Subversion Repositories System09

--=============================================================================--
--                                                                             --
--  System09 - Synthesizable System On a Chip - VHDL FPGA core top level file. --
--                                                                             --
--=============================================================================--
--
--
-- File name      : System09_Digilent_3S200.vhd
--
-- Entity name    : System09
--
-- Purpose        : Top level file for 6809 compatible system on a chip
--                  Designed with Xilinx XC3S200 Spartan 3 FPGA.
--                  Implemented With Digilent Xilinx Starter FPGA board,
--
-- Dependencies   : ieee.Std_Logic_1164
--                  ieee.std_logic_unsigned
--                  ieee.std_logic_arith
--                  ieee.numeric_std
--
-- Uses           : cpu09         (..\VHDL\cpu09.vhd)              CPU core
--                  dat_ram       (..\VHDL\datram.vhd)             Dynamic Address Translation
--                  SYS09BUG_F800 (..\Spartan3\sys093s3_b16.vhd)   Monitor ROM
--                  acia6850      (..\VHDL\acia6850.vhd)           ACIA (UART)
--                  ACIA_Clock    (..\VHDL\ACIA_Clock.vhd)         ACIA Baud Rate Clock Divider
--                  keyboard      (..\VHDL\keyboard.vhd)           PS/2 Keyboard register interface
--                  ps2_keyboard  (..\VHDL\ps2_keyboard.vhd)       PS/2 Keyboard interface logic
--                  keymap_rom    (..\Spartan2\keymap_rom.vhd)     PS/2 Keyboard key code look up table
--                  vdu8          (..\VHDL\vdu8.vhd)		          Video Display Unit
--                                (..\Spartan3\char_rom2K_b16.vhd) Character Generator ROM (B16_RAM)
--                                (..\Spartan3\ram2k_b16.vhd)      Text & Attribute RAM Buffer
--                  seven_segment (..\VHDL\SevenSegment.vhd)       Seven Segment Display
-- 
-- Author         : John E. Kent      
--                  dilbert57@opencores.org      
--
-- Memory Map     :
--
-- $0000 - $DFFF System RAM (256K Mapped via DAT)
-- $E000 - ACIA (SWTPc)
-- $E010 - Reserved for SWTPc FD-01 FD1771 FDC
-- $E020 - Keyboard
-- $E030 - VDU
-- $E040 - Reserved for SWTPc MP-T (was Compact Flash)
-- $E050 - Timer
-- $E060 - Reserved for Bus Trap (Hardware Breakpoint Logic)
-- $E070 - Reserved for Trace Buffer
-- $E080 - Reserved for SWTPc MP-ID 6821 PIA (?)
-- $E090 - Reserved for SWTPc MP-ID 6840 PTM (?)
-- $E0A0 - Switches in / LEDS out
-- $E0B0 - 7 Segment display
-- $E0C0 - Reserved
-- $E0D0 - Reserved
-- $E0E0 - Reserved
-- $E0F0 - Reserved
-- $E100 - $E13F Reserved IDE / Compact Flash Card
-- $E140 - $E17F Reserved for Ethernet MAC (XESS)
-- $E180 - $E1BF Reserved for Expansion Slot 0 (XESS)
-- $E1C0 - $E1FF Reserved for Expansion Slot 1 (XESS)
-- $E200 - $EFFF Reserved for Future I/O
-- $F000 - $F7FF RAM for Sys09bug monitor extensions
-- $F800 - $FFFF Sys09bug ROM (Read only)
-- $FFF0 - $FFFF DAT - Dynamic Address Translation (Write Only)
--
--
--  Copyright (C) 2003 - 2010 John Kent
--
--  This program is free software: you can redistribute it and/or modify
--  it under the terms of the GNU General Public License as published by
--  the Free Software Foundation, either version 3 of the License, or
--  (at your option) any later version.
--
--  This program is distributed in the hope that it will be useful,
--  but WITHOUT ANY WARRANTY; without even the implied warranty of
--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--  GNU General Public License for more details.
--
--  You should have received a copy of the GNU General Public License
--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
--
--===========================================================================--
--
--                              Revision History:
--
--===========================================================================--
--
-- Version 0.1 - 20 March 2003
-- Version 0.2 - 30 March 2003
-- Version 0.3 - 29 April 2003
-- Version 0.4 - 29 June 2003
--
-- Version 0.5 - 19 July 2003
-- prints out "Hello World"
--
-- Version 0.6 - 5 September 2003
-- Runs SBUG
--
-- Version 1.0- 6 Sep 2003 - John Kent
-- Inverted sys_clk
-- Initial release to Open Cores
--
-- Version 1.1 - 17 Jan 2004 - John Kent
-- Updated miniUart.
--
-- Version 1.2 - 25 Jan 2004 - John Kent
-- removed signals "test_alu" and "test_cc" 
-- Trap hardware re-instated.
--
-- Version 1.3 - 11 Feb 2004 - John Kent
-- Designed forked off to produce System09_VDU
-- Added VDU component
--	VDU runs at 25MHz and divides the clock by 2 for the CPU
-- UART Runs at 57.6 Kbps
--
-- Version 2.0 - 2 September 2004 - John Kent
-- ported to Digilent Xilinx Spartan3 starter board
--	removed Compaact Flash and Trap Logic.
-- Replaced SBUG with KBug9s
--
-- Version 2.1 - 21 November 2006 - John Kent
-- Replaced KBug9s with Sys09bug 1.0
-- Inverted bottom nybble of DAT register outputs
-- Changed ROM & I/O decoding to be compatible with SWTPc
-- Upped the serial baud rate to 115.2 KBd
-- added multiple global clock buffers
-- (Uart would not operate correctly)
--
-- Version 2.2 - 22 December 2006 - John Kent
-- Increased CPU clock from 12.5MHz to 25 MHz.
-- Removed some of the global clock buffers
-- Added LED output register
-- Changed address decoding to 4K Blocks
--
-- Version 2.3 - 1 June 2007 - John Kent
-- Updated VDU & ACIA
-- Changed decoding for Sys09Bug
--
-- Version 2.4 - 31 January 2008 - John Kent
--	ACIA does not appear to work.
-- Made RAM OE and WE strobes synchonous to sys_clk
--
-- Version 2.5 - 23rd Feburary 2009 - John Kent
-- Implemented recommendation to remove vga_clk clock buffer
--
-- Version 2.6 - 5th september 2010 - John Kent
-- Renamed ACIA_6850 to acia6850
-- Updated generics on VDU8
-- Shortened the "keyboard" label
-- Fixed up address label on CPU09
-- Removed Flex RAM
-- Map RAM at $F000 - $F7FF
--
--===========================================================================--
library ieee;
   use ieee.std_logic_1164.all;
   use IEEE.STD_LOGIC_ARITH.ALL;
   use IEEE.STD_LOGIC_UNSIGNED.ALL;
   use ieee.numeric_std.all;
 
