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--===========================================================================--
--                                                                           --
--  vdu8_new.vhd - Synthesizable Colour Video Display Unit for System09      --
--                                                                           --
--===========================================================================--
--
--  File name      : vdu8_new.vhd
--
--  Purpose        : Implements a text based Colour Video Display Unit for System09
--                   Supports 2KByte Text buffer and 2KByte Attribute memory
--                   Displays 80 characters across by 25 character rows
--                   Characters are 8 pixels across x 16 lines down.
--                   Character attribute bita for foreground and backgrond colour
--                   1 bit for each Blue Green and Red signal
--                   Supports 2 x 8 chunky graphics character mode.
--                   Uses Generic arguments for setting the video synchronization timing.
--                   (I'm not sure what is "new" about this version.)
--                  
--  Dependencies   : ieee.Std_Logic_1164
--                   ieee.std_logic_arith
--                   ieee.std_logic_unsigned
--                   ieee.numeric_std
--                   unisim.vcomponents
--
--  Uses           : ram_2k (ram2k_b16.vhd)             2KByte Character & Attribute buffer
--                   char_rom (char_rom2k_b16.vhd)      2KByte Character Generator ROM 
--
--  Author         : John E. Kent
--
--  Email          : dilbert57@opencores.org      
--
--  Web            : http://opencores.org/project,system09
--
--  Description    : Display Timing:
--                       800 pixels / line
--                       446 lines / frame
--                       None interlaced
--                       25MHz pixel clock implies 
--                       31.25 KHz line rate
--                       70.067 Hz frame rate   
--                       Timing settable by generics.
--
--                   Display Size:
--                       80 characters across
--                       25 characters down.
--
--                   Character Size:
--                        8 horizontal pixels across
--                       16 vertical scan lines down (2 scan lines/row)
--
--                   Registers:
--                   Base + 0 ASCII character register
--                            Writing to this register writes an 8 bit byte 
--                            into the text buffer at the specified cursor position
--                            Text Mode: ASCII Character (0 to 127)
--                            Chunky Graphics Mode: B0 B1 (0 to 255)
--                                                  B2 B3
--                                                  B4 B5
--                                                  B6 B7
--                   Base + 1 Attibute bit (0 to 255)
--                            Writing to the register writes an 8 bit byte 
--                            into the attribute buffer at the specified cursor position
--                            B7 - 0 => Text Mode / 1 => Chunky Graphics Mode
--                            B6 - 1 => Character Background Blue
--                            B5 - 1 => Character Background Green
--                            B4 - 1 => Character Background Red
--                            B3 - 1 => Character Background & Foreground Alternates
--                            B2 - 1 => Character Foreground Blue
--                            B1 - 1 => Character Foreground Green
--                            B0 - 1 => Character Foreground Red
--                   Base + 2 Cursor Horizontal Position (0 to 79)
--                   Base + 3 Cusror Vertical Position (0 to 24)
--                   Base + 4 Vertical Scroll Offset (0 to 24)
--                            Scrolls the display up by the specified number of character rows
--
--
--  Video Timing :
--
--  Horizontal 800 Pixels/ 25MHz Pixel Clock = 32usec Line period = 31.25 KHz Line Frequency
--  /--------------------------\_____________/---------------\______________/
--      640 Pixels Display       16 Pixel FP    96 Pixel HS     48 Pixel BP
--    
--      VGA_CLK_FREQ           : integer := 25000000; -- HZ
--	     VGA_HOR_FRONT_PORCH    : integer := 16; -- PIXELS 0.64us (0.94us)
--	     VGA_HOR_SYNC           : integer := 96; -- PIXELS 3.84us (3.77us)
--	     VGA_HOR_BACK_PORCH     : integer := 48; -- PIXELS 1.92us (1.89us)
--	     VGA_PIX_PER_CHAR       : integer := 8;  -- PIXELS 0.32us
--	     VGA_HOR_CHARS          : integer := 80; -- CHARACTERS 25.6us
--
--  Vertical 446 Lines * 32 usec Line rate = 14.272ms Frame Period = 70.07Hz Frame frequency  
--  /---------------------------\____________/---------------\______________/
--      400 Line Display          10 Line FP     2 Line VS      34 Line BP
--
--	     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
--	     VGA_LIN_PER_CHAR       : integer := 16; -- LINES 0.512ms
--	     VGA_VER_CHARS          : integer := 25; -- CHARACTERS 12.8ms
--
--  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 Author      Date        Changes
--
-- 0.1     John Kent   2004-09-03  Initial release
--
-- 0.2     Bert Cuzeau 2007-01-16  Modified by for compliance and code cleanliness
--                                 The effort is not over.
--                                 There are still signal initialized, which is BAD.
--
-- 0.3     John Kent   2007-02-07  Added generics for VGA Timing
--
-- 0.4     John Kent   2010-06-16  Added GPL notice. Updated description
--                                 I'm not sure what is "new" about this version
--
 
