ESCAPE FROM THE UNDERWORLD



Blast your way out of the caves of Andromeda in Escape from the

Underworld by J. Dury of Bristol. The program runs on the 48K Spectrum

or Spectrum Plus and sets you the task of destroying five aliens in

each cave before your energy runs out.



[Neither the program or accompanying program notes bothered to say]

[what the control keys are, but they appear to be:                ]

[O/P=left/right, Q=thrust, Z=fire                             JimG]



Lines 5 to 100

Line 5 is a standard Poke to set the machine in CAPS LOCK mode, and

line 20 sets permanent colours in the same way as the colour state-

ments. Dury is an economical programmer, as we shall see later, but

you could do this sort of thing with ordinary statements. Array A() is

set to contain five elements which will correspond to the five aliens.

The rest of the lines read in the data for the user-defined graphics

characters. Variable h holds the hi-score



Lines 100 to 1010

The basic variables are now set up. CAV is the number of the current

cavern, L is the number of lives you have left, SC is the score, SL is

a score modifier used to decide whether you deserve a bonus life and

UP is used to determine whether you are flying up or not. Score, fuel,

lives and other details are printed at the bottom of the screen, and

the program then goes to the routine at 5000 to set up a new cavern.



Lines 1015 to 1220

Some more variables are set up. NA is the number of aliens left, and

FL is your fuel level. X and Y are your starting positions. XN, YN and

DR are used to determine degree of thrust and direction of movement.

Lines 1050 to 1070 position the five aliens on the screen. Line 1060

looks at the colour of the square under examination and if it is not

red, that is, if the ATTR function is not 2, the alien is put there.

A(N) holds the vertical co-ordinate of the alien which is then

printed. N is also used to determine the horizontal position of the

alien initially. 1110 copies the X and Y values into X1 and Y1 for

future use.



Lines 2000 to 2030

Dury starts to get going in the main loop. Instead of using boring old

INKEY$ for the input, he is using the IN instruction which scans the

keyboard via the signals sent out of the port at the back. This is

faster than INKEY$ but difficult to use. BIT is used as a comparison

with the number sent out by the keyboard so the key can be checked. A

value of 254 is given out if no keys are pressed in the appropriate

row, so BIT is subtracted from that to find the actual key pressed.



At line 2020 the IF statement makes the whole of the routine to 2220

into a loop to deal with the aliens - if NA is bigger than 0. If it

isn't there are no aliens, and to avoid trouble when the program

reaches NEXT C that case will be looked after within the loop. This is

Basic programming at its most devious and only recommended to those

with strong nerves. The net result of all those tortuous IN statements

is to decrease horizontal thrust if you move up and down, or remain

stationary.



Lines 2035 to 2080

Line 2035 adds 0.5 to the horizontal thrust if the way is clear, so

you tend to keep moving towards the aliens. Lines 2040 and 2045 work

out all the thrust factors and get X1 and Y1 set to the correct values

for the new position of your man. At 2050 the horizontal thrust is set

to zero if you have bumped into something, and X1 and Y1 revert to

their original values since you aren't going to move at all. At 2070

a space is printed over the old position and UDG is printed in the new

position. Which UDG? DR and UP and FL - the fuel, remember - are used

to decide which version of the man gets printed.



The economical Dury has run all this together, to save speed, but you

could do it with a bundle of IF .... THEN PRINT "???" type statements.

At 2075 the program goes to the routine at 6000 if you have got right

through the cavern. Finally, at 2080 the new co-ordinates become the

old ones, X and Y, in readiness for another waltz around the loop.



Lines 2085 to 2230

Line 2085 invokes the fire routine at line 4000 if the appropriate

keys are pressed. Direction of fire is also worked out and stored in

PX and PY relative to your current position. 2090 ignores the alien

move sequence if there are no aliens. 2095 to 2130 covers the aliens.

They will move more or less in your direction and fire if they are

level with you at line 2130. Line 2210 checks to see if you are out of

fuel, and line 2230 sends you back to the beginning of the main loop

if everything has been successfully negotiated.



Lines 4000 to 4090

The firing routine first checks to see if the next space in the

direction of fire is empty. If so, then at 4032 to 4037 PX and PY are

set to the co-ordinates of the furthest point along the line of fire

which is not blocked by an alien or cavern wall. At 4040 the colour of

that end square - the target - is recorded in ATR. If ATR is 6 the

aliens have been firing at you and routine 5100 is called in for your

death. The line at 4080 handles the demolition of a stalactite.



Lines 4100 to 4192

Lines 4100 to 4120 check for the case where a piece of cavern is

blasted, but a stalactite does not fall down. Lines 4150 to 4192

complete the firing section with the death of an alien. In 4165 the

number of aliens is decreased, and the alien array from which the

alien positions are plotted changes the appropriate element to zero.

Line 4166 gives a little Beep and increases your score. The routine at

4182 to 4192 resets co-ordinates.



Lines 5000 to 5030

The lines draw a cavern at random. Line 5020 is the important one - two

solid squares - in this case red - are printed all the way along the

cavern up to nine deep, top and bottom, depending on the random number

chosen.



Lines 5100 to 5190

Here is the death routine. L is decreased by one life, an appropriate

graphic is chosen from the stock held in A$, and your position is

worked out. You are eliminated, a tune is played, and if you have any

lives left then line 5150 sends you back to the main routine again.



Lines 6000 to 6060

This routine is called when you complete a cavern. A bonus score is

given and a check made for extra lives at line 6040.



Lines 7000 to 7110

Two routine here - the first plays a little tune to congratulate you

and the second, at line 7100, tests for an extra life. You get this if

you have scored 10000 points since your last extra life. SL is

upgraded by 1000 points each time.