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FDC(4)			 BSD Kernel Interfaces Manual			FDC(4)

     fdc — PC architecture floppy disk controller driver

     device fdc

     In /boot/device.hints:

   Device Usage
     This driver provides access to floppy disk drives.	 Floppy disks using
     either FM (single-density) or MFM (double or high-density) recording can
     be handled.

     Floppy disk controllers can connect up to four drives each.  The fdc
     driver can currently handle up to two drives per controller (or four
     drives on ACPI).  Upon driver initialization, an attempt is made to find
     out the type of the floppy controller in use.  The known controller types
     are either the original NE765 or i8272 chips, or alternatively enhanced
     controllers that are compatible with the NE72065 or i82077 chips.	These
     enhanced controllers (among other enhancements) implement a FIFO for
     floppy data transfers that will automatically be enabled once an enhanced
     chip has been detected.  This FIFO activation can be disabled using the
     per-controller flags value of 0x1.

     By default, this driver creates a single device node /dev/fdN for each
     attached drive with number N.  For historical reasons, device nodes that
     use a trailing UFS-style partition letter (ranging from ‘a’ through ‘h’)
     can also be accessed, which will be implemented as symbolic links to the
     main device node.

     Accessing the main device node will attempt to autodetect the density of
     the available medium for multi-density devices.  Thus it is possible to
     use either a 720 KB medium or a 1440 KB medium in a high-density 3.5 inch
     standard floppy drive.  Normally, this autodetection will only happen
     once at the first call to open(2) for the device after inserting the
     medium.  This assumes the drive offers proper changeline support so media
     changes can be detected by the driver.  To indicate a drive that does not
     have the changeline support, this can be overridden using the per-drive
     device flags value of 0x10 (causing each call to open(2) to perform the

     When trying to use a floppy device with special-density media, other
     device nodes can be created, of the form /dev/fdN.MMMM, where N is the
     drive number, and MMMM is a number between one and four digits describing
     the device density.  Up to 15 additional subdevices per drive can be cre‐
     ated that way.  The administrator is free to decide on a policy how to
     assign these numbers.  The two common policies are to either implement
     subdevices numbered 1 through 15, or to use a number that describes the
     medium density in kilobytes.  Initially, each of those devices will be
     configured to the maximal density that is possible for the drive type
     (like 1200 KB for 5.25 inch HD drives or 1440 KB for 3.5 inch HD drives).
     The desired density to be used on that subdevice needs to be configured
     using fdcontrol(8).

     Drive types are configured using the lower four bits of the per-drive
     device flags.  The following values can be specified:

	   1   5.25 inch double-density device with 40 cylinders (360 KB
	       native capacity)

	   2   5.25 inch high-density device with 80 cylinders (1200 KB native

	   3   3.5 inch double-density device with 80 cylinders (720 KB native

	   4   3.5 inch high-density device with 80 cylinders (1440 KB native

	   5   3.5 inch extra-density device with 80 cylinders (2880 KB native
	       capacity, usage currently restricted to at most 1440 KB media)

	   6   Same as type 5, available for compatibility with some BIOSes

     On IA32 architectures, the drive type can be specified as 0 for the
     drives.  In that case, the CMOS configuration memory will be consulted to
     obtain the value for that drive.  The ACPI probe automatically determines
     these values via the _FDE and _FDI methods, but this can be overridden by
     specifying a drive type hint.

     Normally, each configured drive will be probed at initialization time,
     using a short seek sequence.  This is intended to find out about drives
     that have been configured but are actually missing or otherwise not
     responding.  (The ACPI probe method does not perform this seek.)  In some
     environments (like laptops with detachable drives), it might be desirable
     to bypass this drive probe, and pretend a drive to be there so the driver
     autoconfiguration will work even if the drive is currently not present.
     For that purpose, a per-drive device flags value of 0x20 needs to be

   Programming Interface
     In addition to the normal read and write functionality, the fdc driver
     offers a number of configurable options using ioctl(2).  In order to
     access any of this functionality, programmers need to include the header
     file <sys/fdcio.h> into their programs.  The call to open(2) can be per‐
     formed in two possible ways.  When opening the device without the
     O_NONBLOCK flag set, the device is opened in a normal way, which would
     cause the main device nodes to perform automatic media density selection,
     and which will yield a file descriptor that is fully available for any
     I/O operation or any of the following ioctl(2) commands.

