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