NAME

     fd - floppy disk


DESCRIPTION

     The fd* devices refer to the Floppy disk  driver  using  the
     NEC  PD765  floppy  disk  controller.   These  diskettes are
     arrays of 512 byte sectors, although MINIX  3  always  works
     with  two sectors at a time due to its 1024 byte block size.
     You can read or write any number of bytes however,  MINIX  3
     takes   care   of  cutting  and  pasting  incomplete  blocks
     together.

     The driver is normally configured for two floppy  disk  dev-
     ices  fd0  and  fd1.   It  can  handle  two  more, but it is
     unlikely that the average PC can.

     On  the  first  access  to  an  fd  device  (by  open(2)  or
     mount(2)), the driver will execute a series of read tests to
     determine the floppy type.  This works  ok  for  all  floppy
     types  except  the  true  360k type, because it is indistin-
     guishable from the 720k type.  This only means that the size
     of the floppy is not estimated right.

     Bits 2-6 of the minor device number may be set to the floppy
     disk  type  to  make  it  known  to  the driver what type of
     diskette it is reading or  writing.   The  non-auto  devices
     should be used for formatting, or when one wants to be abso-
     lutely sure that the device is accessed right.   These  dev-
     ices exist for drive 0:

         type   device   minor   media
          0      fd0      0      autodetect
          1      pc0      4      360k, 5.25"
          2      at0      8      1.2M, 5.25"
          3      qd0      12     360k in a 720k, 5.25" drive
          4      ps0      16     720k, 3.5"
          5      pat0     20     360k in a 1.2M, 5.25" drive
          6      qh0      24     720k in a 1.2M, 5.25" drive
          7      PS0      28     1.44M, 3.5"

     Type 4 may also be used for  the  rarely  seen  720k,  5.25"
     floppies  (type 2 made them obsolete fast.)  Note that these
     "types" only describe the floppies from a software point  of
     view, type 1 and 4 drives use the same parameters.

     If the format bit (bit 7) is set, then the driver interprets
     write  commands  as track formatting requests.  This is used
     by the format(1) command.

     If the type bits are set to 28, 29,  30,  or  31,  then  the
     driver uses a partition table found in sector 0 to partition
     the floppy.  The partitions of fd0 may be accessed as  fd0p0
     through  fd0p3.   See controller(4) for a description of the
     partition table, and associated ioctl commands.


FILES

     /dev/fd[0-3],  /dev/pc[0-3],   /dev/at[0-3],   /dev/qd[0-3],
     /dev/ps[0-3],   /dev/pat[0-3],  /dev/qh[0-3],  /dev/PS[0-3],
     /dev/fd[0-3]p[0-3]


SEE ALSO

     format(1), controller(4), part(8).


BUGS

     The driver does not know the size of a 360k  diskette  in  a
     360k  5.25"  drive,  because it uses the 720k parameters for
     it.  So it will happily try to read past the end making  all
     kinds of interesting noises.  It's a good thing these drives
     are practically obsolete.

     The partition table is only read when the drive motor is off
     and  only  for  an  auto  or  partition  device.  The driver
     assumes that a floppy in a drive with a running motor  can't
     have been replaced all of a sudden.


AUTHOR

     Kees J. Bot (kjb@cs.vu.nl)