cga mostly done

arch/cga.cc

@@ -32,9 +32,6 @@ void getCursor(unsigned& abs_x, unsigned& abs_y) {
 }
 
 void show(unsigned abs_x, unsigned abs_y, char character, Attribute attrib) {
-  (void)abs_x;
-  (void)abs_y;
-  (void)character;
-  (void)attrib;
+  TEXT_BUFFER_BASE[abs_x + (COLUMNS * abs_y)] = Cell(character, attrib);
 }
 };  // namespace CGA

arch/cga.h

@@ -136,5 +136,5 @@ struct Cell {
 
 /*! \brief Base address for linear text buffer in video memory
  */
-Cell* const TEXT_BUFFER_BASE = nullptr;
+Cell* const TEXT_BUFFER_BASE = reinterpret_cast<CGA::Cell*>(0xb8000);
 };  // namespace CGA