Apple IIe - Page 230

230 Apple lie Users Handbook
Line 10 clears the screen and enables low resolution graphics.
Line 140 sets
the initial position at screen location (20 , 20 ). This is
the center of the screen. The A =65 statement that also appears in
line 140 selects " up " as the initial direction of movement .
(A)
65
90
(Z)
These values represent the ASCII coded values for the controller
keys: A, Z, - and -.
Lines 160-250 set up a loop that monitors the keyboard and acts
accordingly. Line 160 checks for a keypress and sets the direction
variable, A. The expression PEEK (-16384) > 127 is true whenever
a key has been pressed. Lines 170-200 recalculate the postion
variables. Executing the program is the best way to understand
how it operates. By the way, the program will not operate cor-
rectly if an 80-column card is presently active. See Chapter 8 for
instructions about how to deactivate the card.
Recall
from our description of BARACADE, that one of the rules
was that the player was not allowed to collide with his trail. The
SCRN
function will be used to check for collision . If the follow-
ing line is added to the program, collisions will be detected.
210 IF SCRN (X,Y) > 0 THEN 260
The program has not yet been completed . When it is run, an
error occurs after every collision. This is because the computer
does not know where to jump when there is a collision. Let 's tell