Game Port Pinout

  1  XY1 (+5v)
  2  Switch 1
  3  X1
  4  Switch 1 (GND)
  5  Switch 2 (GND)
  6  Y1
  7  Switch 2
  8  N.C.
  9  XY2 (+5v)
 10  Switch 4
 11  X2
 12  Switch 3&4 (GND)
 13  Y2
 14  Switch 3
 15  N.C.

INT 15 - BIOS - JOYSTICK SUPPORT (XT after 11/8/82,AT,XT286,PS)
	AH = 84h
	DX = subfunction
	    0000h read joystick switches
		Return: AL bits 7-4 = switch settings
	    0001h read positions of joysticks
		Return: AX = X position of joystick A
			BX = Y position of joystick A
			CX = X position of joystick B
			DX = Y position of joystick B
Return: CF set on error
	    AH = status
		80h invalid command (PC,PCjr)
		86h function not supported (other)
	CF clear if successful
Notes:	if no game port is installed, subfunction 0000h returns AL=00h (all
	  switches open) and subfunction 0001h returns AX=BX=CX=DX=0000h
	a 250kOhm joystick typically returns 0000h-01A0h