entity system09 is
  port(
    sys_clk     : in  Std_Logic;  -- System Clock input
	 rst_sw    : in  Std_logic;  -- Master Reset input (active high)
	 nmi_sw      : in  Std_logic;
 
    -- Memory Interface signals
    ram_addr    : out Std_Logic_Vector(17 downto 0);
    ram_wen     : out Std_Logic;
    ram_oen     : out Std_Logic;
 
    ram1_cen    : out Std_Logic;
	 ram1_ubn    : out Std_Logic;
	 ram1_lbn    : out Std_Logic;
    ram1_data   : inout Std_Logic_Vector(15 downto 0);
 
    ram2_cen    : out Std_Logic;
	 ram2_ubn    : out Std_Logic;
	 ram2_lbn    : out Std_Logic;
    ram2_data   : inout Std_Logic_Vector(15 downto 0);
 
 	 -- PS/2 Keyboard
	 ps2c        : inout Std_logic;
	 ps2d        : inout Std_Logic;
 
	 -- ACIA Interface
    rxd         : in  Std_Logic;
	 txd         : out Std_Logic;
 
	 -- CRTC output signals
	 vs          : out Std_Logic;
    hs          : out Std_Logic;
    blue        : out std_logic;
    green       : out std_logic;
    red         : out std_logic;
 
