Subversion Repositories System09

--===========================================================================----
--
--  S Y N T H E Z I A B L E    System09 - SOC.
--
--  www.OpenCores.Org - September 2003
--  This core adheres to the GNU public license  
--
-- File name      : System09.vhd
--
-- 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           : mon_rom   (sys09bug_rom4k_b16.vhd) Monitor ROM
--                  cpu09     (cpu09.vhd)              CPU core
--                  dat_ram   (datram.vhd)             Dynamic Address Translation
--                  acia_6850 (acia_6850.vhd)          ACIA (UART)
--                            (acia_rx.vhd)
--                            (acia_tx.vhd)
--                  keyboard  (keyboard.vhd)           PS/2 Keyboard
--                            (ps2_keyboard.vhd)
--                            (keymap_rom)
--                  vdu8      (vdu8.vhd)		          Video Display Unit
--                            (char_rom2K_b16.vhd)
--                            (ram2k_b16.vhd)
--                  seven_segment (SevenSegment.vhd)   Seven Segment Display
-- 
-- Author         : John E. Kent      
--                  dilbert57@opencores.org      
--
--===========================================================================----
--
-- 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 SysClk
-- 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
--
-- Vesrion 2.3 - 1 June 2007 - John Kent
-- Updated VDU & ACIA
-- Changed decoding for Sys09Bug
--
--===========================================================================--
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 My_System09 is
  port(
    SysClk      : in  Std_Logic;  -- System Clock input
	 Reset_sw    : in  Std_logic;  -- Master Reset input (active low)
	 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;
 
	 -- Uart 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 My_System09;
 
-------------------------------------------------------------------------------
-- Architecture for System09
-------------------------------------------------------------------------------
architecture my_computer of My_System09 is
  -----------------------------------------------------------------------------
  -- constants
  -----------------------------------------------------------------------------
  constant SYS_Clock_Frequency  : integer := 50000000;  -- FPGA System Clock
  constant VGA_Clock_Frequency  : integer := 25000000;  -- VGA Pixel Clock
  constant CPU_Clock_Frequency  : integer := 25000000;  -- CPU Clock
  constant BAUD_Rate            : integer := 57600;	  -- Baud Rate
  constant ACIA_Clock_Frequency : integer := BAUD_Rate * 16;
 
  type hold_state_type is ( hold_release_state, hold_request_state );
 
  -----------------------------------------------------------------------------
  -- Signals
  -----------------------------------------------------------------------------
  -- BOOT ROM
  signal rom_cs        : Std_logic;
  signal rom_data_out  : Std_Logic_Vector(7 downto 0);
 
  -- FLEX9 RAM
  signal flex_cs       : Std_logic;
  signal flex_data_out : Std_Logic_Vector(7 downto 0);
 
  -- UART Interface signals
  signal acia_clk      : std_logic;
  signal acia_data_out : Std_Logic_Vector(7 downto 0);  
  signal acia_cs       : Std_Logic;
  signal acia_irq      : Std_Logic;
  signal DCD_n         : Std_Logic;
  signal RTS_n         : Std_Logic;
  signal CTS_n         : Std_Logic;
 
  -- keyboard port
  signal keyboard_data_out : std_logic_vector(7 downto 0);
  signal keyboard_cs       : std_logic;
  signal keyboard_irq      : std_logic;
 
  -- LEDs
  signal leds_data_out : std_logic_vector(7 downto 0);
  signal leds_cs       : std_logic;
 
  -- 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;
 
  -- CPU Interface signals
  signal cpu_reset    : Std_Logic;
  signal cpu_clk      : 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);
 
  -- 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);
 
  -- System Clock Prescaler
  signal clk_count    : std_logic;
 
-----------------------------------------------------------------
--
-- CPU09 CPU core
--
-----------------------------------------------------------------
 
component cpu09
  port (    
	 clk      :	in  std_logic;
    rst      : in  std_logic;
    rw       :	out std_logic;		-- Asynchronous memory interface
    vma      :	out std_logic;
    address  : out std_logic_vector(15 downto 0);
    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;
 
 
----------------------------------------
--
-- 4KByte Block RAM Monitor ROM
--
----------------------------------------
component mon_rom
  Port (
    clk      : in  std_logic;
    rst      : in  std_logic;
    cs       : in  std_logic;
    rw       : in  std_logic;
    addr     : in  std_logic_vector (11 downto 0);
    rdata    : out std_logic_vector (7 downto 0);
    wdata    : in  std_logic_vector (7 downto 0)
    );
end component;
 
