Implement single character write interface

This basic interface just moves the cursor to the next position after
writing a character and supports CR and LF as special chars for now.

terminal.vhd

@@ -15,6 +15,9 @@ entity terminal is
         clk: in std_logic;
         reset: in std_logic;
 
+        write_enable: in std_logic;
+        write_data: in std_logic_vector(7 downto 0);
+
         dvi_d: out std_logic_vector(11 downto 0);
         dvi_clk_p: out std_logic;
         dvi_clk_n: out std_logic;
@@ -31,8 +34,56 @@ architecture syn of terminal is
     signal image_x: std_logic_vector(9 downto 0);
     signal image_y: std_logic_vector(8 downto 0);
     signal pixel_rgb: std_logic_vector(23 downto 0);
+
+    signal fb_write_enable: std_logic;
+    signal fb_write_address: std_logic_vector(12 downto 0);
+    signal fb_write_data: std_logic_vector(7 downto 0);
+
+    signal write_x: unsigned(6 downto 0);
+    signal write_y: unsigned(5 downto 0);
 begin
 
+    fb_write_address <= std_logic_vector(write_x) & std_logic_vector(write_y);
+
+    process(clk)
+        variable next_line: unsigned(5 downto 0);
+    begin
+        if rising_edge(clk) then
+            fb_write_enable <= '0';
+            fb_write_data <= write_data;
+
+            -- calculate next line
+            if write_y = 59 then
+                next_line := (others => '0');
+            else
+                next_line := write_y + 1;
+            end if;
+
+            if write_enable = '1' then
+                fb_write_enable <= '1';
+                if write_x = 79 then
+                    write_x <= (others => '0');
+                    write_y <= next_line;
+                else
+                    write_x <= write_x + 1;
+                end if;
+
+                -- carriage return
+                if write_data = x"0d" then
+                    fb_write_enable <= '0';
+                    write_x <= (others => '0');
+                end if;
+
+                -- line feed (implicit CR)
+                if write_data = x"0a" then
+                    fb_write_enable <= '0';
+                    write_x <= (others => '0');
+                    write_y <= next_line;
+                end if;
+            end if;
+        end if;
+    end process;
+
     dvi_clk_ds: obufds port map (
         I => clk,
         O => dvi_clk_p,
@@ -68,7 +119,10 @@ begin
         clk => clk,
         x => image_x,
         y => image_y,
-        rgb => pixel_rgb
+        rgb => pixel_rgb,
+        write_enable => fb_write_enable,
+        write_address => fb_write_address,
+        write_data => fb_write_data
     );
 
 end syn;