	 -- LEDS & Switches
	 leds        : out std_logic_vector(7 downto 0);
	 switches    : in  std_logic_vector(7 downto 0);
 
	 -- seven segment display
	 segments    : out std_logic_vector(7 downto 0);
	 digits      : out std_logic_vector(3 downto 0)
	 );
end system09;
 
-------------------------------------------------------------------------------
-- Architecture for System09
-------------------------------------------------------------------------------
architecture my_computer of system09 is
  -----------------------------------------------------------------------------
  -- constants
  -----------------------------------------------------------------------------
  constant SYS_CLK_FREQ  : integer := 50000000;  -- FPGA System Clock
  constant VGA_CLK_FREQ  : integer := 25000000;  -- VGA Pixel Clock
  constant CPU_CLK_FREQ  : integer := 25000000;  -- CPU Clock
  constant BAUD_Rate     : integer := 57600;	  -- Baud Rate
  constant ACIA_CLK_FREQ : integer := BAUD_Rate * 16;
 
  type hold_state_type is ( hold_release_state, hold_request_state );
 
  -----------------------------------------------------------------------------
  -- Signals
  -----------------------------------------------------------------------------
 
  -- CPU Interface signals
  signal cpu_clk       : Std_Logic;
  signal cpu_rst       : Std_Logic;
  signal cpu_rw        : std_logic;
  signal cpu_vma       : std_logic;
  signal cpu_halt      : std_logic;
  signal cpu_hold      : std_logic;
  signal cpu_firq      : std_logic;
  signal cpu_irq       : std_logic;
  signal cpu_nmi       : std_logic;
  signal cpu_addr      : std_logic_vector(15 downto 0);
  signal cpu_data_in   : std_logic_vector(7 downto 0);
  signal cpu_data_out  : std_logic_vector(7 downto 0);
 
  -- Dynamic Address Translation
  signal dat_cs        : std_logic;
  signal dat_addr      : std_logic_vector(7 downto 0);
 
  -- BOOT ROM
  signal rom_cs        : Std_logic;
  signal rom_data_out  : Std_Logic_Vector(7 downto 0);
 
  -- ACIA Interface signals
  signal acia_clk      : std_logic;
  signal acia_cs       : Std_Logic;
  signal acia_data_out : Std_Logic_Vector(7 downto 0);  
  signal acia_irq      : Std_Logic;
  signal acia_rxd      : Std_Logic;
  signal acia_txd      : Std_Logic;
  signal acia_dcd_n    : Std_Logic;
--  signal acia_rts_n    : Std_Logic;
  signal acia_cts_n    : Std_Logic;
 
  -- keyboard port
  signal kbd_data_out : std_logic_vector(7 downto 0);
  signal kbd_cs       : std_logic;
  signal kbd_irq      : std_logic;
 
  -- LEDs
  signal leds_data_out : std_logic_vector(7 downto 0);
  signal leds_cs       : std_logic;
 
  -- Video Display Unit
  signal vdu_cs       : std_logic;
  signal vdu_data_out : std_logic_vector(7 downto 0);
  signal vga_clk      : std_logic;
 
  -- 7 Segment Display
  signal seg_cs       : std_logic;
  signal seg_data_out : std_logic_vector(7 downto 0);
 
  -- RAM
  signal ram_cs       : std_logic; -- memory chip select
  signal ram_data_out : std_logic_vector(7 downto 0);
  signal ram1_ce      : std_logic;
  signal ram1_ub      : std_logic;
  signal ram1_lb      : std_logic;
  signal ram2_ce      : std_logic;
  signal ram2_ub      : std_logic;
  signal ram2_lb      : std_logic;
  signal ram_we       : std_logic;
  signal ram_oe       : std_logic;
 
  -- System Clock Prescaler
  signal clk_count    : std_logic;
 
-----------------------------------------------------------------
--
-- CPU09 CPU core
--
-----------------------------------------------------------------
 
component cpu09
  port (    
	 clk      :	in  std_logic;
    rst      : in  std_logic;
    vma      :	out std_logic;
    addr     : out std_logic_vector(15 downto 0);
    rw       :	out std_logic;
    data_in  : in	 std_logic_vector(7 downto 0);
	 data_out : out std_logic_vector(7 downto 0);
	 halt     : in  std_logic;
	 hold     : in  std_logic;
	 irq      : in  std_logic;
	 nmi      : in  std_logic;
	 firq     : in  std_logic
  );
end component;
 
