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FDISK(8)                    System Manager's Manual                   FDISK(8)

     fdisk - MS-DOS partition maintenance program

     fdisk [-aBFfgIiSuv] [-0 | -1 | -2 | -3 | -E number
           [-s [id][/[start][/[size][/[bootmenu]]]]]]
           [-r bootfile | -w bootfile] [-A ptn_alignment[/ptn_0_offset]]
           [-b cylinders/heads/sectors] [-c bootcode] [-T disktype]
           [-t disktab] [-z sectorsize] [device]
     fdisk -l

     The fdisk program is used to display or update the master boot record or
     MBR in the first sector (sector 0) of a disk that uses the MBR style of
     partitioning.  The following NetBSD ports use this style of disk
     partitioning: amd64, arc, bebox, cobalt, hpcarm, hpcmips, hpcsh, i386,
     macppc, mvmeppc, netwinder, ofppc, playstation2, and prep.

     The MBR contains bootable code, a partition table, an indication of which
     partition is `active', and (optionally, depending on the boot code) a
     menu for selecting a partition to be booted.  There can be at most 4
     partitions defined in sector 0, one of which can be an extended partition
     which can be split into any number of sub-partitions (then called logical

     The boot code in the MBR is usually invoked by the BIOS or firmware, and
     the MBR passes control to the next stage boot code stored in the first
     sector of the partition to be booted (the partition boot record or PBR).

     After booting, NetBSD does not use the partitioning done by fdisk,
     instead it uses a NetBSD disklabel saved in sector 1 of the NetBSD
     partition.  See mbrlabel(8) for a way of using information from the MBR
     to construct a NetBSD disklabel.

     The standard MBR boot code will only boot the `active' partition.
     However, NetBSD contains additional boot programs which allow the user to
     interactively select which of the partitions to boot.  The `mbr_ext' code
     will also boot NetBSD from an extended partition but will not work on old
     systems that do not support LBA reads, the `mbr_com0' and `mbr_com0_9600'
     will read and write from a serial port.  At the start the fdisk program
     will determine whether the disk sector 0 is valid as a boot sector.
     (This is determined by checking the magic number.)  If not, fdisk will
     initialise the boot code as well as the partition table.  During this,
     all four partitions will be marked empty.

     The flags -a, -i or -u are used to indicate that the partition data is to
     be updated.  The fdisk program will enter an interactive conversational
     mode.  This mode is designed not to change any data unless you explicitly
     tell it to; fdisk selects defaults for its questions to guarantee that

     If partition data is going to be updated and the disk carries GUID
     Partition Tables, fdisk will remove both primary and backup GPT headers
     from the disk unless the -g flag is specified.  See gpt(8) for
     information on how to manipulate GUID Partition Tables.

     fdisk will calculate the correct cylinder, head, and sector values for
     any partition you edit.  If you specify -v you will be asked whether you
     want to specify them yourself.

     Finally, when all the data for the first sector has been accumulated,
     fdisk will ask if you really want to write the new partition table.  Only
     if you reply affirmatively to this question will fdisk write anything to
     the disk.

     Available options:

     -0      Specify partition slot 0 to be printed or updated.

     -1      Specify partition slot 1 to be printed or updated.

     -2      Specify partition slot 2 to be printed or updated.

     -3      Specify partition slot 3 to be printed or updated.

     -A ptn_alignment[/ptn_0_offset]
             Specify the alignment for all partitions and optionally the
             offset for the first partition of the disk and of logical
             partitions.  If ptn_alignment is specified and ptn_0_offset is
             not specified, then the offset is set to the alignment.  If -A is
             not specified, then the alignment of the first partition is
             inspected.  If it ends on a 2048 sector boundary, then the
             alignment is set to 2048, if the start is a power of 2 less than,
             or equal to 2048 then the offset is set to the start sector.  If
             the first partition is not defined then the alignment and offset
             for disks larger than 128GB is set to 2048 (1MB).  In all other
             cases the alignment default to a cylinder and the offset to a
             track (both using the BIOS geometry).  The 1MB alignment is the
             same as that used by recent windows versions.

