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fdc — PC architecture floppy disk controller driver
This driver provides access to floppy disk drives. Floppy disks using
either FM (single-density) or MFM (double or high-density) recording can
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
created 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
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
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
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
performed 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
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
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
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-
density 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
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
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
argument 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,
controller-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
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
configuration 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.