----------------------------------------
--
-- Dynamic Address Translation Registers
--
----------------------------------------
component dat_ram
  port (
    clk      : in  std_logic;
	 rst      : in  std_logic;
	 cs       : in  std_logic;
	 rw       : in  std_logic;
	 addr_lo  : in  std_logic_vector(3 downto 0);
	 addr_hi  : 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 component;
 
----------------------------------------
--
-- 4KByte Block RAM Monitor ROM
--
----------------------------------------
component SYS09BUG_F800
  Port (
    clk      : in  std_logic;
    rst      : in  std_logic;
    cs       : in  std_logic;
    rw       : in  std_logic;
    addr     : in  std_logic_vector (10 downto 0);
    data_in  : in  std_logic_vector (7 downto 0);
    data_out : out std_logic_vector (7 downto 0)
    );
end component;
 
-----------------------------------------------------------------
--
-- 6850 ACIA
--
-----------------------------------------------------------------
 
component acia6850
  port (
    clk      : in  Std_Logic;  -- System Clock
    rst      : in  Std_Logic;  -- Reset input (active high)
    cs       : in  Std_Logic;  -- ACIA Chip Select
    rw       : in  Std_Logic;  -- Read / Not Write
    addr     : in  Std_Logic;  -- Register Select
    data_in  : in  Std_Logic_Vector(7 downto 0); -- Data Bus In 
    data_out : out Std_Logic_Vector(7 downto 0); -- Data Bus Out
    irq      : out Std_Logic;  -- Interrupt
    RxC      : in  Std_Logic;  -- Receive Baud Clock
    TxC      : in  Std_Logic;  -- Transmit Baud Clock
    RxD      : in  Std_Logic;  -- Receive Data
    TxD      : out Std_Logic;  -- Transmit Data
	 DCD_n    : in  Std_Logic;  -- Data Carrier Detect
    CTS_n    : in  Std_Logic;  -- Clear To Send
    RTS_n    : out Std_Logic   -- Request To send
  );
end component;
 
-----------------------------------------------------------------
--
-- ACIA Clock divider
--
-----------------------------------------------------------------
 
component ACIA_Clock
  generic (
     SYS_CLK_FREQ  : integer :=  SYS_CLK_FREQ;
	  ACIA_CLK_FREQ : integer := ACIA_CLK_FREQ
  );   
  port (
     clk      : in  Std_Logic;  -- System Clock Input
	  acia_clk : out Std_logic   -- ACIA Clock output
  );
end component;
 
----------------------------------------
--
-- PS/2 Keyboard
--
----------------------------------------
 
component keyboard
  generic(
    KBD_CLK_FREQ : integer := CPU_CLK_FREQ
  );
  port(
    clk             : in    std_logic;
    rst             : in    std_logic;
    cs              : in    std_logic;
    addr            : in    std_logic;
    rw              : in    std_logic;
    data_in         : in    std_logic_vector(7 downto 0);
    data_out        : out   std_logic_vector(7 downto 0);
    irq             : out   std_logic;
    kbd_clk         : inout std_logic;
    kbd_data        : inout std_logic
  );
end component;
 
