INT 33,19 - Get User Alternate Interrupt Address

	AX = 19h
	CX = user interrupt call mask (see below)


	on return:
	BX:DX = user interrupt vector
	CX = user interrupt call mask or zero if not found

		│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

	- returns vector to function defined by INT 33,18
	- searches the event handlers defined by INT 33,18 for a routine
	  with a call mask matching CX