INT 15,20 - PRINT.COM Critical Region Flag (undocumented DOS 3.x+)

	AH = 20h
	AL = 00  disable user critical region flag
	   = 01  enable user critical region flag
	ES:BX = pointer to user critical region flag byte (AH = 1)


	returns nothing


	- provides PRINT.COM with the address of a byte to increment on
	  entry to DOS functions
	- see bibliography reference for "Undocumented DOS"