----------------------------------------
--
-- Video Display Unit.
--
----------------------------------------
component vdu8
  generic(
    VGA_CLK_FREQ           : integer := VGA_CLK_FREQ; -- HZ
	 VGA_HOR_CHARS          : integer := 80; -- CHARACTERS 25.6us
	 VGA_HOR_CHAR_PIXELS    : integer := 8;  -- PIXELS 0.32us
	 VGA_HOR_FRONT_PORCH    : integer := 16; -- PIXELS 0.64us
	 VGA_HOR_SYNC           : integer := 96; -- PIXELS 3.84us
	 VGA_HOR_BACK_PORCH     : integer := 48; -- PIXELS 1.92us
	 VGA_VER_CHARS          : integer := 25; -- CHARACTERS 12.8ms
	 VGA_VER_CHAR_LINES     : integer := 16; -- LINES 0.512ms
	 VGA_VER_FRONT_PORCH    : integer := 10; -- LINES 0.320ms
	 VGA_VER_SYNC           : integer := 2;  -- LINES 0.064ms
	 VGA_VER_BACK_PORCH     : integer := 34  -- LINES 1.088ms
  );
  port(
    -- control register interface
    vdu_clk      : in  std_logic;	 -- CPU Clock - 12.5MHz
    vdu_rst      : in  std_logic;
    vdu_cs       : in  std_logic;
    vdu_rw       : in  std_logic;
    vdu_addr     : in  std_logic_vector(2 downto 0);
    vdu_data_in  : in  std_logic_vector(7 downto 0);
    vdu_data_out : out std_logic_vector(7 downto 0);
 
    -- vga port connections
    vga_clk      : in  std_logic;	-- VGA Pixel Clock - 25 MHz
    vga_red_o    : out std_logic;
    vga_green_o  : out std_logic;
    vga_blue_o   : out std_logic;
    vga_hsync_o  : out std_logic;
    vga_vsync_o  : out std_logic
  );
end component;
 
----------------------------------------
--
-- Seven Segment Display driver
--
----------------------------------------
 
component seven_segment is
  port (	
    clk         : in  std_logic;
    rst         : in  std_logic;
    cs          : in  std_logic;
    addr        : in  std_logic_vector(1 downto 0);
    rw          : in  std_logic;
    data_in     : in  std_logic_vector(7 downto 0);
	 data_out    : out std_logic_vector(7 downto 0);
	 segments    : out std_logic_vector(7 downto 0);
	 digits	     : out std_logic_vector(3 downto 0)
  );
end component;
 
component BUFG 
  port (
    i            : in  std_logic;
    o            : out std_logic
  );
end component;
 
begin
  -----------------------------------------------------------------------------
  -- Instantiation of internal components
  -----------------------------------------------------------------------------
 
my_cpu : cpu09  port map (    
	 clk	     => cpu_clk,
    rst       => cpu_rst,
    vma       => cpu_vma,
    addr      => cpu_addr(15 downto 0),
    rw	     => cpu_rw,
    data_in   => cpu_data_in,
	 data_out  => cpu_data_out,
	 halt      => cpu_halt,
	 hold      => cpu_hold,
	 irq       => cpu_irq,
	 nmi       => cpu_nmi,
	 firq      => cpu_firq
    );
 
my_dat : dat_ram port map (
    clk       => cpu_clk,
	 rst       => cpu_rst,
	 cs        => dat_cs,
	 addr_hi   => cpu_addr(15 downto 12),
	 addr_lo   => cpu_addr(3 downto 0),
	 rw        => cpu_rw,
    data_in   => cpu_data_out,
	 data_out  => dat_addr(7 downto 0)
	 );
 
my_rom : SYS09BUG_F800 port map (
    clk       => cpu_clk,
    rst       => cpu_rst,
	 cs        => rom_cs,
	 rw        => '1',
    addr      => cpu_addr(10 downto 0),
    data_in   => cpu_data_out,
    data_out  => rom_data_out
    );
 
my_acia  : acia6850 port map (
	 clk	     => cpu_clk,
	 rst       => cpu_rst,
    cs        => acia_cs,
    addr      => cpu_addr(0),
	 rw        => cpu_rw,
	 data_in   => cpu_data_out,
	 data_out  => acia_data_out,
    irq       => acia_irq,
	 RxC       => acia_clk,
	 TxC       => acia_clk,
	 RxD       => acia_rxd,
	 TxD       => acia_txd,
	 DCD_n     => acia_dcd_n,
	 CTS_n     => acia_cts_n,
	 RTS_n     => open
    );
 
 
----------------------------------------
--
-- ACIA Clock
--
----------------------------------------
my_ACIA_Clock : ACIA_Clock
  generic map(
    SYS_CLK_FREQ  => SYS_CLK_FREQ,
	 ACIA_CLK_FREQ => ACIA_CLK_FREQ
  ) 
  port map(
    clk        => sys_clk,
    acia_clk   => acia_clk
  ); 
 
