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