Subversion Repositories open8_urisc

-- 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
-- ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
-- THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
--
-- VHDL Units :  o8_ram_1k
-- Description:  Provides a wrapper layer for a 1kx8 RAM model
 
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(
  Reset_Level           : std_logic;
  Address               : ADDRESS_TYPE
);
port(
  Clock                 : in  std_logic;
  Reset                 : in  std_logic;
  --
  Bus_Address           : in  ADDRESS_TYPE;
  Wr_Enable             : in  std_logic;
  Wr_Data               : in  DATA_TYPE;
  Rd_Enable             : in  std_logic;
  Rd_Data               : out DATA_TYPE
);
end entity;
 
architecture behave of o8_ram_4k is
 
  constant User_Addr    : std_logic_vector(15 downto 12)
                          := Address(15 downto 12);
  alias Comp_Addr       is Bus_Address(15 downto 12);
  alias RAM_Addr        is Bus_Address(11 downto 0);
 
  signal Addr_Match     : std_logic := '0';
  signal Wr_En          : std_logic := '0';
  signal Rd_En          : std_logic := '0';
  signal Rd_Data_i      : DATA_TYPE := OPEN8_NULLBUS;
 
begin
 
  -- This decode needs to happen immediately, to give the RAM a chance to
  --  do the lookup before we have to set Rd_Data
  Addr_Match            <= '1' when Comp_Addr = User_Addr else '0';
  Wr_En                 <= Addr_Match and Wr_Enable;
 
  -- 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,
    wren                => Wr_En,
    q                   => Rd_Data_i
  );
 
  RAM_proc: process( Reset, Clock )
  begin
    if( Reset = Reset_Level )then
      Rd_En             <= '0';
      Rd_Data           <= OPEN8_NULLBUS;
    elsif( rising_edge(Clock) )then
      Rd_En             <= Addr_Match and Rd_Enable;
      Rd_Data           <= OPEN8_NULLBUS;
      if( Rd_En = '1' )then
        Rd_Data         <= Rd_Data_i;
      end if;
    end if;
  end process;
 
end architecture;