Amiga Computing - Commodore Is Awesome

HI South shows you
ow to emulate
ore arcade game
chniques using the
of so cheesy Amos
logs, eh? I bet you don't think about them
or give them a second thought, do you?
You don't have the slightest idea what I'm
talking about, do you? Well, o flog, for
the purposes of this discourse, is a vanwhich
is set to indicate to a program that a
in state exists So when the flog is set, some.
ng has happened that the program should know
, and more importantly doesn't need to check
again. Until, that is, the state changes, in which
se the flag is changed and the program then
s checking for that state again. Confused?
y iers use on example.
In arcade games, you fire a bullet from your
ship/loser/whatever. The program checks the
buton and if its on it activates and moves a
sprite across the screen away from the gun, ii
let the sound effect of the gun going off, and
ks the bullet across the screen until it hits
ing.
So you make o loop to check for the button on
joystick being pressed. Simple enough, yes?
el, the first thing you notice when you write a
ine like this from scratch is that when you press
Duton the loop checking for the button press
round about 50 times a second, so when you
t you get a horrible staccato buzzing noise
the bullet doesn't ocivaly leave or the buzzing
se stop) until you've released the button.
The buzzing noise is the bullet sound, the bong,
lig played many times over and over very fast,
reason the bullet doesn't leave the barrel is that
skirts its journey every time the button is sensed to
ir the down position. So how do you moke the
If you ca re fully
hallow Phil South's If Tire(I) And MISSFLAI4 Then Sol Ploy 1
instructions you will NISSFLASal : Ildb 2,SX,S1,2 : NIzSX : NY.SY
soon ha y. ships If 1I55ILIG=1 Then Add OX,S : Bob 2,10,17,2 : If
shooting trom lb. hip
- or some whe re
/DOWN Then lob Off 2 : RIS5U6.0
ton press once and only once? Now you're
MEM I N N
•
t
-AMIGA COMPUTING
FEBRUARY 1996
Nol with
a bullet
OVER TO YOU
Why not send me your examples of Hags, and interesting uses to which they can be
put. Making a process easier, showing the program that states have changed, even
some interesting flip-flop effects (('II be going into those another time) - anything
really as long as it's about 10 lines of code or less and it Mcludes a flag or two. I'm
particularly interested in graphic elleas, but timing and more mundane task are
acceptable. Perhaps there is on interface problem you can solve with these flags?
Look into it. I'll look at the best in a month or two. In the meantime see you
next month for another arcade game routine which you'll find handy when you
design your own version of an arcade classic.
getting the idea You use a flog Take the following
example:
If firt(l) Then Sao Play 1 : lob 2,SI,STa
MPH : POW
Add O0,5 : lob 2,
11
Oft 2
IoNT,
2 :
I f
O U P I T
T I D O
T h
e n
B
h
a
That's simple enough Sense the joystick, make a
bang [in this case a nice sample], then move the
sprite Iwo pixels at a time from the barrel to the
edge of the screen in a straight line. If it gets 100
pixels away from the ship without hitting something
it gets turned off. But this routine suffers from
buzzing and no bulets. The better way of doing it
is:
In this version, when the button is pressed the flag
•
•
MISSFLAG is set from 0 to 1. In the next line the missile
is moved only if MISSFLAG is still 1. Until the missile
moves 100 pixels away from the barrel, the flag is
stil set, so the fire button is dead. You only fire one
shot. When the first missile vanishes [you could
explode it, but in this case it just vanishes), you con
then shoot another. Obviously, you could vary the timing
so that you can add a multiple shot, perhaps, or
make it a second (or 50/50ths of a second' or even
half a second between shots, enabling the player to
get his finger off the button.
This is an extremely useful technique and one
which answers a lot of those questions you ask, like
'how do you do that? Nine times Out of ten the
answer is either flogs, subroutines or vectors. A side
benefit of using flogs in your Amos programming is
that these techniques ore used quite a lot in the worlds
of C and assembly programming. So as and when
you move on to those languages, as you might in the
course of time, you will be prepared for at least one
cool trick.
WRITE STUFF
If you have any other Amos programs or
queries about Amos, please write to the
usual address, which is: Phil South, Amos
Column, Amiga Computing, Media House,
Adlington Park, Macclesfield SKI° ANP.
Please send routines on an Amiga disk
with notes on how the program works
on paper, not as text files on the disk.
,Make the routines short enough to
appear in print, i.e. no more than about
30-40 lines of code and, if possible,
make them use no external graphics, or
if they can't be used without them then
be sure to provide them on the disk in
native f FF formal, and the same goes for
sound files. Follow these guidelines and
you'll be sure of making me a happy
man if nothing else.
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