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;
library unisim;
   use unisim.vcomponents.all;
 
Entity vdu8 is
  generic(
        VGA_CLK_FREQ           : integer := 25000000; -- 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;       -- 12.5/25 MHz CPU Clock
    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;       -- 25MHz clock
    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 vdu8;
 
Architecture RTL of vdu8 is
  --
  -- Synchronisation constants
  --
  -- Displayed Characters per row
  constant HOR_DISP_CHR : integer := VGA_HOR_CHARS;
  -- Last horizontal pixel displayed
  constant HOR_DISP_END : integer := HOR_DISP_CHR * VGA_HOR_CHAR_PIXELS - 1;
  -- Start of horizontal synch pulse
  constant HOR_SYNC_BEG : integer := HOR_DISP_END + VGA_HOR_FRONT_PORCH;
  -- End of Horizontal Synch pulse
  constant HOR_SYNC_END : integer := HOR_SYNC_BEG + VGA_HOR_SYNC;
  -- Last pixel in scan line
  constant HOR_SCAN_END : integer := HOR_SYNC_END + VGA_HOR_BACK_PORCH;
 
  -- Displayed Characters per Column
  constant VER_DISP_CHR   : integer := VGA_VER_CHARS;
  -- last row displayed
  constant VER_DISP_END   : integer := VER_DISP_CHR * VGA_VER_CHAR_LINES - 1;
  -- start of vertical synch pulse
  constant VER_SYNC_BEG   : integer := VER_DISP_END + VGA_VER_FRONT_PORCH;
  -- end of vertical synch pulse
  constant VER_SYNC_END   : integer := VER_SYNC_BEG + VGA_VER_SYNC;
  -- Last scan row in the frame
  constant VER_SCAN_END   : integer := VER_SYNC_END + VGA_VER_BACK_PORCH;
 
  constant BLINK_PERIOD   : integer := 500; -- Blink Rate in msec
  constant BLINK_RATE     : integer := BLINK_PERIOD * (VGA_CLOCK_FREQUENCY/1000);
 
  signal vga_rst       : std_logic;
  signal horiz_sync    : std_logic := '0';
  signal vert_sync     : std_logic := '0';
  signal cursor_on_v   : std_logic := '0';
  signal cursor_on_h   : std_logic := '0';
  signal video_on_v    : std_logic := '0';
  signal video_on_h    : std_logic := '0';
  signal h_count       : natural range 0 to HOR_SCAN_END := 0;
  signal v_count       : natural range 0 to VER_SCAN_END	:= 0;
  signal p_count       : natural range 0 to VGA_PIXELS_PER_CHAR-1 := 0;
  signal l_count       : std_logic_vector(3 downto 0) := "0000";
  signal c_count       : std_logic_vector(6 downto 0) := "0000000";
  signal r_count       : std_logic_vector(5 downto 0) := "000000";
  signal blink_count   : natural range 0 to BLINK_RATE := 0;
  --
  -- Character generator ROM
  --
  signal char_addr     : std_logic_vector(10 downto 0);
  signal char_data_out : std_logic_vector(7 downto 0);
 
