ZX Computings - OpenLibra

the blank to remove the black
blob. This blank is printed just
before the screen SCROLLS upward,
to move all the zeroes upward.
Your aim in the game —
needless to say — is to avoid
the hailstones, as you can see
from diagram two. You'll
remember that at the start of
this article we mentioned that
the RND function was fairly
slow. To minimise the delay,
we've used a single pair of random
numbers (generated in
lines 60 and 65) four times in
line 70.
Lines 1 00 and 110 are very
interesting, and are crucial for
producing a worthwhile program
using the SCROLL facility.
Line 100 moves the PRINT AT
position to A + C, B which is a
line down from where the black
blob is at present. Using line
110, the computer then looks
up this position in the display
5
10
15
£ 0
25
27
LET S =VRL *"0"
LET R = URL "8"
LET T = UftL "30
LET B = R
LET C = B / B
LET D =C +c
file, to see if anything is there. If
it finds the code for' 0" in that
position — that is one line
below the black blob — it
knows the black blob is going to
hit a hailstone, and prints out
your score, the value of the
variable S. The Q at the end of
this line causes the ZX81 to
stop with a 2 error code, meaning
an unassigned variable has
been encountered. This is
'cheaper', in memory terms,
than running the line twice,
with THEN STOP at the end of
the second line, or of saying
IF . . .THEN GOTO 1020,
where line 1020 reads PRINT
S. Other memory-saving
devices you can glean from this
listing include the use of VAL a
string, instead of using a
number. Believe it or not, LET P
= 1 uses up more memory,
because of the way the ZX81
memory works, than LET P =
30 PRINT RT '
4-0 LET X=R
50 LET Y=6
60 LET M =RND*6 +11
65 LET N=PND*13
70 PRINT RT M+D,N-C, "0"; RT M,N
j"0";RT M - N . N ; " 0 ' ; P T M,M+N;"0"
60 LET B=B+D* (INKEY$ = "8" .RNT> f
v'T) -D* ( INKEY$-"5" RND B >D)
90 LET S = S+C
100 PRINT RT R+C,B;
110 IF PEEK (PEEK 16398 +256 frPEf
K 16399) =CODE "0" THEN PRINT 5, CV
990 PRINT RT X^Y;" "
1000 SCROLL
1010 GOTO T
• Program five
Moving graphics
VAL "1", and CODE "0" is
cheaper than the number 28.
In our final program,
SQUASH (program number
six), a ball (which looks
remarkably like an asterisk) is
bounced off a black blob at the
bottom of the screen, which
you move right and left by using
the "Q" and "P" keys. If you
manage to 'bounce it', your
score (shown in the top right
hand corner of the screen) increases
by one, and the game
continues. Miss the ball, and
the fun is over. Enter and run
this program to see how it
works. Note that because a random
number does not have to
be generated between successive
reprints off the ball, the
time the screen is blank is very
short (only while the computer
processes lines 65 and 70), so
a second pair of variables to
'hold the ball' are not needed.
0 0 0
0
0 00 0
0
000
0
0 00
0 0 0
0 0
00 0 0
0 0
0 0
0
0
0
Diagram two
5 LET Y -
10 LET R =
P • 15 LET U =
• » • • 20 LET X —
• * • • 2 5 LET D =
• • 3 0 LET U =
• • • 31 PRINT
0 • 3 5 FOR C =
m 4-0 IP INK
ft 1
• • 4 5 IF INK
• a 1
• ft 5© PRINT
* * — 1; "
• • 5 5 IF X-0
ft * D
• * • 6 0 P R I N T l
« • • 6 6 LET Y =
m • 7 0 LET X=
• • ft 7 5 PR INT (
• v
8 0 NEXT C
8 5 IF RBS
• Diagram one • Program six
0
mm
Line 5 initialises the vertical
position of the ball, and line 10
sets up the player (black blob)
starting position. Line 1 5 starts
the count at - 1, which quickly
becomes zero at line 30, before
the first score is printed. Line
20 initialises the ball's horizontal
position, and 25 initialises
the horizontal direction of the
ball. Minus one sends it left,
zero sends it straight up and
plus one sends it right. Line 30
increments, the score count.
Line 85 checks to see if the ball
and the bat are fairly close, and
if they are, allows the game to
continue.
There are a number of simple
ideas presented in this article
for you to implement in your
own moving graphics games on
the ZX81. We'd be pleased to
see your favourite program
which you've developed after
reading this article.
00
1 6 "
0 0
0
0
0 0
0
0
0 0
(RND*25 +3)