Commodore VIC-20 User Manual page 181

Chapter 5: Game Controllers
167
CHOOSING THE KEYS
We will first select the keys that will make up thejoystick. Nine keys are
needed: eight for the various pointing directions and one for the Fire button.
The keys should be arranged so that they are easy to use. The arrangement
below uses the keys that are naturally under the right hand when touch-
typing. Their circular pattern is easy to learn and remember.
DESIGNING THE INTERFACE
TO THE MAIN PROGRAM
Now let's take a look at the programming necessary to interpret the
keys and simulate the joystick. The subroutine should be compatible with
the one described for the actualjoystick so that you can write programs that
work with either. This subroutine will set the same three variables for the
main program.
XI The X Increment. Controls movement to the right or left.
YI The Y Increment. Controls movement up or down.
FB Tells whether the Fire Button was pressed.
XI is set to -1 for left,
+
1 for right, and 0 for neither. YI is set to -1 for
down,
+
1 for up, and 0 for neither. If the
K
key is pressed, FB will be a 1;
otherwise, it will be a O.
LOADING THE TABLES
The program will interpret the keys by looking them up in a table. It
uses other tables for the XI and YI values. These tables will be stored as