  --
  -- Control Registers
  --
  signal reg_character : std_logic_vector(7 downto 0);
  signal reg_colour    : std_logic_vector(7 downto 0);
  signal reg_hcursor   : std_logic_vector(6 downto 0);   -- 80 columns
  signal reg_vcursor   : std_logic_vector(4 downto 0);   -- 25 rows
  signal reg_voffset   : std_logic_vector(4 downto 0);   -- 25 rows
  --
  -- Video Shift register
  --
  signal vga_shift     : std_logic_vector(7 downto 0);
  signal vga_fg_colour : std_logic_vector(2 downto 0);
  signal vga_bg_colour : std_logic_vector(2 downto 0);
  signal cursor_on     : std_logic := '0';
  signal cursor_on1    : std_logic := '0';
  signal video_on      : std_logic := '0';
  signal video_on1     : std_logic := '0';
  signal video_on2     : std_logic := '0';
  --
  -- vga character ram access bus
  --
  signal col_addr      : std_logic_vector(6 downto 0) := (others=>'0'); -- 0 to 79
  signal row_addr      : std_logic_vector(5 downto 0) := (others=>'0'); -- 0 to 49 (25 * 2 -1)
  signal col1_addr     : std_logic_vector(6 downto 0) := (others=>'0'); -- 0 to 79
  signal row1_addr     : std_logic_vector(5 downto 0) := (others=>'0'); -- 0 to 49 (25 * 2 - 1)
  signal hor_addr      : std_logic_vector(6 downto 0) := (others=>'0'); -- 0 to 79
  signal ver_addr      : std_logic_vector(6 downto 0) := (others=>'0'); -- 0 to 124
  signal vga0_cs       : std_logic;
  signal vga0_rw       : std_logic;
  signal vga1_cs       : std_logic;
  signal vga1_rw       : std_logic;
  signal vga2_cs       : std_logic;
  signal vga2_rw       : std_logic;
  signal vga_cs        : std_logic;
  signal vga_rw        : std_logic;
  signal vga_addr      : std_logic_vector(10 downto 0) := (others=>'0');  -- 2K byte character buffer
  signal vga_data_out  : std_logic_vector(7 downto 0);
  signal attr_data_out : std_logic_vector(7 downto 0);
 
  --
  -- Character write handshake signals
  --
  signal req_write     : std_logic;     -- request character write
  signal ack_write     : std_logic;
 
  --
  -- Block Ram Character gen
  --
  component char_rom
    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);
      wdata : in std_logic_vector (7 downto 0);
      rdata : out std_logic_vector (7 downto 0)
      );
  end component;
 
  component ram_2k
    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);
      wdata : in  std_logic_vector (7 downto 0);
      rdata : out std_logic_vector (7 downto 0)
      );
  end component;
 
begin
 
--
-- instantiate Character generator ROM
--
vdu_char_rom : char_rom port map(
    clk   => vga_clk,
	 rst   => vga_rst,
	 cs    => '1',
	 rw    => '1',
    addr  => char_addr,
    wdata => "00000000",
    rdata => char_data_out
    );
 
--
-- Character buffer RAM
--
char_buff_ram : ram_2k port map(
    clk   => vga_clk,
    rst   => vga_rst,
    cs    => vga_cs,
    rw    => vga_rw,
    addr  => vga_addr,
    wdata => reg_character,
    rdata => vga_data_out
    );
 
--
-- Attribute buffer RAM
--
  attr_buff_ram : ram_2k port map(
    clk   => vga_clk,
    rst   => vga_rst,
    cs    => vga_cs,
    rw    => vga_rw,
    addr  => vga_addr,
    wdata => reg_colour,
    rdata => attr_data_out
    );
 
