library ieee;

use ieee.std_logic_1164.all;

use work.mlite_pack.all;

entity mem_ctrl is

   generic(ACCURATE_TIMING : boolean := false);

   port(clk          : in std_logic;

        reset_in     : in std_logic;

        pause_in     : in std_logic;

        nullify_op   : in std_logic;

        address_pc   : in std_logic_vector(31 downto 0);

        opcode_out   : out std_logic_vector(31 downto 0);

        address_data : in std_logic_vector(31 downto 0);

        mem_source   : in mem_source_type;

        data_write   : in std_logic_vector(31 downto 0);

        data_read    : out std_logic_vector(31 downto 0);

        pause_out    : out std_logic;

        mem_address  : out std_logic_vector(31 downto 0);

        mem_data_w   : out std_logic_vector(31 downto 0);

        mem_data_r   : in std_logic_vector(31 downto 0);

        mem_byte_sel : out std_logic_vector(3 downto 0);

        mem_write    : out std_logic);

end; --entity mem_ctrl

architecture logic of mem_ctrl is

   --"00" = big_endian; "11" = little_endian

   constant little_endian  : std_logic_vector(1 downto 0) := "00";

   signal opcode_reg       : std_logic_vector(31 downto 0);

   signal next_opcode_reg  : std_logic_vector(31 downto 0);

   signal address_reg      : std_logic_vector(31 downto 0);

   subtype mem_state_type is std_logic_vector(1 downto 0);

   signal mem_state_reg  : mem_state_type;

   constant STATE_FETCH  : mem_state_type := "00";

   constant STATE_ADDR   : mem_state_type := "01";

   constant STATE_WRITE  : mem_state_type := "10";

   constant STATE_PAUSE  : mem_state_type := "11";

   --ACCURATE_TIMING notes:

   --The VHDL compiler's timing calculation isn't able to realize that

   --memory reads take two clock cycles.  It notices that reg_bank:reg_dest

   --is dependent on mem_ctrl:mem_data_r which is dependent on 

   --mem_ctrl:mem_address which is dependent on alu:c_alu.  However,

   --this dependency is only true for memory read or write cycles

   --which are multiple clock cycles.  Enabling ACCURATE_TIMING

   --creates an additional 32-bit register that does nothing other

   --than letting the VHDL compiler accurately predict the maximum

   --clock speed.

begin

mem_proc: process(clk, reset_in, pause_in, nullify_op,

                  address_pc, address_data, mem_source, data_write,

                  mem_data_r,

                  opcode_reg, next_opcode_reg, mem_state_reg,

                  address_reg)

   variable data           : std_logic_vector(31 downto 0);

   variable opcode_next    : std_logic_vector(31 downto 0);

   variable byte_sel       : std_logic_vector(3 downto 0);

   variable write_line     : std_logic;

   variable mem_state_next : mem_state_type;

   variable pause          : std_logic;

   variable address        : std_logic_vector(31 downto 0);

   variable bits           : std_logic_vector(1 downto 0);

   variable mem_data_w_v   : std_logic_vector(31 downto 0);

begin

   byte_sel := "0000";

   write_line := '0';

   pause := '0';

   mem_state_next := mem_state_reg;

   address := address_pc;

   data := ZERO;

   mem_data_w_v := ZERO;

   case mem_source is

   when mem_read32 =>

      data := mem_data_r;

   when mem_read16 | mem_read16s =>

      if address_reg(1) = little_endian(1) then

         data(15 downto 0) := mem_data_r(31 downto 16);

      else

         data(15 downto 0) := mem_data_r(15 downto 0);

      end if;

      if mem_source = mem_read16 or data(15) = '0' then

         data(31 downto 16) := ZERO(31 downto 16);

      else

         data(31 downto 16) := ONES(31 downto 16);

      end if;

   when mem_read8 | mem_read8s =>

      bits := address_reg(1 downto 0) xor little_endian;

