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--===========================================================================--
--
--  S Y N T H E Z I A B L E    datram - Dynamic Address Translation core
--
--  www.OpenCores.Org - September 2003
--  This core adheres to the GNU public license  
--
-- File name      : datram.vhd
--
-- Entity name    : dat_ram
--
-- Purpose        : Maps address bits A12 to A15 of an 8 bit
--                  Microprocessor to access 1Mbyte of RAM
--                  The 8 data output pins form addresses
--                  PA12 to PA19. 16 x 4K pages are selected
--                  by writing the high order physical address
--                  of memory into the 16 registers.
--                  Low order CPU addres bits A11 .. A0
--                  connect directly through to the Phyical
--                  address bits PA11 .. PA0
--                  Register 0  is for A15..A12 = "0000"
--                  Register 1  is for A15..A12 = "0001"
--                  Register 2  is for A15..A12 = "0010"
--                  Register 15 is for A15..A12 = "1111"
--                  Registers are pre-initialised to select
--                  the bottom 64K of physical address space
--
-- Dependencies   : ieee.std_logic_1164
--                  ieee.std_logic_unsigned
--
-- Uses           : Nothing
--
-------------------------------------------------------------------------------
-- Revision list
--
-- Revision 0.1 - 10 November 2002 - John Kent
--
-- Revision 1.0 - 7 September - John Kent
-- Initial release to open cores
--
 
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
 
entity dat_ram is
	port (	
	 clk       : in  std_logic;
    rst       : in  std_logic;
    cs        : in  std_logic;
    rw        : in  std_logic;
    addr_hi   : in  std_logic_vector(3 downto 0);
    addr_lo   : in  std_logic_vector(3 downto 0);
    data_in   : in  std_logic_vector(7 downto 0);
	 data_out  : out std_logic_vector(7 downto 0));
end;
 
architecture datram_arch of dat_ram is
signal dat_reg0 : std_logic_vector(7 downto 0);
signal dat_reg1 : std_logic_vector(7 downto 0);
signal dat_reg2 : std_logic_vector(7 downto 0);
signal dat_reg3 : std_logic_vector(7 downto 0);
signal dat_reg4 : std_logic_vector(7 downto 0);
signal dat_reg5 : std_logic_vector(7 downto 0);
signal dat_reg6 : std_logic_vector(7 downto 0);
signal dat_reg7 : std_logic_vector(7 downto 0);
signal dat_reg8 : std_logic_vector(7 downto 0);
signal dat_reg9 : std_logic_vector(7 downto 0);
signal dat_reg10 : std_logic_vector(7 downto 0);
signal dat_reg11 : std_logic_vector(7 downto 0);
signal dat_reg12 : std_logic_vector(7 downto 0);
signal dat_reg13 : std_logic_vector(7 downto 0);
signal dat_reg14 : std_logic_vector(7 downto 0);
signal dat_reg15 : std_logic_vector(7 downto 0);
 
begin
 
 
--------------------------------
--
-- read I/O port
--
--------------------------------
 
 
---------------------------------
--
-- Write DAT RAM
--
---------------------------------
 
dat_write : process( clk, rst, addr_lo, cs, rw, data_in )
begin
  if clk'event and clk = '0' then
    if rst = '1' then
      dat_reg0 <= "00000000";
      dat_reg1 <= "00000001";
      dat_reg2 <= "00000010";
      dat_reg3 <= "00000011";
      dat_reg4 <= "00000100";
      dat_reg5 <= "00000101";
      dat_reg6 <= "00000110";
      dat_reg7 <= "00000111";
      dat_reg8 <= "00001000";
      dat_reg9 <= "00001001";
      dat_reg10 <= "00001010";
      dat_reg11 <= "00001011";
      dat_reg12 <= "00001100";
      dat_reg13 <= "00001101";
      dat_reg14 <= "00001110";
      dat_reg15 <= "00001111";
    else
	   if cs = '1' and rw = '0' then
        case addr_lo is
	     when "0000" =>
		    dat_reg0 <= data_in;
	     when "0001" =>
		    dat_reg1 <= data_in;
	     when "0010" =>
		    dat_reg2 <= data_in;
	     when "0011" =>
		    dat_reg3 <= data_in;
	     when "0100" =>
		    dat_reg4 <= data_in;
	     when "0101" =>
		    dat_reg5 <= data_in;
	     when "0110" =>
		    dat_reg6 <= data_in;
	     when "0111" =>
		    dat_reg7 <= data_in;
	     when "1000" =>
		    dat_reg8 <= data_in;
	     when "1001" =>
		    dat_reg9 <= data_in;
	     when "1010" =>
		    dat_reg10 <= data_in;
	     when "1011" =>
		    dat_reg11 <= data_in;
	     when "1100" =>
		    dat_reg12 <= data_in;
	     when "1101" =>
		    dat_reg13 <= data_in;
	     when "1110" =>
		    dat_reg14 <= data_in;
	     when "1111" =>
		    dat_reg15 <= data_in;
        when others =>
		    null;
		  end case;
	   end if;
	 end if;
  end if;
end process;
 
dat_read : process(  addr_hi,
                     dat_reg0, dat_reg1, dat_reg2, dat_reg3,
                     dat_reg4, dat_reg5, dat_reg6, dat_reg7,
                     dat_reg8, dat_reg9, dat_reg10, dat_reg11,
                     dat_reg12, dat_reg13, dat_reg14, dat_reg15 )
begin
      case addr_hi is
	     when "0000" =>
		    data_out <= dat_reg0;
	     when "0001" =>
		    data_out <= dat_reg1;
	     when "0010" =>
		    data_out <= dat_reg2;
	     when "0011" =>
		    data_out <= dat_reg3;
	     when "0100" =>
		    data_out <= dat_reg4;
	     when "0101" =>
		    data_out <= dat_reg5;
	     when "0110" =>
		    data_out <= dat_reg6;
	     when "0111" =>
		    data_out <= dat_reg7;
	     when "1000" =>
		    data_out <= dat_reg8;
	     when "1001" =>
		    data_out <= dat_reg9;
	     when "1010" =>
		    data_out <= dat_reg10;
	     when "1011" =>
		    data_out <= dat_reg11;
	     when "1100" =>
		    data_out <= dat_reg12;
	     when "1101" =>
		    data_out <= dat_reg13;
	     when "1110" =>
		    data_out <= dat_reg14;
	     when "1111" =>
		    data_out <= dat_reg15;
        when others =>
		    null;
		end case;
end process;
 
end datram_arch;