BIPODS for Sinclair ZX-81
by D.E. Thorpe
from Your Computer, July 1984

D.E. Thorpe with a program that makes excellent use of the ZX-81's
limited graphics. Your mission - to destroy the Bipods before they
destroy you.


[The article inconveniently neglects to specify the control keys for]
[moving the missile launcher; they are: 5 - rotate left; 8 - rotate ]
[right; 7 - move forward; 6 - move backward; plus 0 to fire.        ]
[Once all the Bipods are active it's possible to get an unlimited   ]
[score by moving the missile launcher so that a barrier is directly ]
[between it and the remaining Bipods; they will just sit there and  ]
[the fuel trucks can pass across the screen unhindered.         JimG]


Martian pods have landed in your battle zone. At first they are
protected by force fields, but to attack they divide into deadly Bipods.
You must try to destroy them using a missile launcher controlled by
the cursor keys.

Don't be too trigger happy - fire is 0 - since there may be a re-
loading delay at a crucial moment! The missile launcher will move until
the opposite key is pressed to stop or reverse it.

You score 20 points for a Bipod and 10 points every time the fuel
tanker successfully crosses the screen. There are bonus points at the
end of each game level.

The game occupies just over 2K of machine code and is entered as
follows. First, lower RAMtop by:
	POKE 16389,90    (N/L)
	NEW              (N/L)
Now enter a REM statement line 1 of 97 zeroes and line 2 of 128 zeroes. 
Edit line 2 to make lines 3 to 17 then enter the commands
	POKE 16511,194   (N/L)
	POKE 16512,8     (N/L)
	POKE 16510,0     (N/L)
Now immediately enter line 2:
	SAVE "BIPODS"
You now have a long REM statement at line 0. Now enter the hex loader
program and run it from line 100, enter the machine code and when
finished add the the following Basic lines:
	5 RAND USR 16592
	8 STOP
Now save the program before running it. Run from line 5 and the screen
should show the pods, fuel tanker, missile units, obstacles and zero
score.

Press any key to start the game and key S at the end of a game to
return to Basic or key Newline to play again.

If you have a sound board it is easy to add sound to this game. There
is space to add to your machine-code sound routines after the main
program and these can be called from:
	40D0  to set up the sound registers
	4115  to produce the sounds

The sounds to be produced are indicated by Bits being set in Byte
4021. After creating a sound the relevant Bits must be reset. If your
generator uses the AY-3-8910 sound chip the sounds can be produced
simultaneously by loading Byte 4021 into Register 7 to enable the
sounds channels. See Table 1.


Table 1. Byte 4021 comprises of:

BIT    7       6       5       4       3       2       1       0
     SPARE    UNIT   BOUND    POD     FUEL    DIV     POD     UNIT
              HIT     HIT     HOT     HIT    -IDE     FIRE    FIRE

