INT 21,64 - Set Device Driver Look Ahead (Undocumented)

	AH = 64h
	AL = value for switch
	     00   causes DOS to perform look ahead to the device driver
		  before execution of INT 21,1, INT 21,8 and INT 21,A
	     other - no look ahead is performed

	returns ???

	- DOS internal; should not be called by user program
	- switch defaults to zero
	- undocumented; since DOS 3.3
	- some other subfunctions are know to have existed since DOS 3.2 but
	  details on their functionality are incomplete