      case bits is

      when "00" => data(7 downto 0) := mem_data_r(31 downto 24);

      when "01" => data(7 downto 0) := mem_data_r(23 downto 16);

      when "10" => data(7 downto 0) := mem_data_r(15 downto 8);

      when others => data(7 downto 0) := mem_data_r(7 downto 0);

      end case;

      if mem_source = mem_read8 or data(7) = '0' then

         data(31 downto 8) := ZERO(31 downto 8);

      else

         data(31 downto 8) := ONES(31 downto 8);

      end if;

   when mem_write32 =>

      write_line := '1';

      mem_data_w_v := data_write;

      byte_sel := "1111";

   when mem_write16 =>

      write_line := '1';

      mem_data_w_v := data_write(15 downto 0) & data_write(15 downto 0);

      if address_data(1) = little_endian(1) then

         byte_sel := "1100";

      else

         byte_sel := "0011";

      end if;

   when mem_write8 =>

      write_line := '1';

      mem_data_w_v := data_write(7 downto 0) & data_write(7 downto 0) &

                  data_write(7 downto 0) & data_write(7 downto 0);

      bits := address_data(1 downto 0) xor little_endian;

      case bits is

      when "00" =>

         byte_sel := "1000";

      when "01" =>

         byte_sel := "0100";

      when "10" =>

         byte_sel := "0010";

      when others =>

         byte_sel := "0001";

      end case;

   when others =>

   end case;

   opcode_next := opcode_reg;

   if mem_source = mem_fetch then --opcode fetch

      mem_state_next := STATE_FETCH;

      if pause_in = '0' then

         opcode_next := mem_data_r;

      end if;

   else  --data read or write (not opcode fetch)

      --State Machine

      case mem_state_reg is

      when STATE_FETCH =>

         write_line := '0';

         pause := '1';

         byte_sel := "0000";

         if pause_in = '0' then

            mem_state_next := STATE_ADDR;

         end if;

      when STATE_ADDR =>  --address lines pre-hold

         address := address_reg;

         if write_line = '1' and address_reg(31) = '1' then

            pause := '1';

            byte_sel := "0000";

            if pause_in = '0' then

               mem_state_next := STATE_WRITE;    --4 cycle access

            end if;

         else

            if pause_in = '0' then

               opcode_next := next_opcode_reg;

               mem_state_next := STATE_FETCH;    --2 cycle access

            end if;

         end if;

      when STATE_WRITE =>

         pause := '1';

         address := address_reg;

         if pause_in = '0' then

            mem_state_next := STATE_PAUSE;

         end if;

      when OTHERS =>  --STATE_PAUSE address lines post-hold

         address := address_reg;

         byte_sel := "0000";

         if pause_in = '0' then

            opcode_next := next_opcode_reg;

            mem_state_next := STATE_FETCH;

         end if;

      end case;

   end if;

   if nullify_op = '1' then

      opcode_next := ZERO;  --NOP

   end if;

   if reset_in = '1' then

      mem_state_reg <= STATE_FETCH;

      opcode_reg <= ZERO;

      next_opcode_reg <= ZERO;

      write_line := '0';

   elsif rising_edge(clk) then

      mem_state_reg <= mem_state_next;

      opcode_reg <= opcode_next;

      if mem_state_reg = STATE_FETCH then

         next_opcode_reg <= mem_data_r;

      end if;

   end if;

   if rising_edge(clk) then

      if ACCURATE_TIMING then

         address_reg <= address_data;

      end if;

   end if;

   if not ACCURATE_TIMING then

      address_reg <= address_data;

   end if;

   opcode_out <= opcode_reg;

   data_read <= data;

   pause_out <= pause;

   mem_byte_sel <= byte_sel;

   mem_address <= address;

   mem_write <= write_line;

   if write_line = '1' then

      mem_data_w <= mem_data_w_v;

   else

      mem_data_w <= HIGH_Z; --ZERO;

   end if;

end process; --data_proc

end; --architecture logic