Virtual Screen
==============

The eight by forty characters on the LCD are only one sixth of the
number of characters contained within the virtual screen. The LCD acts
as a window onto the virtual screen. The contents of the virtual screen
may be inspected at will by using the cursor control keys in their
shifted state.

Screen Size
-----------

HUNTER's virtual screen is 80 characters by 24 lines. The virtual screen
behaves very much like a terminal with automatic scrolling. When the
cursor reaches the bottom line, the rest scrolls up losing the top line.

Moving the Window
-----------------

The window may be moved over the virtual screen either manually by the
user, or automatically by the virtual screen handler resident in the
operating system and as such is transparent to the user.

Manual Movement
~~~~~~~~~~~~~~~

#. **Shifted state cursor control keys (looking keys)**

   By using the cursor control keys in their shifted state, the window
   can be made to move in the direction marked on the key, i.e. press
   the 'shift' key, hold it down, and then press the desired cursor
   control key. When this operation is performed it will be noticed that
   the cursor on the LCD (a blank flashing block) will change its
   position on the LCD and quite often disappear. This will happen
   because the cursor position refers to the position in the virtual
   screen, and when the window is moved the cursor position relative to
   the window position in the virtual screen will change.

#. **Function "cursor control keys"**

   By pressing CTL/FN and either horizontal arrow key, then either the
   left or right 40 characters of the screen can be seen. This function
   is useful for easy reading of wide screen formats.

#. **'INS' Key**

   Pressing the INS key will return the window to its original position
   prior to use of the looking keys, as outlined above.

#. **Transparent Movement**

   The transparent movement of the virtual screen window is primarily
   controlled by inputs, both from the keyboard and the serial port. If
   HUNTER is required to accept input from, say, the keyboard then the
   window will automatically be moved to allow the viewing of the
   immediate area of the virtual screen centred around the present
   cursor position, thus displaying any prompts which may have been
   required by either a Basic program or a CP/M type program.

   It must be noted that printing text onto the virtual screen will not
   move the window. However, if any part of the virtual screen which is
   presently in view on the LCD is changed by, say, a
   :doc:`../basic-functions/print` statement, then these changes
   will be reflected on the LCD, thus always giving a true
   representation of the virtual screen.

   If HUNTER is accepting input from the keyboard and the cursor reaches
   the forty-first position of the virtual screen, then the window will
   be moved laterally so that the cursor is now positioned mid-way
   across the LCD, as soon as the forty-first character is entered, thus
   displaying the previous 20 virtual screen characters and the
   following 20 virtual screen characters.

   If HUNTER is waiting for input and the looking keys are used to move
   the window, a valid input will cause the window to automatically
   revert to its original position prior to the use of the looking keys.
   This facility does not force the user to press the INS key in order
   to restore the original position of the window when in this mode.

   For further information on cursor addressing see
   :doc:`../basic-functions/locate`.

   .. Note::

      The virtual screen window does not operate in Graphics mode.

Moving The Cursor
-----------------

To move the cursor in the required direction it is only necessary to
press the appropriate cursor control key.

.. image:: virtual-screen.png

.. Note::

   The shifted cursor control keys move the window over the
   virtual screen in the direction of the arrows.