     When opening the device with O_NONBLOCK set, automatic media density
     selection will be bypassed, and the device remains in a half-opened
     state.  No actual I/O operations are possible, but many of the ioctl(2)
     commands described below can be performed.	 This mode is intended for
     access to the device without the requirement to have an accessible media
     present, like for status inquiries to the drive, or in order to format a
     medium.  O_NONBLOCK needs to be cleared before I/O operations are possi‐
     ble on the descriptor, which requires a prior specification of the den‐
     sity using the FD_STYPE command (see below).  Operations that are not
     allowed on the half-opened descriptor will cause an error value of

     The following ioctl(2) commands are currently available:

     FD_FORM	Used to format a floppy disk medium.  Third argument is a
		pointer to a struct fd_formb specifying which track to format,
		and which parameters to fill into the ID fields of the floppy
		disk medium.

     FD_GTYPE	Returns the current density definition record for the selected
		device.	 Third argument is a pointer to struct fd_type.

     FD_STYPE	Adjusts the density definition of the selected device.	Third
		argument is a pointer to struct fd_type.  For the fixed-den‐
		sity subdevices (1 through 15 per drive), this operation is
		restricted to a process with superuser privileges.  For the
		auto-selecting subdevice 0, the operation is temporarily
		allowed to any process, but this setting will be lost again
		upon the next autoselection.  This can be used when formatting
		a new medium (which will require to open the device using
		O_NONBLOCK, and thus to later adjust the density using

     FD_GOPTS	Obtain the current drive options.  Third argument is a pointer
		to int, containing a bitwise union of the following possible
		flag values:

		FDOPT_NORETRY	Do not automatically retry operations upon

		FDOPT_NOERRLOG	Do not cause “hard error” kernel logs for
				failed I/O operations.

		FDOPT_NOERROR	Do not indicate I/O errors when returning from
				read(2) or write(2) system calls.  The caller
				is assumed to use FD_GSTAT calls in order to
				inquire about the success of each operation.
				This is intended to allow even erroneous data
				from bad blocks to be retrieved using normal
				I/O operations.

		FDOPT_AUTOSEL	Device performs automatic density selection.
				Unlike the above flags, this one is read-only.

     FD_SOPTS	Set device options, see above for their meaning.  Third argu‐
		ment is a pointer to int.  Drive options will always be
		cleared when closing the descriptor.

     FD_DEBUG	Set the driver debug level.  Third argument is a pointer to
		int, level 0 turns off all debugging.  Only applicable if the
		driver has been configured with options FDC_DEBUG.

     FD_CLRERR	Clear the internal low-level error counter.  Normally, con‐
		troller-level I/O errors are only logged up to FDC_ERRMAX
		errors (currently defined to 100).  This command resets the
		counter.  Requires superuser privileges.

     FD_READID	Read one sector ID field from the floppy disk medium.  Third
		argument is a pointer to struct fdc_readid, where the read
		data will be returned.	Can be used to analyze a floppy disk

     FD_GSTAT	Return the recent floppy disk controller status, if available.
		Third argument is a pointer to struct fdc_status, where the
		status registers (ST0, ST1, ST2, C, H, R, and N) are being
		returned.  EINVAL will be caused if no recent status is avail‐

     FD_GDTYPE	Returns the floppy disk drive type.  Third argument is a
		pointer to enum fd_drivetype.  This type is the same as being
		used in the per-drive configuration flags, or in the CMOS con‐
		figuration data or ACPI namespace on IA32 systems.

     /dev/fd*  floppy disk device nodes

     fdformat(1), fdread(1), fdwrite(1), ioctl(2), open(2), read(2), write(2),

     This man page was initially written by Wilko Bulte, and later vastly
     rewritten by Jörg Wunsch.

BSD				 May 11, 2006				   BSD

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