     -a      Change the active partition.  In interactive mode this question
             will be asked after the partitions have been processed.

     -B      On an i386 or amd64 system, interactively update the boot
             selector settings.  These include the default boot partition and
             the timeout value for the prompt.  (The boot selector permits the
             user to interactively select the boot partition, and thus which
             operating system is run, at system boot time; see x86/mbr(8) for
             more information.)

     -b cylinders/heads/sectors
             Specify the BIOS geometry parameters for cylinders, heads, and
             sectors.  It is used only in conjunction with the -u flag.  If
             not specified the BIOS geometry will be obtained using sysctl
             (i386 and amd64) or by solving the simultaneous equations from
             the existing partition information.  If that fails then either
             the geometry from the disklabel or 63 sectors and 16 heads is
             used.  For modern disks larger than about 8GB, and where the BIOS
             is configured to use LBA-Assisted translation, a setting of -b
             1023/255/63 is likely to work.

     -c bootcode
             Specify the filename that fdisk should read the bootcode from.
             If the name of a directory is specified, then fdisk will look for
             files with the default names in that directory.  The default is
             to read from /usr/mdec/mbr, /usr/mdec/mbr_bootsel or
             /usr/mdec/mbr_ext depending on whether bootmenu was specified for
             any partitions on an i386 machine, and leave the bootcode empty
             for other machines.

     -E number
             Specify logical partition number to be printed or updated.  If
             the specified logical partition does not exist on updating
             partition data an additional logical partition will be created.

     -F      Indicate that device is a regular file.  Unless the geometry of
             device is told to fdisk by -T disktype, fdisk will count the
             512-byte sectors in device and produce a fake geometry.  If
             device is a regular file, -F will be used implicitly.

     -f      Run fdisk in a non-interactive mode.  In this mode, you can only
             change the disk parameters by using the -b flag.  This is
             provided only so scripts or other programs may use fdisk as part
             of an automatic installation process.

             Using the -f flag with -u makes it impossible to specify the
             starting and ending cylinder, head, and sector fields (only start
             and size can be specified by -s option).  They will be
             automatically computed using the BIOS geometry.

     -g      Preserve existing GPT headers when updating partitions.

     -I      Ignore errors from overlapping partitions.  Some devices (cameras
             CHDK) require overlapping partitions to support bigger than 4GB
             cards.  The -I flag ignores overlapping error checks and does not
             fix them, allowing these incorrect configurations to be used.

     -i      Explicitly request initialisation of the master boot code
             (similar to what fdisk /mbr does under MS-DOS), even if the magic
             number in the first sector is ok.  The partition table is left
             alone by this (but see above).

     -l      Lists known sysid values and exit.

     -r bootfile
             Read the boot record from file bootfile instead of the specified
             disk.  The geometry information used is still that of the disk
             volume.  Any changes are written back to the file.

     -S      When used with no other flags print a series of /bin/sh commands
             for setting variables to the partition information.  This could
             be used by installation scripts.

     -s [id][/[start][/[size][/[bootmenu]]]]
             Specify the partition id, start, size, and bootmenu.  If the
             optional arguments are not provided, they stay as before or use
             the same defaults as the interactive mode, if new.  This flag
             requires the use of a partition selection flag (-0, -1, -2, -3,
             or -E number).

     -T disktype
             Use the disklabel disktype instead of the disklabel on device.

     -t disktab
             Read disktype from the named disktab(5) file instead of from

     -u      Update partition data, including id, start, and size.  Unless -f
             option (non-interactive mode) is specified, fdisk will display
             the partitions and interactively ask which one you want to edit.
             fdisk will step through each field showing the old value and
             asking for a new one.  The start and size can be specified in
             blocks (NN), cylinders (NNc or NNcyl), megabytes (NNm or NNMB),
             or gigabytes (NNg or NNGB), values in megabytes and gigabytes
             will be rounded to the nearest cylinder boundary.  The size may
             be specified as $ in which case the partition will extend to the
             end of the available free space.