----------------------------------------
--
-- 8KBytes Block RAM for FLEX9
-- $C000 - $DFFF
--
----------------------------------------
component flex_ram
  Port (
    clk      : in  std_logic;
    rst      : in  std_logic;
    cs       : in  std_logic;
    rw       : in  std_logic;
    addr     : in  std_logic_vector (12 downto 0);
    rdata    : out std_logic_vector (7 downto 0);
    wdata    : in  std_logic_vector (7 downto 0)
    );
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;
 
-----------------------------------------------------------------
--
-- Open Cores Mini UART
--
-----------------------------------------------------------------
 
component ACIA_6850
  port (
    clk      : in  Std_Logic;  -- System Clock
    rst      : in  Std_Logic;  -- Reset input (active high)
    cs       : in  Std_Logic;  -- miniUART Chip Select
    rw       : in  Std_Logic;  -- Read / Not Write
    irq      : out Std_Logic;  -- Interrupt
    Addr     : in  Std_Logic;  -- Register Select
    DataIn   : in  Std_Logic_Vector(7 downto 0); -- Data Bus In 
    DataOut  : out Std_Logic_Vector(7 downto 0); -- Data Bus Out
    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_Clock_Frequency  : integer :=  SYS_Clock_Frequency;
	  ACIA_Clock_Frequency : integer := ACIA_Clock_Frequency
  );   
  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_Clock_Frequency : integer := CPU_Clock_Frequency
  );
  port(
  clk             : in    std_logic;
  rst             : in    std_logic;
  cs              : in    std_logic;
  rw              : in    std_logic;
  addr            : 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(
        VDU_CLOCK_FREQUENCY    : integer := CPU_Clock_Frequency; -- HZ
        VGA_CLOCK_FREQUENCY    : integer := VGA_Clock_Frequency; -- HZ
	     VGA_HOR_CHARS          : integer := 80; -- CHARACTERS
	     VGA_VER_CHARS          : integer := 25; -- CHARACTERS
	     VGA_PIXELS_PER_CHAR    : integer := 8;  -- PIXELS
	     VGA_LINES_PER_CHAR     : integer := 16; -- LINES
	     VGA_HOR_BACK_PORCH     : integer := 40; -- PIXELS
	     VGA_HOR_SYNC           : integer := 96; -- PIXELS
	     VGA_HOR_FRONT_PORCH    : integer := 24; -- PIXELS
	     VGA_VER_BACK_PORCH     : integer := 13; -- LINES
	     VGA_VER_SYNC           : integer := 1;  -- LINES
	     VGA_VER_FRONT_PORCH    : integer := 36  -- LINES
      );
      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;
     rw          : in  std_logic;
     addr        : in  std_logic_vector(1 downto 0);
     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_reset,
    rw	     => cpu_rw,
    vma       => cpu_vma,
    address   => cpu_addr(15 downto 0),
    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_rom : mon_rom port map (
    clk       => cpu_clk,
    rst       => cpu_reset,
	 cs        => rom_cs,
	 rw        => '1',
    addr      => cpu_addr(11 downto 0),
    rdata     => rom_data_out,
    wdata     => cpu_data_out
    );
 
my_flex : flex_ram port map (
    clk       => cpu_clk,
    rst       => cpu_reset,
	 cs        => flex_cs,
	 rw        => cpu_rw,
    addr      => cpu_addr(12 downto 0),
    rdata     => flex_data_out,
    wdata     => cpu_data_out
    );
 
my_dat : dat_ram port map (
    clk       => cpu_clk,
	 rst       => cpu_reset,
	 cs        => dat_cs,
	 rw        => cpu_rw,
	 addr_hi   => cpu_addr(15 downto 12),
	 addr_lo   => cpu_addr(3 downto 0),
    data_in   => cpu_data_out,
	 data_out  => dat_addr(7 downto 0)
	 );
 
my_acia  : ACIA_6850 port map (
	 clk	     => cpu_clk,
	 rst       => cpu_reset,
    cs        => acia_cs,
	 rw        => cpu_rw,
    irq       => acia_irq,
    Addr      => cpu_addr(0),
	 Datain    => cpu_data_out,
	 DataOut   => acia_data_out,
	 RxC       => acia_clk,
	 TxC       => acia_clk,
	 RxD       => rxd,
	 TxD       => txd,
	 DCD_n     => dcd_n,
	 CTS_n     => cts_n,
	 RTS_n     => rts_n
	 );
 
