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FDC(4)		       FreeBSD Kernel Interfaces Manual			FDC(4)

     fdc -- PC architecture floppy disk	controller driver

     device fdc

     In	/boot/device.hints:"isa"

   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 de-
     vice 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 de-
     vice flags.  The following	values can be specified:

	   1   5.25 inch double-density	device with 40 cylinders (360 KB na-
	       tive 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 re-
     sponding.	(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 ac-
     cess 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 se-
     lection 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 possible on the
     descriptor, which requires	a prior	specification of the density using the
     FD_STYPE command (see below).  Operations that are	not allowed on the
     half-opened descriptor will cause an error	value of EAGAIN.

     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 al-
		lowed 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_CLRERR	Clear the internal low-level error counter.  Normally, con-
		troller-level I/O errors are only logged up to FDC_ERRMAX er-
		rors (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 re-
		turned.	 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

     fdread(1),	fdwrite(1), ioctl(2), open(2), read(2),	write(2),
     fdcontrol(8), fdformat(8)

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

FreeBSD	13.0			 April 7, 2017			  FreeBSD 13.0


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