FDCONTROL(8) FreeBSD System Manager's Manual FDCONTROL(8)


fdcontroldisplay and modify floppy disk parameters


fdcontrol [ -F][ -d dbg][ -f fmt][ -s fmtstr][ -v] device


The fdcontrol utility allows the modification of the run-time behavior of the fdc(4) driver for the device specified by device.

Commands are implemented to query the current device density settings as well as the underlying device hardware as registered with the driver, to manipulate debugging levels, and to adjust the device density settings. All the operations that manipulate the kernel settings are restricted to the superuser (by the device driver), while all inquiry requests only require read access to device.

The device argument should always be given as a full path name, e.g. /dev/fd0.

Inquiry Commands

Running the fdcontrol utility without any of the optional flags will report the drive type that is registered with the device driver. In the shortest form, a single string describing the drive type will be returned. Possible values are: “ 360K”, “ 1.2M”, “ 720K”, “ 1.44M”, “ 2.88M”, or “ unknown”. This information is primarily intended to be easily parsable by scripts.

In order to add some descriptive text that makes the output better human readable, the flag -v can be added.

Specifying flag -F will report the device's density settings in a form that is suitable as input to the -s fmtstr option (see below). Again, together with -v, some more text will be returned, including the total capacity of the density settings in kilobytes.

Debug Control

If the fdc(4) driver was configured with the FDC_DEBUG option, by default, device debugging information is still disabled since it could produce huge amounts of kernel messages. It needs to be turned on using fdcontrol together with “ -d 1”, usually immediately before starting an operation on the respective device the debug information is wanted for, and later turned off again using “ -d 0”. Note that debugging levels are a driver's global option that will affect any drives and controllers using the fdc(4) driver, regardless which device was specified on the fdcontrol command line.

Density Control

The fdc(4) control utilities support two different options how to specify device density settings. The first form uses -f fmt to specify the format of the medium in kilobytes. Depending on the underlying drive type, the value is compared against a table of known commonly used device density settings for that drive, and if a match is found, those settings will be used. Currently, the following values for the respective drive types are acceptable:
  • 2.88M and 1.44M drives:
    KB sectrac secsize ncyls speed heads flags
    1721 21 2 (512) 82 500 2 MFM
    1476 18 2 (512) 82 500 2 MFM
    1440 18 2 (512) 80 500 2 MFM
    1200 15 2 (512) 80 500 2 MFM
    820 10 2 (512) 82 250 2 MFM
    800 10 2 (512) 80 250 2 MFM
    720 9 2 (512) 80 250 2 MFM
  • 1.2M drives:
    KB sectrac secsize ncyls speed heads flags
    1200 15 2 (512) 80 500 2 MFM
    1232 8 3 (1024) 77 500 2 MFM
    1476 18 2 (512) 82 500 2 MFM
    1440 18 2 (512) 80 500 2 MFM
    1200 15 2 (512) 80 500 2 MFM
    820 10 2 (512) 82 300 2 MFM
    800 10 2 (512) 80 300 2 MFM
    720 9 2 (512) 80 300 2 MFM
    360 9 2 (512) 40 300 2 MFM,2STEP
    640 8 2 (512) 80 300 2 MFM
  • 720K drives:
    KB sectrac secsize ncyls speed heads flags
    720 9 2 (512) 80 250 2 MFM
  • 360K drives:
    KB sectrac secsize ncyls speed heads flags
    360 9 2 (512) 40 250 2 MFM

The second form to specify a device density uses -s fmtstr to explicitly specify each parameter in detail. The argument fmtstr is a comma-separated list of values of the form:

sectrac, secsize, datalen, gap, ncyls, speed, heads, f_gap, f_inter, offs2, flags

The meaning of the parameters is:

The number of sectors per track.
The sector size code, 0 = 128 bytes (or less), 1 = 256 bytes, 2 = 512 bytes, 3 = 1024 bytes.
The actual sector size if the size code is 0, or the (ignored) value 0xFF for larger size codes.
The length of the gap 3 parameter for read/write operations.
The number of cylinders.
The transfer speed in kilobytes per second. Can be 250, 300, 500, or 1000, but each drive type only supports a subset of these values.
The number of heads.
The length of the gap 3 when formatting media.
The sector interleave to be applied when formatting. 0 means no interleave, 1 means 1:1 etc.
The offset of the sector numbers on side 2 (i.e., head number 1). Normally, sector numbering on both sides starts with 1.
A list from one of the following flag values:

Use MFM encoding.
Use FM (single-density) encoding.
Use 2 steps per each cylinder (for accessing 40-cylinder media in 80-cylinder drives).
Do not use 2 steps per cylinder, i.e., access each physical cylinder of the drive.
Use perpendicular recording (for 2.88 MB media, currently not supported).
Use longitudinal recording.

For any missing parameter, the current value will be used, so only actual changes need to be specified. Thus to turn off a flag bit (like +mfm which is the default for all drive types), the form with a leading minus sign must explicitly be used.


A simple inquiry about the drive type:

$ fdcontrol /dev/fd0 

Same as above, but with verbose output. Note that the result is about the drive type, as opposed to a device density, so it is independent from the actual subdevice being used for device.

$ fdcontrol -v /dev/fd0 
/dev/fd0: 1.44M drive (3.5" high-density)

Inquiry about the density settings:

$ fdcontrol -F /dev/fd0 

The verbose flag makes this human readable:

/dev/fd0: 1440 KB media type 
        Format:         18,512,0xff,0x1b,80,500,2,0x6c,1,0,+mfm 
        Sector size:    512 
        Sectors/track:  18 
        Heads/cylinder: 2 
        Cylinders/disk: 80 
        Transfer rate:  500 kbps 
        Sector gap:     27 
        Format gap:     108 
        Interleave:     1 
        Side offset:    0 
        Flags           <MFM>

As indicated, trailing commas in the parameter list may be omitted.

In order to access archaic 160 KB single-density (FM encoded) 5.25 media in a modern 1.2M drive, something like the following definition would be needed. (Note that not all controller hardware is actually capable of handling FM encoding at all.)

# fdcontrol -s 16,128,0x80,0x2,40,300,,0x10,,,-mfm,+2step /dev/fd1.1

It is still possible to hook up 8" drives to most modern floppy controllers, given the right cable magic. (On PC hardware, tell the BIOS that it is a 5.25" drive.) The classical 128/26/2/77 format can be read with this entry

fdcontrol -s  26,128,0x80,0x2,77,500,2,0x10,,,-mfm /dev/fd0




The fdcontrol utility appeared in FreeBSD 2.0, and was vastly overhauled in FreeBSD 5.0.


The program and this man page was contributed by Jörg Wunsch, Dresden.
December 25, 2001 FreeBSD