 
----------------------------------------
--
-- PS/2 Keyboard Interface
--
----------------------------------------
my_keyboard : keyboard
   generic map (
	  KBD_CLK_FREQ => CPU_CLK_FREQ
	) 
   port map(
	  clk          => cpu_clk,
	  rst          => cpu_rst,
	  cs           => kbd_cs,
	  addr         => cpu_addr(0),
	  rw           => cpu_rw,
	  data_in      => cpu_data_out(7 downto 0),
	  data_out     => kbd_data_out(7 downto 0),
	  irq          => kbd_irq,
	  kbd_clk      => ps2c,
	  kbd_data     => ps2d
	);
 
----------------------------------------
--
-- Video Display Unit instantiation
--
----------------------------------------
my_vdu : vdu8 
  generic map(
    VGA_CLK_FREQ           => VGA_CLK_FREQ, -- 25MHZ
    VGA_HOR_CHARS          => 80, -- CHARACTERS 25.6us
    VGA_HOR_CHAR_PIXELS    => 8,  -- PIXELS 0.32us
    VGA_HOR_FRONT_PORCH    => 16, -- PIXELS 0.64us
    VGA_HOR_SYNC           => 96, -- PIXELS 3.84us
    VGA_HOR_BACK_PORCH     => 48, -- PIXELS 1.92us
    VGA_VER_CHARS          => 25, -- CHARACTERS 12.8ms
    VGA_VER_CHAR_LINES     => 16, -- LINES 0.512ms
    VGA_VER_FRONT_PORCH    => 10, -- LINES 0.320ms
    VGA_VER_SYNC           => 2,  -- LINES 0.064ms
    VGA_VER_BACK_PORCH     => 34  -- LINES 1.088ms
  )
  port map(
 
    -- Control Registers
    vdu_clk       => cpu_clk,					 -- 12.5 MHz System Clock in
    vdu_rst       => cpu_rst,
    vdu_cs        => vdu_cs,
    vdu_addr      => cpu_addr(2 downto 0),
    vdu_rw        => cpu_rw,
    vdu_data_in   => cpu_data_out,
    vdu_data_out  => vdu_data_out,
 
    -- vga port connections
    vga_clk       => vga_clk,					 -- 25 MHz VDU pixel clock
    vga_red_o     => red,
    vga_green_o   => green,
    vga_blue_o    => blue,
    vga_hsync_o   => hs,
    vga_vsync_o   => vs
  );
 
 
----------------------------------------
--
-- Seven Segment Display instantiation
--
----------------------------------------
 
my_seg : seven_segment port map (
    clk        => cpu_clk,
	 rst        => cpu_rst,
	 cs         => seg_cs,
	 rw         => cpu_rw,
	 addr       => cpu_addr(1 downto 0),
    data_in    => cpu_data_out,
	 data_out   => seg_data_out,
	 segments   => segments,
	 digits     => digits
	 );
 
--vga_clk_buffer : BUFG port map(
--    i => clk_count,
--	 o => vga_clk
--    );
 
vga_clk <= cpu_clk;
 
cpu_clk_buffer : BUFG port map(
    i => clk_count,
	 o => cpu_clk
    );	 
 
--
-- Clock divider
-- Assumes 50 MHz system clock
-- 25MHz pixel clock
-- 25MHz CPU clock
--
sys09_clock : process( sys_clk, clk_count )
begin
	if sys_clk'event and sys_clk='1' then
	   clk_count <= not clk_count;
   end if;
end process;
 
