CAVERN BLAST
by Peter Fothergill
from C&VG, February 1984


Aliens have discovered your underground
fortress - and they are making an all out
attempt to destroy it. You are the commander
of the fortress defence systems and control a
laser blaster which you must use to shoot
down the attacking aliens. A force shield
protects the fortress - and you must protect
the force shield. If an alien manages to blast it
three times then the fortress will be
destroyed.

	The aliens come in two different shapes.
The slow, green ones are battle cruisers. The
fast yellow ones are star-fighters. You score
30 points for a green alien, 50 for a yellow
one. At 1,000 and 2,500 you'll get an extra life
plus a 200 point bonus. A high score table is
included to record your best efforts.

[The next two paragraphs are erroneous. There was]
[no such code in the published listing.      JimG]

	The program includes machine code sound
effects. These are POKEd in from lines 75 to 90
and called using LET L=USR 65000 and LET
L=USR 65030. From these two calls an array
of sounds is produced by POKEing in
subsequent numbers throughout the
program.

	On levels two and three when the two
millipedes appear and drop bombs some of
the machine code sound effects are called
and the program stops. This can be cured by
omitting line 3270.


Variables:
ax = vertical co-ordinate of your cannon
bx = horizontal co-ordinate of your cannon
cx = vertical co-ordinate of fast alien ship
dx = horizontal co-ordinate of fast alien ship
ex = vertical co-ordinate of slow alien ship
fx = horizontal co-ordinate of slow alien ship
sc = score
li = lives left
