====================================
                     Inv  -  Space Invaders in 256 bytes.
                     ====================================

                                    by      


                            James David Chapman.                  
                                                           
                            
                     

   Inv.  Even its name has been cut down.  Bloody hell, 256 bytes isn't very
 much to work with.  I have had to cut out almost everything I wanted to
 to put into the thing. 

                                    
   There are still savings to be made of course, the graphics, while only
 text mode, are a bit OTT, but if you take *everything* nice out then you are
 just left with uninspiring kloz, err rubbish, err, poor graphics.




                     ====================================
                          Inv  -  Playing the game.              
                     ====================================



     The program runs in dos, or a full screen dos box in windows.

     The game is simple enough to control:

     Use the left and right <SHIFT> buttons to move your space ship.

     And either <ALT> button to fire.

     To quit back to dos press one of the <CONTROL> buttons.


  The aim of the game, is to shoot all the invaders down, however since
 this is very difficult, they come back again to give you another try!

   In fact, they keep on coming till you have shot them all down.  The object
 of the game is to destroy all the invaders in the shortest number of 'waves'.


  The level at which to play (really just the game speed), can be set from
 the command line when you begin.  To set level 1 (the fastest) simply enter
 "Inv".  To set speed level 2 enter "Inv ", for level 3, type "Inv  " ie the
 speed is equal to the number of spaces you type after "Inv" command before
 you press <Enter>.



                     ====================================
                            Inv  -  The Program.                
                     ====================================


   In terms of programming, there are a few tricks in the code, such as
 reading the keyboard flags and self modifying code.


   As well as these, the invaders control array is held in the command line,
 using 7FH to flag a destroyed invader... this value is impossible to enter
 from the keyboard, (even with ALT), so the array should not be affected by
 a silly command line entered by the user.


   Words are INCreased wherever possible, since these encode 1 byte shorter
 than byte INCreases, and of course XOR is used to zero registers.


   There are a few suspect bits of code still in there, and I expect that it
is possible to stuff more into the game, but for now I simply do not have the
time.


   Not much else to say, other than I tried to concentrate on the look and
 feel of the original rather than the functions, the coding of which, were
 surprisingly byte-hungry and added little to the overall impression of the
 game.  For example, there is no score function or 'invaded' check, but there
 are pretty sprites and proper invader side to side movement.  A program which
 uses single characters for sprites will undoubtedly be able to fit more
 game functions in, but hopefully will not look as good.


   Having said all that, the game *is* quite challenging, it is not easy to
 complete... especially on the first (green) wave.


                     ====================================                                                                                                                       
                         Inv - Implemented Features.
                     ====================================

       Coloured sprites.
       Keyboard input.
       Variable level/speed setting (via command line).
       Ufo.
       Correctly advancing invaders,  (well, almost).
       Space ship.
       Fire function.
       Invaders disappear when hit.
       Ufo disappears when hit.      
       Space ship flickers when earth 'invaded'.
       Invaders come back around giving you another chance to zap them.
       Invaders colours change.



          ============================================================
             Bits of code (which take up space but are not pretty)
          ============================================================


      + CLS function to tidy screen up a little, on start *and* end of game.
      + Vertical retrace wait(s).
      + Fire top of screen check.
      + Fire only allows one bomb on screen at once... like the original.      
      + Invaders off screen check.



                       =================================
                          Not enough space for...  :(
                       =================================


      x Animated sprites (a simple XOR of a memory location would have done).
      x Increase number of lines of invaders in each new wave.
      x Score.
      x Screen beautification.



                       =================================
                                Disclaimer:
                       =================================


   I place this program into the public domain.  Anyone is free to use,
  modify, rework, or re-release it, providing you credit me as the original
  author, and make no financial profit from it.


   In return, you must accept that I will accept no liability for
  damage caused to you or your equipment by this program.






   The 256 compo was a good idea, I'll be interested to see what the others
 have come up with.  Take care.
                                        
                                        
 James.           


 [James David Chapman jchap@globalnet.co.uk
 http://www.users.globalnet.co.uk/~jchap/]