The count down is on!
By KIM WEST and PAUL PASSANT
DENARY Derrick is designed to help young children to get practice
with numbers, and in so doing, get used to a computer keyboard.
The program will play two games. The first requires the defender
to fire rockets by pressing the key which has the same number
as Derrick is carrying.
Once the correct key is depressed a rocket is launched, and
can be guided by the Left and Right keys.
It is not very difficult to hit Derrick once the rocket is fired.
The aim of the game is to stop him, but the essential tactic is
to get the correct number.
Once the first game is mastered the major role of Derrick can
be used. In the system of counting we use 10 is very important.
The columns in base 10 (denary), represent units, tens of units,
tens of tens, and so on.
It can be seen that relating numbers to 10 is very important.
The numerical difference between a number and 10 is fundamental
in subtraction, and thus in many more complex calculations. This
is where Denary Derrick comes in.
The second game is almost identical to the first but rockets
are only launched if the key representing the difference between
Derrick's number and 10 is pressed.
For example, if Derrick has a 1 the rocket will only be fired
if 9 is pressed.
If Derrick is hit he is merely teleported back to the top of
the screen where he will collect a different number. A hit scores
10 points.
When Derrick lands for the third time a score card is displayed
showing the last score and the total overall. The program then
returns to the rule sheet and the player can reselect game one
or two.
The program itself has a simple structure. PROCintro gives the
idea of the game while PROCdefine sets up tables of data and defines
characters.
Then there are three nested REPEAT loops. The outermost, a continuous
loop, will rerun the game until BREAK or ESCAPE is pressed.
The second loop allows game one or two to be selected before
each go, ending when Derrick has landed for his third time.
Inside this is a loop which keeps Derrick moving down until
he is hit or has landed.
PROCbase checks to make sure the launch pad can be moved, and
if so prints it in its new position.
Similarly PROCspider does the same for Derrick (note line 400
- VDU48+N% is used to print the required figure on Derrick).
FNhit acts as a shield around Derrick, checking if the rocket
has reached him.
All the machine code for printing Derrick is assembled in PROCdefine
along with all the data and characters.
To ensure Derrick carries different numbers on subsequent descents
PROCinit sets NI% to the old value of N% (Derrick's number), and
picks random values of N% until it is different to NI%.
KEY% is the Ascii value of the number which should be pressed.
Lines 1530 and 1540 calculate KEY% depending on which version
of the game is being played.
Several colours are changed with VDU19, the flash rate increased
so Derrick's eyes move faster, and printing is sent to the graphics
cursor ready to print Derrick.
PROCupdate briefly returns printing to the text cursor and prints
the score.
On landing all colours are changed to blue except one for the
printing, PROCland also plays a tune and then resets the colours
ready for the next descent.
PROCanother gives the sheet, last score and running total. Finally
PROCexplode teleports Derrick with the aid of some interesting
machine code which fragments his part of the screen. Denary Derrick
is a long listing, but a very educational program for youngsters.
