Game I/O Adapter Port

 Port 201H is designated as the Game Adapter Port or Joystick Port.  It is
 not supported by the PC/XT BIOS, but AT BIOS service INT 15H 84H provides
 an easy-to-use interface for the adapter.

 It is an analog-to-digital converter which can be used by scientific
 measuring tools and other devices which present an analog (resistive)
 input to the interface.  It accepts up to four digital inputs (on/off data
 such as the press of a button) and up to four resistive inputs (such as an
 X-ordinate of a joystick or a temperature value from an electronic

 The value obtained by an IN from 201H is as diagrammed:

   ╚════╦════╝ ╚═════════╩═► Coordinates (resistive, time-dependent inputs)
        ╚══════════════════► Buttons/Triggers (digital inputs)

 You can read the buttons (digital inputs)  with:

        mov   dx,201H
        out   dx,al      ;initiate transaction; AL=anything
        in    al,dx      ;read bits 4-7 for buttons: 0=pressed, 1=open

 The joysticks, paddles, measuring devices (resistive inputs) are read by
 tracking the time that an X or Y bit stays high (1) after an OUT 201,xxx.
 To read an individual resistive input (e.g., joystick A, X ordinate):

        mov   dx,201H
        out   dx,al      ;initiate transaction  AL=anything
        mov   cx,-1      ;set resistance counter for first loop
 again: in    al,dx      ;read the settings
        inc   cx         ;bump counter
        test  al,1       ;has X-ordinate for joystick A gone low?
        jnz   again      ;loop until it does (when bit=0, we're done)

 The delay value accumulated in CX will indicate the X-ordinate position of
 joystick A.  The resulting value is CPU-dependent.  To ensure accuracy, it
 would be better to use a hardware timer, rather than a loop counter.

See Also: INT 15H 84H (joystick support)
          I/O Port Map