 
----------------------------------------
--
-- ACIA Clock
--
----------------------------------------
my_ACIA_Clock : ACIA_Clock
  generic map(
    SYS_Clock_Frequency  => SYS_Clock_Frequency,
	 ACIA_Clock_Frequency => ACIA_Clock_Frequency
  ) 
  port map(
    clk        => SysClk,
    acia_clk   => acia_clk
  ); 
 
 
----------------------------------------
--
-- PS/2 Keyboard Interface
--
----------------------------------------
my_keyboard : keyboard
   generic map (
	KBD_Clock_Frequency => CPU_Clock_frequency
	) 
   port map(
	clk          => cpu_clk,
	rst          => cpu_reset,
	cs           => keyboard_cs,
	rw           => cpu_rw,
	addr         => cpu_addr(0),
	data_in      => cpu_data_out(7 downto 0),
	data_out     => keyboard_data_out(7 downto 0),
	irq          => keyboard_irq,
	kbd_clk      => ps2c,
	kbd_data     => ps2d
	);
 
----------------------------------------
--
-- Video Display Unit instantiation
--
----------------------------------------
my_vdu : vdu8 
  generic map(
      VDU_CLOCK_FREQUENCY    => CPU_Clock_Frequency, -- HZ
      VGA_CLOCK_FREQUENCY    => VGA_Clock_Frequency, -- HZ
	   VGA_HOR_CHARS          => 80, -- CHARACTERS
	   VGA_VER_CHARS          => 25, -- CHARACTERS
	   VGA_PIXELS_PER_CHAR    => 8,  -- PIXELS
	   VGA_LINES_PER_CHAR     => 16, -- LINES
	   VGA_HOR_BACK_PORCH     => 40, -- PIXELS
	   VGA_HOR_SYNC           => 96, -- PIXELS
	   VGA_HOR_FRONT_PORCH    => 24, -- PIXELS
	   VGA_VER_BACK_PORCH     => 13, -- LINES
	   VGA_VER_SYNC           => 1,  -- LINES
	   VGA_VER_FRONT_PORCH    => 36  -- LINES
  )
  port map(
 
		-- Control Registers
		vdu_clk       => cpu_clk,					 -- 12.5 MHz System Clock in
      vdu_rst       => cpu_reset,
		vdu_cs        => vdu_cs,
		vdu_rw        => cpu_rw,
		vdu_addr      => cpu_addr(2 downto 0),
		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_reset,
	 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
    );
 
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( SysClk, clk_count )
begin
	if SysClk'event and SysClk='0' then
	   clk_count <= not clk_count;
   end if;
end process;
 
----------------------------------------------------------------------
--
-- Process to decode memory map
--
----------------------------------------------------------------------
 
mem_decode: process( cpu_clk, Reset_sw,
                     cpu_addr, cpu_rw, cpu_vma,
					      dat_cs, dat_addr,
					      rom_data_out,
						   acia_data_out,
							keyboard_data_out,
							vdu_data_out,
							seg_data_out,
							leds_data_out,
							flex_data_out,
							ram_data_out
							)
begin
    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
        acia_cs     <= '0';
        keyboard_cs <= '0';
        vdu_cs      <= '0';
        seg_cs      <= '0';
        leds_cs     <= '0';
        flex_cs     <= '0';
        ram_cs      <= '0';
	 else
      case dat_addr(3 downto 0) is
	   --
		-- Monitor ROM $F000 - $FFFF
		--
		when "1111" => -- $XF000 - $XFFFF
        cpu_data_in <= rom_data_out;
        dat_cs      <= '0';              -- write DAT
        rom_cs      <= cpu_vma;          -- read  ROM
        acia_cs     <= '0';
        keyboard_cs <= '0';
        vdu_cs      <= '0';
        seg_cs      <= '0';
        leds_cs     <= '0';
        flex_cs     <= '0';
        ram_cs      <= '0';
 
