Space Invaders-style programming demo, 27-Oct-97

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NOTE: This is currently not a working game. It only demos

 a scrolling method.



 Recently someone asked me how games that have more than

40 moving objects on screen might be implemented on a

GameBoy given the fact that GameBoy only supports a max-

imum of 40 sprites simultaneously on screen.



 The game Space Invaders (SI) by Taito for GameBoy is a good

example of such a game. It has 40 "invaders" along with

a space ship and missle launcher. All of these have the

ability to be moving simultaneously.



 A quick disassembly of SI seems to suggest that they

have separate tiles for each invader. Given the fact that

each invader is 8x8 pixels, it appears they move them

two pixels at a time using tiles similar to these :





        +--------+--------+   +--------+--------+

        |        |   XX   |   |        | XX     |

        |        |  XXXX  |   |        |XXXX    |

        |        | XXXXXX |   |       X|XXXXX   |

        |        |XX XX XX|   |      XX| XX XX  |

        |        |XXXXXXXX|   |      XX|XXXXXX  |

        |        | X    X |   |       X|    X   |

        |        |X      X|   |      X |     X  |

        |        | X    X |   |       X|    X   |

        +--------+--------+   +--------+--------+



        +--------+--------+   +--------+--------+

        |       X|X       |   |     XX |        |

        |      XX|XX      |   |    XXXX|        |

        |     XXX|XXX     |   |   XXXXX|X       |

        |    XX X|X XX    |   |  XX XX |XX      |

        |    XXXX|XXXX    |   |  XXXXXX|XX      |

        |     X  |  X     |   |   X    |X       |

        |    X   |   X    |   |  X     | X      |

        |     X  |  X     |   |   X    |X       |

        +--------+--------+   +--------+--------+



 Given the fact that separate tiles are required to show

smooth movement across the screen, seven times as many

tiles are required of the original invader tile. Not only

does this consume large amounts of ROM but valuable tiles

are used up as well.



 One method of conserving tiles is to have dynamic tiles.

Dynamic tiles are tiles that are modified during a VBlank

interrupt to show movement. Still large amounts of ROM

are often required to update these dynamic tiles.





 This demo source & ROM code for GameBoy shows another

method for smooth scrolling of a large number of on screen

objects. It does the scrolling not by modifying the tile

map memory but rather by modifying the BackGround X scroll

register on a line by line basis.



 So how does the code know when a new scan line is present?

By setting and using the LYC scan interrupt. It interrupts

at every scan line allowing the scroll register to be

modified.



 This demo ROM code will work on the real GameBoy, VGB 1.0

or later, and possibly other emulators. It does NOT work

on VGB 0.71 and possibly others.



 The source code was compiled using RGBDS V1.07.

All screen images were created using TILE256. The

'tiles.dat' file is the original TILE256 data file.





  Jeff