--
-- CPU Write interface
--
  vga_cpu_write : process(vdu_clk, vdu_rst)
  begin
    if vdu_rst = '1' then
      reg_character <= "00000000";
      reg_colour    <= "00000111";
      reg_hcursor   <= "0000000";
      reg_vcursor   <= "00000";
      reg_voffset   <= "00000";
      req_write     <= '0';
 
    elsif falling_edge(vdu_clk) then
      if (vdu_cs = '1') and (vdu_rw = '0') then
        case vdu_addr is
          when "000" =>
            reg_character <= vdu_data_in;
            req_write     <= '1';
          when "001" =>
            reg_colour    <= vdu_data_in;
          when "010" =>
            reg_hcursor   <= vdu_data_in(6 downto 0);
          when "011" =>
            reg_vcursor   <= vdu_data_in(4 downto 0);
          when others =>
            reg_voffset   <= vdu_data_in(4 downto 0);
        end case;
      else
 
        if (req_write = '1') and (ack_write = '1') then
          req_write <= '0';
        else
          req_write <= req_write;
        end if;
 
      end if;
    end if;
  end process;
  --
  -- CPU Read interface
  --
  vga_cpu_read : process(vdu_addr, vdu_cs,
                         reg_character, reg_colour,
                         reg_hcursor, reg_vcursor, reg_voffset)
  begin
    case vdu_addr is
      when "000" =>
        vdu_data_out <= reg_character;
      when "001" =>
        vdu_data_out <= reg_colour;
      when "010" =>
        vdu_data_out <= "0" & reg_hcursor;
      when "011" =>
        vdu_data_out <= "000" & reg_vcursor;
      when others =>
        vdu_data_out <= "000" & reg_voffset;
    end case;
  end process;
 
  --
  -- Video memory access
  --
  vga_addr_proc : process(vga_clk, vga_rst)
  begin
 
    if vga_rst = '1' then
      vga0_cs   <= '0';
      vga0_rw   <= '1';
      row_addr  <= "000000";
      col_addr  <= "0000000";
      --
      vga1_cs   <= '0';
      vga1_rw   <= '1';
      row1_addr <= "000000";
      col1_addr <= "0000000";
      --
      vga2_cs   <= '0';
      vga2_rw   <= '1';
      ver_addr  <= "0000000";
      hor_addr  <= "0000000";
      --
      vga_cs    <= '0';
      vga_rw    <= '1';
      vga_addr  <= "00000000000";
 
    elsif falling_edge(vga_clk) then
      --
      -- on h_count = 0 initiate character write.
      -- all other cycles are reads.
      --
      case p_count is
        when 0 =>                   -- pipeline character write
          vga0_cs  <= req_write;
          vga0_rw  <= '0';
          col_addr <= reg_hcursor(6 downto 0);
          row_addr <= ("0" & reg_vcursor(4 downto 0)) + ("0" & reg_voffset(4 downto 0));
        when others =>                  -- other 7 cycles free
          vga0_cs  <= '1';
          vga0_rw  <= '1';
          col_addr <= c_count;
          row_addr <= r_count + ("0" & reg_voffset(4 downto 0));
      end case;
 
      --
      -- on vga_clk + 1 round off row address
      --
      vga1_cs <= vga0_cs;
      vga1_rw <= vga0_rw;
      if row_addr < VER_DISP_CHR then
        row1_addr <= row_addr;
      else
        row1_addr <= row_addr - VER_DISP_CHR;
      end if;
      col1_addr <= col_addr;
      --
      -- on vga_clk + 2 calculate vertical address
      --
      vga2_cs   <= vga1_cs;
      vga2_rw   <= vga1_rw;
      ver_addr  <= ("00" & row1_addr(4 downto 0)) + (row1_addr(4 downto 0) & "00");
      hor_addr  <= col1_addr;
      --
      -- on vga_clk + 3 calculate memory address
      --
      vga_cs    <= vga2_cs;
      vga_rw    <= vga2_rw;
      vga_addr  <= ("0000" & hor_addr) + (ver_addr & "0000");
    end if;
  end process;
--
-- Video shift register
--
  vga_shift_proc : process( vga_clk, vga_rst)
  begin
    if vga_rst = '1' then
      ack_write     <= '0';
      video_on2     <= '0';
      video_on      <= '0';
      cursor_on     <= '0';
      vga_bg_colour <= "000";
      vga_fg_colour <= "111";
      vga_shift     <= "00000000";
      vga_red_o     <= '0';
      vga_green_o   <= '0';
      vga_blue_o    <= '0';
      -- Put all video signals through DFFs to elminate any delays that cause a blurry image
 