             In a non-interactive mode (specified by -f option), partition
             data should be specified by -s option.  A partition selection
             option (-0, -1, -2, -3, or -E number) should also be specified to
             select a partition slot to be updated.

             fdisk will not allow you to create partitions which overlap.  If
             -u and -s are specified in a non-interactive mode then the
             details of the specified partition will be changed.  Any other
             partitions which overlap the requested part of the disk will be
             silently deleted.

             If bootmenu is specified for any partition fdisk will determine
             whether the installed boot code supports the bootselect code, if
             it does not you will be asked whether you want to install the
             required boot code.  To remove a bootmenu label, simply press
             <space> followed by <return>.

     -v      Be more verbose, specifying -v more than once may increase the
             amount of output.

             Using -v with -u allows the user to change more parameters than
             normally permitted.

     -w bootfile
             Write the modified partition table to file bootfile instead of
             the disk.

     -z sectorsize
             Specify a sector size other than 512, for devices that only
             support larger sector sizes.  The sector size needs to be a power
             of two greater than 512.

     When called with no arguments, it prints the partition table.  An example

         Disk: /dev/rwd0d
         NetBSD disklabel disk geometry:
         cylinders: 16383, heads: 16, sectors/track: 63 (1008 sectors/cylinder)
         total sectors: 40032696

         BIOS disk geometry:
         cylinders: 1023, heads: 255, sectors/track: 63 (16065 sectors/cylinder)
         total sectors: 40032696

         Partition table:
         0: NetBSD (sysid 169)
             bootmenu: net 1.5.
             start 4209030, size 8289540 (4048 MB, Cyls 262-778), Active
         1: Primary DOS with 32 bit FAT (sysid 11)
             bootmenu: win98
             start 63, size 4208967 (2055 MB, Cyls 0-262)
         2: NetBSD (sysid 169)
             bootmenu: current
             start 32515560, size 7517136 (3670 MB, Cyls 2024-2491/234/40)
         3: Ext. partition - LBA (sysid 15)
             start 12498570, size 20016990 (9774 MB, Cyls 778-2024)
         Extended partition table:
         E0: NetBSD (sysid 169)
             bootmenu: test
             start 12498633, size 12305727 (6009 MB, Cyls 778-1544)
         E1: Primary DOS with 32 bit FAT (sysid 11)
             start 24804423, size 4096512 (2000 MB, Cyls 1544-1799)
         E2: Primary DOS with 32 bit FAT (sysid 11)
             start 28900998, size 3614562 (1765 MB, Cyls 1799-2024)
         Bootselector enabled, infinite timeout.
         First active partition: 0

     This example disk is divided into four partitions, the last of which is
     an extended partition.  The logical partitions of the extended partition
     are also shown.  In this case there is no free space in either the disk
     or in the extended partition.

     The various fields in each partition entry are:
         ptn_number: id_name (sysid id_number)
             bootmenu: bootmenu
             start start, size size (MB MB, Cyls first-next) [, Active]

               is the number of the partition.

     id_name   is the name of the filesystem type or operating system that
               uses this partition.

               is the number that identifies the partition type.  169 decimal
               is used for NetBSD partitions, 15 decimal to create an extended
               partition and 0 to mark a partition as unused.  Use fdisk -l to
               list the known partition types.

     bootmenu  is the menu prompt output by the interactive boot code for this
               partition.  This line is omitted if the prompt is not defined.

     start, size
               are the start address and size of the partition in sectors.

     MB        is the size of the partition in megabytes.

     first, next
               are the bounds of this partition displayed as
               cylinder/head/sector.  If the partition starts (or ends) on a
               cylinder boundary the head and sector values are omitted.  If
               -v is not specified the start of logical partitions and the
               first partition on the disk are rounded down to include the
               mandatory red tape in the preceding track.

     Active    is output if this is the active partition.

