INT 21 - Stacker - GET DEVICE DRIVER ADDRESS AND SET VOLUME NUMBER
        AX = 4404h
        BL = drive number (00h = default, 01h = A:, etc)
        CX = 0004h
        DS:DX -> DWORD buffer to receive device driver address
Note:   In addition to returning the address of the Stacker device driver,
          this call also sets the volume number at offset 3Eh in the device
          driver (see INT 25/AX=CDCDh)