Commodore VIC-20 User Manual page 174

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The VIC 20 User Guide
There is also a Fire button, which has its own switch. Using the method
described in the following sections, your program can tell whether these
switches are open or closed and thus determine which way the joystick is
pointing. By moving the joystick, you can direct elements on the screen.
The VIA Chips
The joystick is connected to the VIC through integrated circuits called
Versatile Interface Adapters (VIAs). Certain pins of the VIA chip connect to
the "outside world." These pins receive a signal sent to them (input) or send a
signal out to another device (output). Circuitry in the VIA chip enables the
VIC to set or examine the signals on these pins using memory locations. The
signals can be read and controlled through PEEKs and POKEs in BASIC.
Testing the Joystick SWItches
The VIA's input/ output pins are divided into two groups of eight. Each
group can be set or examined through a single memory location, with one bit
in that location representing each pin. The Up, Down, Left, and Fire
switches are connected to one group of pins, and the Right switch is in
another group.
Memory
Location
37137
37152
Bits
I
1 1321161 814 1
•
I
1 I
Joystick
Switch Direction
Up
Down
Left
Fire
fl-__________
Right