The -i and -u flags are used to indicate that the slice data is to be
     updated.  Unless the -f option is also given, fdisk will enter a conver-
     sational mode.  In this mode, no changes will be written to disk unless
     you explicitly tell fdisk to.

     The fdisk utility will display each slice and ask whether you want to
     edit it.  If you say yes, fdisk will step through each field, show you
     the old value, and ask you for a new one.  When you are done with the
     slice, fdisk will display it and ask you whether it is correct.  It will
     then proceed to the next entry.

     Getting the cyl, sector, and head fields correct is tricky, so by
     default, they will be calculated for you; you can specify them if you
     choose to though.

     After all the slices are processed, you are given the option to change
     the ``active'' slice.  Finally, when all the new data for sector 0 has
     been accumulated, you are asked to confirm whether you really want to re-
     write it.