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-- Copyright (c)2013, 2020 Jeremy Seth Henry

-- All rights reserved.

--

-- Redistribution and use in source and binary forms, with or without

-- modification, are permitted provided that the following conditions are met:

--     * Redistributions of source code must retain the above copyright

--       notice, this list of conditions and the following disclaimer.

--     * Redistributions in binary form must reproduce the above copyright

--       notice, this list of conditions and the following disclaimer in the

--       documentation and/or other materials provided with the distribution,

--       where applicable (as part of a user interface, debugging port, etc.)

--

-- THIS SOFTWARE IS PROVIDED BY JEREMY SETH HENRY ``AS IS'' AND ANY

-- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

-- WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

-- DISCLAIMED. IN NO EVENT SHALL JEREMY SETH HENRY BE LIABLE FOR ANY

-- DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES

-- (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

-- LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND

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--

-- VHDL Units :  o8_ram_4k

-- Description:  Provides a wrapper layer for a 4kx8 RAM model with interface

--            :   logic for the Open8 CPU. Also provides an optional write

--            :   enable register that prevents regions from being written

--            :   by non-ISR code (uses the I flag) as a way to prevent tasks

--            :   from inadvertently writing outside of their designated

--            :   memory space.

--            :  When enabled, the write mask logically divides the memory into

--            :   32, 128 byte regions, corresponding to the 32 bits in the WPR

--            :   register.

--

-- WP Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x00  AAAAAAAA Region Enables  7:0                  (RW)

--   0x01  AAAAAAAA Region Enables 15:8                  (RW)

--   0x02  AAAAAAAA Region Enables 23:16                 (RW)

--   0x03  AAAAAAAA Region Enables 31:24                 (RW)

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      04/16/20 Revision block added

-- Seth Henry      05/12/20 Added write protect logic

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;

library work;

  use work.open8_pkg.all;

entity o8_ram_4k is

generic(

  Write_Protect              : boolean := FALSE;

  Default_Mask               : std_logic_vector(31 downto 0) := x"00000000";

  Address_WPR                : ADDRESS_TYPE := x"1000";

  Address_RAM                : ADDRESS_TYPE

);

port(

  Open8_Bus                  : in  OPEN8_BUS_TYPE;

  Rd_Data                    : out DATA_TYPE;

  Write_Fault                : out std_logic

);

end entity;

architecture behave of o8_ram_4k is

  alias  Clock               is Open8_Bus.Clock;

  alias  Reset               is Open8_Bus.Reset;

  alias  ISR_En              is Open8_Bus.GP_Flags(EXT_ISR);

  alias  Wr_En               is Open8_Bus.Wr_En;

  alias  Rd_En               is Open8_Bus.Rd_En;

  constant WPR_User_Addr     : std_logic_vector(15 downto 2)

                               := Address_WPR(15 downto 2);

  constant RAM_User_Addr     : std_logic_vector(15 downto 12)

                               := Address_RAM(15 downto 12);

  alias  WPR_Comp_Addr       is Open8_Bus.Address(15 downto 2);

  signal WPR_Addr_Match      : std_logic := '0';

  alias  WPR_Reg_Sel_d       is Open8_Bus.Address(1 downto 0);

  signal WPR_Reg_Sel_q       : std_logic_vector(1 downto 0) :=

                                (others => '0');

  alias  Wr_Data_d           is Open8_Bus.Wr_Data;

  signal WPR_Wr_Data_q       : DATA_TYPE := x"00";

  signal Write_Mask          : std_logic_vector(31 downto 0) :=

                                x"00000000";

  alias  Write_Mask_0        is Write_Mask( 7 downto  0);

  alias  Write_Mask_1        is Write_Mask(15 downto  8);

  alias  Write_Mask_2        is Write_Mask(23 downto 16);

  alias  Write_Mask_3        is Write_Mask(31 downto 24);

  signal WPR_Wr_En_d         : std_logic := '0';

  signal WPR_Wr_En_q         : std_logic := '0';

  signal WPR_Rd_En_d         : std_logic := '0';

  signal WPR_Rd_En_q         : std_logic := '0';

  alias  RAM_Base_Addr       is Open8_Bus.Address(15 downto 12);

  alias  RAM_Addr            is Open8_Bus.Address(11 downto 0);

  alias  RAM_Rgn_Addr        is Open8_Bus.Address(11 downto 7);

  signal RAM_Region_Match    : std_logic := '0';

  signal RAM_Addr_Match      : std_logic := '0';

  signal RAM_Wr_En_d         : std_logic := '0';

  signal RAM_Rd_En_d         : std_logic := '0';

  signal RAM_Rd_En_q         : std_logic := '0';

  signal RAM_Rd_Data         : DATA_TYPE := OPEN8_NULLBUS;

  signal Write_Fault_d       : std_logic := '0';

begin

Write_Protect_On : if( Write_Protect )generate

  WPR_Addr_Match             <= '1' when WPR_Comp_Addr = WPR_User_Addr else '0';