      --
		-- IO Devices $E000 - $EFFF
		--
		when "1110" => -- $XE000 - $XEFFF
		   dat_cs      <= '0';
			rom_cs      <= '0';
		   case cpu_addr(7 downto 4) is
			--
			-- UART / ACIA ($E000 - $E00F)
			--
			when "0000" =>
		     cpu_data_in <= acia_data_out;
			  acia_cs     <= cpu_vma;
			  keyboard_cs <= '0';
			  vdu_cs      <= '0';
			  seg_cs      <= '0';
			  leds_cs     <= '0';
 
			--
			-- Reserved - FD1771 FDC ($E010 - $E01F) (SWTPC)
         --
 
			--
         -- Keyboard port ($E020 - $E02F)
			--
			when "0010" =>
           cpu_data_in <= keyboard_data_out;
			  acia_cs     <= '0';
			  keyboard_cs <= cpu_vma;
			  vdu_cs      <= '0';
			  seg_cs      <= '0';
			  leds_cs     <= '0';
 
         --
         -- VDU port ($E030 - $E03F)
			--
			when "0011" =>
           cpu_data_in <= vdu_data_out;
			  acia_cs     <= '0';
			  keyboard_cs <= '0';
			  vdu_cs      <= cpu_vma;
			  seg_cs      <= '0';
			  leds_cs     <= '0';
 
         --
			-- Reserved - Compact Flash ($E040 - $E04F) (B5-X300)
			--
 
         --
         -- 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;
			  acia_cs     <= '0';
			  keyboard_cs <= '0';
			  vdu_cs      <= '0';
			  seg_cs      <= '0';
			  leds_cs     <= cpu_vma;
 
         --
         -- 7 segment display port ($E0B0 - $E0BF)
			--
			when "1011" =>
           cpu_data_in <= seg_data_out;
			  acia_cs     <= '0';
			  keyboard_cs <= '0';
			  vdu_cs      <= '0';
			  seg_cs      <= cpu_vma;
			  leds_cs     <= '0';
 
 
			when others => -- $EXC0 to $EXFF
           cpu_data_in <= "00000000";
			  acia_cs     <= '0';
			  keyboard_cs <= '0';
			  vdu_cs      <= '0';
			  seg_cs      <= '0';
			  leds_cs     <= '0';
		   end case;
         flex_cs     <= '0';
			ram_cs      <= '0';
	   --
		-- FLEX RAM $0C000 - $0DFFF
		--
		when "1100" | "1101" => -- $0C000 - $0DFFF
		  if dat_addr(7 downto 4) = "0000" then
          cpu_data_in <= flex_data_out;
          flex_cs     <= cpu_vma;
          ram_cs      <= '0';
		  else
          cpu_data_in <= ram_data_out;
          flex_cs     <= '0';
          ram_cs      <= cpu_vma;
		  end if;
        dat_cs      <= '0';
        rom_cs      <= '0';
        acia_cs     <= '0';
        keyboard_cs <= '0';
        vdu_cs      <= '0';
        seg_cs      <= '0';
        leds_cs     <= '0';
		--
		-- Everything else is RAM
		--
		when others =>
		  cpu_data_in <= ram_data_out;
		  rom_cs      <= '0';
		  ram_cs      <= cpu_vma;
		  dat_cs      <= '0';
		  acia_cs     <= '0';
		  keyboard_cs <= '0';
		  vdu_cs      <= '0';
		  seg_cs      <= '0';
        leds_cs     <= '0';
        flex_cs     <= '0';
      end case;
    end if;
end process;
 
 
--
-- 1M byte SRAM Control
-- Processes to read and write memory based on bus signals
--
ram_process: process( cpu_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_we   <= (not cpu_rw) and cpu_clk;
	 ram_oe   <= cpu_rw and cpu_clk;
	 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_reset, switches )
begin
	if cpu_reset = '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(	Reset_sw, 
								acia_irq,
								keyboard_irq,
								nmi_sw
							 )
begin
	cpu_reset <= Reset_sw; -- CPU reset is active high
	cpu_firq  <= keyboard_irq;
	cpu_nmi   <= nmi_sw;
	cpu_irq   <= acia_irq;
	cpu_halt  <= '0';
	cpu_hold  <= '0';
 
	DCD_n     <= '0';
	CTS_n     <= '0';
end process;
 
 
end my_computer; --===================== End of architecture =======================--