    elsif falling_edge(vga_clk) then
	   --
      -- p_count = 0 load pixel shift register
		--
      if p_count = 0 then
        if (req_write = '1') and (ack_write = '0') then
          ack_write <= '1';
        elsif (req_write = '0') and (ack_write = '1') then
          ack_write <= '0';
        else
          ack_write <= ack_write;
        end if;
 
		  --
		  -- Pipeline video enable
		  --
        video_on2     <= video_on1;
        video_on      <= video_on2;
		  --
		  -- Blink Cursor
		  --
		  if blink_count > (BLINK_RATE/2) then
           cursor_on  <= cursor_on1 or attr_data_out(3);
        else
		     cursor_on  <= '0';
        end if;
		  --
		  -- Set forground and background colours
		  --
        vga_fg_colour <= attr_data_out(2 downto 0);
        vga_bg_colour <= attr_data_out(6 downto 4);
		  --
		  -- Attribute bit 7
		  --   0 => text
		  --   1 => chunky graphics
		  --
        if attr_data_out(7) = '0' then
          vga_shift <= char_data_out;
        else
          case l_count(3 downto 2) is
            when "00" =>
              vga_shift(7 downto 4) <= vga_data_out(0) & vga_data_out(0) & vga_data_out(0) & vga_data_out(0);
              vga_shift(3 downto 0) <= vga_data_out(1) & vga_data_out(1) & vga_data_out(1) & vga_data_out(1);
            when "01" =>
              vga_shift(7 downto 4) <= vga_data_out(2) & vga_data_out(2) & vga_data_out(2) & vga_data_out(2);
              vga_shift(3 downto 0) <= vga_data_out(3) & vga_data_out(3) & vga_data_out(3) & vga_data_out(3);
            when "10" =>
              vga_shift(7 downto 4) <= vga_data_out(4) & vga_data_out(4) & vga_data_out(4) & vga_data_out(4);
              vga_shift(3 downto 0) <= vga_data_out(5) & vga_data_out(5) & vga_data_out(5) & vga_data_out(5);
            when others =>
              vga_shift(7 downto 4) <= vga_data_out(6) & vga_data_out(6) & vga_data_out(6) & vga_data_out(6);
              vga_shift(3 downto 0) <= vga_data_out(7) & vga_data_out(7) & vga_data_out(7) & vga_data_out(7);
          end case;
        end if;
      else
		  --
		  -- p_count /= 0 shift out pixel shift register
		  --
        vga_shift <= vga_shift(6 downto 0) & '0';
      end if;
 
      --
      -- Colour mask is
      --  7  6  5  4  3  2  1  0
      --  X BG BB BR  X FG FB FR
      --
      if vga_shift(7) = (not cursor_on) then
        vga_red_o   <= video_on and vga_fg_colour(0);
        vga_green_o <= video_on and vga_fg_colour(1);
        vga_blue_o  <= video_on and vga_fg_colour(2);
      else
        vga_red_o   <= video_on and vga_bg_colour(0);
        vga_green_o <= video_on and vga_bg_colour(1);
        vga_blue_o  <= video_on and vga_bg_colour(2);
      end if;
    end if;
  end process;
 
