Add LCD control structure.

Also add the Bin module to easily deal with binary.

.merlin

@@ -1,3 +1,3 @@
 S src
 
-B _build
+B _build/src

src/bit.ml

@@ -0,0 +1,4 @@
+let is_set num i =
+  if i < 0 then failwith "Invalid bit index."
+  (num lsr i) land 0b00000001 <> 0
+

src/screen.ml

@@ -11,6 +11,40 @@ type t = {
   data   : pixel array;
 }
 
+type control = {
+  lcd_display_enable    : bool;
+  tile_map_select       : int;
+  window_display_enable : bool;
+  tile_data_select      : int;
+  bg_tile_map_select    : int;
+  sprite_size           : int;
+  sprite_enable         : bool;
+  bg_display            : bool;
+}
+
+let get_lcd_control mem =
+  let b = Memory.get mem 0xFF40 |> int_of_char in
+
+  let lcd_display_enable = Bit.is_set b 7 in
+  let tile_map_select = if not (Bit.is_set b 6) then 0x9800 else 0x9c00 in
+  let window_display_enable = Bit.is_set b 5 in
+  let tile_data_select = if not (Bit.is_set b 4) then 0x8800 else 0x8000 in
+  let bg_tile_map_select = if not (Bit.is_set b 3) then 0x9800 else 0x9c00 in
+  let sprite_size = if not (Bit.is_set b 2) then 8 else 16 in
+  let sprite_enable = Bit.is_set b 1 in
+  let bg_display = Bit.is_set b 0 in
+
+  {
+    lcd_display_enable;
+    tile_map_select;
+    window_display_enable;
+    tile_data_select;
+    bg_tile_map_select;
+    sprite_size;
+    sprite_enable;
+    bg_display;
+  }
+
 let init_screen w h =
   {
     width = w;