  WPR_Wr_En_d                <= WPR_Addr_Match and Wr_En and ISR_En;

  WPR_Rd_En_d                <= WPR_Addr_Match and Rd_En;

  RAM_Addr_Match             <= '1' when RAM_Base_Addr = RAM_User_Addr else '0';

  RAM_Region_Match           <= Write_Mask(conv_integer(RAM_Rgn_Addr)) or

                                ISR_En;

  RAM_Rd_En_d                <= RAM_Addr_Match and Rd_En;

  RAM_Wr_En_d                <= RAM_Addr_Match and RAM_Region_Match and Wr_En;

  Write_Fault_d              <= RAM_Addr_Match and (not RAM_Region_Match) and Wr_En;

  RAM_proc: process( Reset, Clock )

  begin

    if( Reset = Reset_Level )then

      WPR_Reg_Sel_q          <= (others => '0');

      WPR_Wr_Data_q          <= x"00";

      WPR_Wr_En_q            <= '0';

      WPR_Rd_En_q            <= '0';

      Write_Mask             <= Default_Mask;

      RAM_Rd_En_q            <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

      Write_Fault            <= '0';

    elsif( rising_edge(Clock) )then

      WPR_Reg_Sel_q          <= WPR_Reg_Sel_d;

      WPR_Wr_En_q            <= WPR_Wr_En_d;

      WPR_Wr_Data_q          <= Wr_Data_d;

      if( WPR_Wr_En_q = '1' )then

        case( WPR_Reg_Sel_q )is

          when "00" =>

            Write_Mask_0     <= WPR_Wr_Data_q;

          when "01" =>

            Write_Mask_1     <= WPR_Wr_Data_q;

          when "10" =>

            Write_Mask_2     <= WPR_Wr_Data_q;

          when "11" =>

            Write_Mask_3     <= WPR_Wr_Data_q;

          when others =>

            null;

        end case;

      end if;

      WPR_Rd_En_q            <= WPR_Rd_En_d;

      RAM_Rd_En_q            <= RAM_Rd_En_d;

      Rd_Data                <= OPEN8_NULLBUS;

      if( RAM_Rd_En_q = '1' )then

        Rd_Data              <= RAM_Rd_Data;

      elsif( WPR_Rd_En_q = '1'  )then

        case( WPR_Reg_Sel_q )is

          when "00" =>

            Rd_Data          <= Write_Mask_0;

          when "01" =>

            Rd_Data          <= Write_Mask_1;

          when "10" =>

            Rd_Data          <= Write_Mask_2;

          when "11" =>

            Rd_Data          <= Write_Mask_3;

          when others =>

            null;

        end case;

      end if;

      Write_Fault            <= Write_Fault_d;

    end if;

  end process;

end generate;

Write_Protect_Off : if( not Write_Protect )generate

  Write_Fault                <= '0';

  RAM_Addr_Match             <= '1' when RAM_Base_Addr = RAM_User_Addr else '0';

  RAM_Rd_En_d                <= RAM_Addr_Match and Open8_Bus.Rd_En;

  RAM_Wr_En_d                <= RAM_Addr_Match and Open8_Bus.Wr_En;

  RAM_proc: process( Reset, Clock )

  begin

    if( Reset = Reset_Level )then

      RAM_Rd_En_q            <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

    elsif( rising_edge(Clock) )then

      RAM_Rd_En_q            <= RAM_Rd_En_d;

      Rd_Data                <= OPEN8_NULLBUS;

      if( RAM_Rd_En_q = '1' )then

        Rd_Data              <= RAM_Rd_Data;

      end if;

    end if;

  end process;

end generate;

  -- Note that this RAM should be created without an output FF (unregistered Q)

  U_RAM : entity work.ram_4k_core

  port map(

    address                  => RAM_Addr,

    clock                    => Clock,

    data                     => Wr_Data_d,

    wren                     => RAM_Wr_En_d,

    q                        => RAM_Rd_Data

  );

end architecture;