INT 2F U - DOS 3+ internal - GET CURRENT DIRECTORY STRUCTURE FOR DRIVE
        AX = 1217h
        SS = DOS DS (must be using a DOS internal stack)
        STACK: WORD drive (0 = A:, 1 = B:, etc)
Return: CF set on error
            (drive > LASTDRIVE)
        CF clear if successful
            DS:SI -> current directory structure for specified drive
        STACK unchanged