VIC 20 JOYSTICK/PADDLES/LIGHT PEN

 The following documentation is taken from the "VIC 20 Programmers Reference
Manual".

USING A JOYSTICK ON THE VIC

 Like all other input and output, the joysticks are controlled using the
VIC'S 6522 Versatile Interface Adapters (VIAs). The 6522 is a very versatile
and complex device. Fortunately, it isn't necessary to delve deeply into the
mysteries of the 6522 VIA to read the joysticks.
 Each 6522 has two Input/Output ports, called port A and port B. Each of the
ports has a control register attached, called the DATA DIRECTION REGISTER
(DDR). This highly inportant register controls the direction of the port. By
using this register you can use the port for input, output, or both at the
same time. To set one bit of the port to output, set the corresponding bit
of the Data Direction Register to 1. To set a bit of the port for input, set
the corresponding bit of the DDR to 0. For example, to set bit 7 of port A to
input, and the rest of the bits to output, poke 127 in the DDR for port A.
 To read the joystick, one port (and one DDR) of each of the 6522 VIAs on the
VIC must be used.
 The joystick switches are arranged as follows:

                                 TOP
         FIRE BUTTON
         Switch 4 (FR)    
                              Switch 0
                                (S0)
                                 |
                                 |
                                 |
                  Switch 2 ------ ------ Switch 3
                    (S2)         |         (S3)
                                 |
                                 |
                                (S1)
                              Switch 1

 Switch 0, Switch 1, Switch 2, and the Fire button can be read from VIA #1,
which is located beginning at location $9110. Switch 3 must be read from the
other 6522 (VIA #2) which is located beginning at location $9120.
 Now the key locations for the joystick are as follows:

 HEX     DECIMAL    PURPOSE
 $9113   37139      Data direction register for I/O port A on VIA #1

 $9111   37137      Output register A
                    Bit 2 Joy switch 0
                    Bit 3 Joy switch 1
                    Bit 4 Joy switch 2
                    Bit 5 Fire Button

 $9122   37154      Data direction register for I/O port B on VIA #2

 $9120   37152      Output register B
                    Bit 7 Joy switch 3


IMPORTANT

 The relevant bit is one when a switch is open and zero when a switch is
closed.
                     1 = open
                     0 = closed   

 A switch is closed when the joystick is pointing in that direction and open
if it isn't pointing in that direction.


JOYSTICK NOTES

 It is important to note that the bits are one when the switch is open and
not zero. Some VIC 20 manuals state that it is the other way around which
is not the case. 
 To read the joystick inputs, a VIC 20 program will firstly set the ports
to input mode by setting the DDR to 0. Then the values of the switches can
be read by the program. VIA #2 is also used for reading the keyboard. Setting
the DDR can mess up the keyscan rather badly. So VIC 20 programs will make
sure that they restore the DDR to the original condition.