 
--
-- Sync generator & timing process
-- Generate Horizontal and Vertical Timing Signals for Video Signal
--
  vga_sync : process(vga_clk, vga_rst)
  begin
    if vga_rst = '1' then
        h_count     <= 0;
		  c_count     <= "0000000";
		  p_count     <= 0;
        v_count     <= 0;
		  l_count     <= "0000";
		  r_count     <= "000000";
        horiz_sync  <= '0';
        vert_sync   <= '0';
        video_on_h  <= '0';
        video_on_v  <= '0';
        cursor_on_h <= '0';
        cursor_on_v <= '0';
		  blink_count <= BLINK_RATE;
    elsif falling_edge(vga_clk) then
      --
      -- H_count counts pixels (640 + extra time for sync signals)
      --
      --  Horiz_sync  -----------------------------__________--------
      --  H_count       0                640      659       755    799
      --
      if h_count = HOR_SCAN_END then
        h_count <= 0;
		  c_count <= "0000000";
		  p_count <= 0;
      else
        h_count <= h_count + 1;
		  p_count <= p_count + 1;
		  if p_count = VGA_PIXELS_PER_CHAR-1 then
		    p_count <= 0;
		    c_count <= c_count + "0000001";
		  end if; 
      end if;
 
      --
      -- V_count counts rows of pixels
      -- 400 lines + extra time for sync signals
      -- 25 rows * 16 scan lines
      --
      --  Vert_sync      ---------------------------------_______------------
      --  V_count         0                       400    413     414        444
      --
      if (v_count = VER_SCAN_END) and (h_count = HOR_SCAN_END) then
        v_count <= 0;
		  l_count <= "0000";
		  r_count <= "000000";
      elsif h_count = HOR_SYNC_END then
        v_count <= v_count + 1;
        l_count <= l_count + "0001";
		  if l_count = VGA_LINES_PER_CHAR-1 then
		    l_count <= "0000";
		    r_count <= r_count + "000001";
		  end if; 
      end if;
 
      --
      -- Generate Horizontal Sync Signal
      --
      if h_count = HOR_SYNC_BEG then
        horiz_sync <= '1';
      elsif h_count = HOR_SYNC_END then
        horiz_sync <= '0';
      end if;
 
      --
      -- Generate Vertical Sync Signal
      --
      if v_count = VER_SYNC_BEG then
        vert_sync <= '1';
      elsif v_count = VER_SYNC_END then
        vert_sync <= '0';
      end if;
 
		--
      -- Horizontal display enable
		--
      if h_count = 0 then
        video_on_h <= '1';
      elsif h_count = HOR_DISP_END then
        video_on_h <= '0';
      end if;
 
		--
      -- Verical display enable
		--
      if v_count = 0 then
        video_on_v <= '1';
      elsif v_count = VER_DISP_END then
        video_on_v <= '0';
      end if;
 
		--
		-- Horizontal cursor on
		--
      if c_count = reg_hcursor(6 downto 0) then
        cursor_on_h <= '1';
      else
        cursor_on_h <= '0';
      end if;
 
		--
		-- Vertical cursor on
		--
      if r_count = ("0" & reg_vcursor(4 downto 0)) then
        cursor_on_v <= '1';
      else
        cursor_on_v <= '0';
      end if;
 
      -- cursor_on is only active when on selected character
		if blink_count = 0 then
		  blink_count <= BLINK_RATE;
		else
        blink_count <= blink_count - 1;
      end if;
    end if;
 
  end process;
  --
  -- VGA reset = VDU reset
  -- VGA = timing section
  -- VDU = CPU registers
  --
  vga_rst <= vdu_rst;
  --
  -- VGA Sync for 640 pixels x 400 lines
  -- negative horizonal sync
  -- positive verical sync
  --
  vga_hsync_o <= not horiz_sync;
  vga_vsync_o <= vert_sync;
  --
  -- video_on is high only when RGB data is displayed
  --
  video_on1   <= video_on_H and video_on_V;
  cursor_on1  <= cursor_on_h and cursor_on_v;
 
--
-- Here to look up character ROM
-- This will take one clock cycle
-- and should be performed on p_count = "111"
--
  char_addr(10 downto 4) <= vga_data_out(6 downto 0);
  char_addr(3 downto 0)  <= l_count;
 
end RTL;