----------------------------------------------------------------------
--
-- Process to decode memory map
--
----------------------------------------------------------------------
 
mem_decode: process( cpu_addr, cpu_rw, cpu_vma,
					      dat_cs, dat_addr,
					      rom_data_out,
						   acia_data_out,
							kbd_data_out,
							vdu_data_out,
							seg_data_out,
							leds_data_out,
							ram_data_out
							)
begin
      cpu_data_in <= (others=>'0');
      dat_cs      <= '0';
      rom_cs      <= '0';
      acia_cs     <= '0';
      kbd_cs      <= '0';
      vdu_cs      <= '0';
      seg_cs      <= '0';
      leds_cs     <= '0';
      ram_cs      <= '0';
--	     timer_cs    <= '0';
--      trap_cs     <= '0';
--	     pb_cs       <= '0';
--      ide_cs      <= '0';
--      ether_cs    <= '0';
--	     slot1_cs    <= '0';
--      slot2_cs    <= '0';
 
      if cpu_addr( 15 downto 8 ) = "11111111" then
	     cpu_data_in <= rom_data_out;
        dat_cs      <= cpu_vma;              -- write DAT
        rom_cs      <= cpu_vma;              -- read  ROM
	   --
		-- Sys09Bug Monitor ROM $F000 - $FFFF
		--
	   elsif dat_addr(3 downto 0) = "1111" then -- $XF000 - $XFFFF
        if cpu_addr(11) = '1' then
          --
		    -- Monitor ROM $F800 - $FFFF
		    --
          cpu_data_in <= rom_data_out;
          rom_cs      <= cpu_vma;          -- read  ROM
        else
          --
		    -- SRAM $F000 - $F7FF
		    --
          cpu_data_in <= ram_data_out;
          ram_cs      <= cpu_vma;
        end if;
      --
		-- IO Devices $E000 - $EFFF
		--
		elsif dat_addr(3 downto 0) = "1110" then -- $XE000 - $XEFFF
			case cpu_addr(11 downto 8) is
			--
			-- SWTPC peripherals from $E000 to $E0FF
			--
			when "0000" =>
		     case cpu_addr(7 downto 4) is
			  --
			  -- ACIA ($E000 - $E00F)
			  --
			  when "0000" =>
		       cpu_data_in <= acia_data_out;
			    acia_cs     <= cpu_vma;
 
	        --
			  -- Reserved - FD1771 FDC ($E010 - $E01F) (SWTPC)
           --
 
			  --
           -- Keyboard port ($E020 - $E02F)
			  --
			  when "0010" =>
             cpu_data_in <= kbd_data_out;
			    kbd_cs <= cpu_vma;
 
           --
           -- VDU port ($E030 - $E03F)
			  --
			  when "0011" =>
             cpu_data_in <= vdu_data_out;
			    vdu_cs      <= cpu_vma;
 
           --
			  -- Reserved - SWTPc MP-T ($E040 - $E04F)
			  --
 
           --
           -- Reserved - Timer ($E050 - $E05F) (B5-X300)
			  --
 
           --
           -- Reserved - Bus Trap Logic ($E060 - $E06F) (B5-X300)
			  --
 
           --
           -- Reserved - I/O port ($E070 - $E07F) (B5-X300)
			  --
 
			  --
			  -- Reserved - PTM 6840 ($E080 - $E08F) (SWTPC)
			  --
 
			  --
			  -- Reserved - PIA Timer ($E090 - $E09F) (SWTPC)
			  --
 
           --
			  -- Read Switched port ($E0A0 - $E0AF)
			  -- Write LEDS
			  --
			  when "1010" =>
             cpu_data_in <= leds_data_out;
			    leds_cs     <= cpu_vma;
 
           --
           -- 7 segment display port ($E0B0 - $E0BF)
			  --
			  when "1011" =>
             cpu_data_in <= seg_data_out;
			    seg_cs      <= cpu_vma;
 
 
			  when others => -- $EXC0 to $EXFF
			    null;
		     end case;
         --
			--	$E100 to $EFFF reserved for future use
			--
        	when others =>
			  null;
         end case;
		--
		-- Everything else is RAM
		--
		else
		  cpu_data_in <= ram_data_out;
		  ram_cs      <= cpu_vma;
    end if;
end process;
 
