Joystick Functions for C02 Programs

This module contains constants and a function for reading digital joysticks
(the type used on Atari and Commodore 8-bit computers). For systems that use
analog joysticks (such as the Apple II), the paddle module should be used.

The constants and assembly code vary by system, so when invoking the 
cross-compilers, the command line option -s must be used to ensure that
the correct header files are included.

At the beginning of the program use the directives

  #include <joystk.h02>

The following constants are defined:

  #JYSTKS       Maximum number of joysticks supported by system.

                For systems with no joystick support, this will be 0.

  #JOYUP        Joystick Switch Bitmask - Up
  #JOYDN        Joystick Switch Bitmask - Down
  #JOYLF        Joystick Switch Bitmask - Left
  #JOYRT        Joystick Switch Bitmask - Right
  #JOYB0        Joystick Switch Bitmask - Fire Button

The following functions are defined:

  j = joystk(n);        Returns the current state of joystick n.

                        If n is greater than or equal to #JYSTKS then $FF
                        will be returned. Otherwise, a byte representing the
                        state of the joystick switches will be returned.

                        Each switch in the joystick has a corresponding bit 
                        in the result which will be 1 if the switch is 
                        depressed or 0 if it is not.

                        The specific assignment of bits varies by system, so
                        the joystick bitmask constants should be used to check
                        the state of the individual switches. 
                        
                        The state of an individual switch can be determined by  
                        doing a bitwise and of the result with the corresponding
                        bitmask constant, which will result in a non-zero value
                        if and only if that switch is depressed.

Note: This library has no external dependencies.
