ALIEN ATTACK

by Paul Hayward



Here's one for all you trigger-happy alien-zappers.



You are Earth-bound but, luckily, provided with weapons, when a mother

ship full of strange creatures and small craft approaches the planet.

Now's your chance to save the world by stopping them from landing.



In this version of the arcade game you aim with the 'Z' and 'X' keys

and fire with the Space key. Instead of bunkers there is an

'atmosphere' which will provide you with a limited amount of

protection, and rapid random firing is prevented since continual

firing will abort previously fired missiles. For each alien or ship

destroyed you score 10 points, and you gain another 100 points by

destroying the mother ship.



Variables

H	High score

S	Score

X	Vertical base coordinate

Y	Horizontal base coordinate

A	Vertical coordinate of first row of aliens

B	Horizontal coordinate of alien rows

L	Counter (starts at 0; when 9 is reached aliens move down

	vertically one row)

Z	Either 1 or -1, increments or decrements b

a()	Switch (0 or 1) status is determined by whether there are

	aliens left in each row

Fi,Fa	Switches tell whether a missile has been fired

Fx,Fy	Coordinates of Earth missile

Ax,Ay	Coordinates of alien missile

F	Allotted for general use



Structure

10-60	UDGs, variables and set-up routines

100	PRINTS missile base

110	Scans keyboard for base movement

120	Checks if space bar has been used to fire

122	Erases previous missile

124	Sets new missile coordinates

125	Checks whether there is a missile on the screen

130,140	Updates missile

150-200	Prints aliens

210-320	Alien fires, various checks

400	Update alien's horizontal coordinates

500	Loops back to 100



Subroutines

1000	Earth missile has hit something, determines what and acts

	accordingly

2000	Alien missile has hit something, determines what and acts

	accordingly

5000	Die routine, new game option

6000	Instructions

8000	Set up variables, string variables, 8090 draws atmosphere

9000	UDGs, data for music