 
--
-- 1M byte SRAM Control
-- Processes to read and write memory based on bus signals
--
ram_process: process( cpu_rst, sys_clk,
                      cpu_addr, cpu_rw, cpu_vma, cpu_data_out,
					       dat_addr, ram_cs,
                      ram1_ce, ram1_ub, ram1_lb, ram1_data,
                      ram2_ce, ram2_ub, ram2_lb, ram2_data,
							 ram_we, ram_oe )
begin
    --
    -- ram_hold signal helps 
    --
    if( cpu_rst = '1' ) then
	   ram_we   <= '0';
	   ram_oe   <= '0';
    --
	 -- Clock Hold on rising edge
	 --
    elsif( sys_clk'event and sys_clk='1' ) then
	   if (ram_cs = '1') and (ram_we = '0') and (ram_oe = '0') then
	     ram_we   <= not cpu_rw;
	     ram_oe   <=     cpu_rw;
      else
	     ram_we   <= '0';
	     ram_oe   <= '0';
      end if;
    end if;
 
	 ram_wen  <= not ram_we;
	 ram_oen  <= not ram_oe;
 
    ram1_ce   <= ram_cs and (not cpu_addr(1));
    ram1_ub   <= not cpu_addr(0);
    ram1_lb   <= cpu_addr(0);
    ram1_cen  <= not ram1_ce;
    ram1_ubn  <= not ram1_ub;
    ram1_lbn  <= not ram1_lb;
 
    ram2_ce   <= ram_cs and cpu_addr(1);
    ram2_ub   <= not cpu_addr(0);
    ram2_lb   <= cpu_addr(0);
    ram2_cen  <= not ram2_ce;
    ram2_ubn  <= not ram2_ub;
    ram2_lbn  <= not ram2_lb;
 
	 ram_addr(17 downto 10) <= dat_addr(7 downto 0);
	 ram_addr(9 downto 0) <= cpu_addr(11 downto 2);
 
    if ram_we = '1' and ram1_ce = '1' and ram1_lb = '1' then
		ram1_data(7 downto 0) <= cpu_data_out;
	 else
      ram1_data(7 downto 0)  <= "ZZZZZZZZ";
	 end if;
 
    if ram_we = '1' and ram1_ce = '1' and ram1_ub = '1' then
		ram1_data(15 downto 8) <= cpu_data_out;
	 else
      ram1_data(15 downto 8)  <= "ZZZZZZZZ";
	 end if;
 
    if ram_we = '1' and ram2_ce = '1' and ram2_lb = '1' then
		ram2_data(7 downto 0) <= cpu_data_out;
	 else
      ram2_data(7 downto 0)  <= "ZZZZZZZZ";
	 end if;
 
    if ram_we = '1' and ram2_ce = '1' and ram2_ub = '1' then
		ram2_data(15 downto 8) <= cpu_data_out;
	 else
      ram2_data(15 downto 8)  <= "ZZZZZZZZ";
	 end if;
 
	 case cpu_addr(1 downto 0) is
	 when "00" =>
      ram_data_out <= ram1_data(15 downto 8);
	 when "01" =>
      ram_data_out <= ram1_data(7 downto 0);
	 when "10" =>
      ram_data_out <= ram2_data(15 downto 8);
    when others =>
      ram_data_out <= ram2_data(7 downto 0);
    end case;
end process;
 
--
-- LEDS output register
--
leds_output : process( cpu_clk, cpu_rst, switches )
begin
	if cpu_rst = '1' then
		leds <= "00000000";
	elsif cpu_clk'event and cpu_clk='0' then
		if	leds_cs = '1' and cpu_rw = '0' then
			leds <= cpu_data_out;
		end if;
	end if;
	leds_data_out <= switches;
end process;
 
--
-- Interrupts and other bus control signals
--
interrupts : process(	sys_clk, rst_sw, 
								acia_irq, kbd_irq, nmi_sw )
begin
   if sys_clk'event and sys_clk = '1' then
	  cpu_rst  <= rst_sw; -- CPU reset is active high
   end if;
	cpu_firq   <= kbd_irq;
	cpu_nmi    <= nmi_sw;
	cpu_irq    <= acia_irq;
	cpu_halt   <= '0';
	cpu_hold   <= '0';
end process;
 
--
-- ACIA pin assignments
--
acia_assignments : process( rxd, acia_txd )
begin
	acia_dcd_n <= '0';
	acia_cts_n <= '0';
	acia_rxd   <= rxd;
	txd        <= acia_txd;
end process;
 
 
end my_computer; --===================== End of architecture =======================--