ALIEN BLASTER
by Alan Green

In this enthralling little arcade game, you must save the planet Earth
from a group of alien spacecraft who are trying to colonise it. If you
allow five alien spacecraft to land, then the Earth has been colonised
and you have lost. The game makes particularly good use of the Spectrum's
sound, colour and graphics facilities.

Lines
   5       Call introductory music subroutine.
  35       Set high score to zero.
  40-60    Set up user-defined graphics to represent the alien spacecraft.
  70-80    Set score, inital missile base X and Y.
  90-97    If instructions are required, print them.
 100-120   Print the screen.
 150-155   Select a random starting position, direction of flight and
           type for the next alien attack.
 160       Print missile base.
 170       Determine plotting X co-ordinate of base.
 180       Move base right or left.
 190       Move plotting cursor to base's new position.
 200       If you have fired, draw the missile track.
 205       Move alien according to its direction of travel.
 210       Move alien down; if it lands, check to see if you have lost.
 212-214   Move the missile base again.
 220       Repeat the main program loop.
 230-250   Print the alien who has landed below the line and, if five
           have landed, you lose.
 260-290   See if your missile hits the alien.
 300-340   Print game over; see if the player wants to try again.
 350-370   Data for user-defined graphics.
 380-510   Instructions.
 520-640   Introductory music and picture.
1000-3000  Move alien according to direction of travel.
