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