INT 33,18 - Set alternate subroutine call mask and address

	AX = 18h
	DX = offset to function
	CX = user interrupt mask:

		|F-8|7|6|5|4|3|2|1|0| user interrupt mask in CX
		  |  | | | | | | | `--- alt key pressed during event
		  |  | | | | | | `---- ctrl key pressed during event
		  |  | | | | | `----- shift key pressed during event
		  |  | | | | `------ right button up event
		  |  | | | `------- right button down event
		  |  | | `-------- left button up event
		  |  | `--------- left button down event
		  |  `---------- cursor moved
		  `------------ unused

	- entire mask is set to zero when INT 33,0 is called
	- up to three handlers may be defined with this call
	- mask should be cleared before program exit
	- when handler is called it 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