INT 33,C - Set Mouse User Defined Subroutine and Input Mask

	AX = 0C
	ES:DX = far pointer to user interrupt
	CX = user interrupt mask:

	   │F-5│4│3│2│1│0│ user interrupt mask in CX
	     │	│ │ │ │ └─── cursor position changed
	     │	│ │ │ └──── left button pressed
	     │	│ │ └───── left button released
	     │	│ └────── right button pressed
	     │	└─────── right button released
	     └───────── unused

	returns nothing


	- routine at ES:DX is called if an event occurs and the
	  corresponding bit specified in user mask is set
	- routine at ES:DX receives parameters in the following
	  registers:

	  AX = condition mask causing call
	  CX = horizontal cursor position
	  DX = vertical cursor position
	  DI = horizontal counts
	  SI = vertical counts
	  DS = mouse driver data segment
	  BX = button state:

	     │F-2│1│0│
	       │  │ └─── left button (1 = pressed)
	       │  └──── right button (1 = pressed)
	       └────── unused

	- initial call mask and user routine should be restore on exit
	  from user program
	- user program may need to set DS to it's own segment
	- see	INT 33,14