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FD(4)                      Kernel Interfaces Manual                      FD(4)

       fd - floppy disk

       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

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

       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  indistinguishable 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  absolutely  sure that the device is accessed right.
       These devices 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)

       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.

       /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]

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

       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.

       Kees J. Bot (