AH = 44h AL = 0F BL = new drive number (0=default, 1=A:, 2=B:, 3=C:, ...) on return AX = error code if CF set (see DOS ERROR CODES) AL = logical drive number (should be equal to BL input) = 0 if drive accessed by only one drive specifier = 1 if drive A:, 2 if drive B:, etc - changes the logical drive designator of the physical drive to be accessed next - implemented from DOS 3.2