Function  33h  Control-Break Check 
     Get or set control-break checking at CON 
entry   AH      33h 
	AL      00h     to test for break checking 
		01h     to set break checking 
			DL      00h     to disable break checking 
			        01h     to enable break checking 
		02h     internal, called by PRINT.COM (DOS 3.1) 
		03h     unknown 
		04h     unknown 
		05h     boot drive (DOS 4.0+) 
return  DL      00h     if break=off  
		01h     if break=on 
		(if AL=05h) boot drive, A=1, B=2, etc) 
	AL      0FFh    error