     If the -v flag is specified, the beginning and end of each partition are
     also displayed as follows:
         beg: cylinder cylinder, head head, sector sector
         end: cylinder cylinder, head head, sector sector

     cylinder, head, sector
               are the beginning or ending address of a partition.

               Note: these numbers are read from the bootblock, so are the
               values calculated by a previous run of fdisk.

     fdisk attempts to check whether each partition is bootable, by checking
     the magic number and some other characteristics of the first sector of
     each partition (the PBR).  If the partition does not appear to be
     bootable, fdisk will print a line containing "PBR is not bootable"
     followed by an error message.  If the partition is bootable, and if the
     -v flag is specified, fdisk will print "PBR appears to be bootable".  If
     the -v flag is specified more than once, fdisk will print the heading
     "Information from PBR:" followed by one or more lines of information
     gleaned from the PBR; this additional information may be incorrect or
     misleading, because different operating systems use different PBR
     formats.  Note that, even if no errors are reported, an attempt to boot
     from the partition might fail.  NetBSD partitions may be made bootable
     using installboot(8).

     This program is only available (and useful) on systems with PC-platform-
     style MBR partitioning.

     Traditionally the partition boundaries should be on cylinder boundaries
     using the BIOS geometry, with the exception of the first partition, which
     traditionally begins in the second track of the first cylinder (cylinder
     0, head 1, sector 1).  Although the BIOS geometry is typically different
     from the geometry reported by the drive, neither will match the actual
     physical geometry for modern disks (the actual geometry will vary across
     the disk).  Keeping the partition boundaries on cylinder boundaries makes
     partitioning a driver easier as only relatively small numbers need be

     The automatic calculation of the starting cylinder and other parameters
     uses a set of figures that represent what the BIOS thinks is the geometry
     of the drive.  The default values should be correct for the system on
     which fdisk is run; however, if you move the disk to a different system,
     the BIOS of that system might use a different geometry translation.

     If you run the equivalent of fdisk on a different operating system then
     the bootmenu strings associated with extended partitions may be lost.

     Editing an existing partition is risky, and may cause you to lose all the
     data in that partition.

     You should run this program interactively once or twice to see how it
     works.  This is completely safe as long as you answer the last question
     in the negative.  You can also specify -w bootfile to write the output to
     a file and later specify -r bootfile to read back the updated
     information.  This can be done without having write access to the disk

     /usr/mdec/mbr          Default location of i386 bootcode
     /usr/mdec/mbr_bootsel  Default location of i386 bootselect code
     /usr/mdec/mbr_ext      Default location of i386 bootselect for extended
                            partitions (i.e., NetBSD on logical partitions)

     Update MBR partition data of /dev/rwd0d in interactive mode:

           fdisk -u /dev/rwd0d

     Change active MBR partition of /dev/rwd0d in interactive mode:

           fdisk -a /dev/rwd0d

     Install MBR bootcode /usr/mdec/mbr_bootsel into /dev/rwd0d:

           fdisk -c /usr/mdec/mbr_bootsel /dev/rwd0d

     Set MBR partition data for slot 0 of /dev/rwd0d specifying values without

           fdisk -f -u -0 -s 169/63/2097089 /dev/rwd0d

     Make partition slot 0 of /dev/rwd0d active without prompt:

           fdisk -f -a -0 /dev/rwd0d

     Initialize and create MBR partition data using bootcode
     destdir/usr/mdec/mbr without prompt against 1GB disk image file diskimg:

           fdisk -f -i -b 130/255/63 -c destdir/usr/mdec/mbr -F diskimg

     Create MBR partition data for slot 0 which has an active NetBSD partition
     using whole disk without prompt against 1GB disk image file diskimg:

           fdisk -f -a -u -0 -s 169/63/2097089 -F diskimg

     disktab(5), boot(8), disklabel(8), gpt(8), installboot(8), mbrlabel(8),

     A version of fdisk first appeared in the Mach Operating System.  It was
     subsequently ported to 386BSD.

     fdisk for Mach Operating System was written by Robert Baron
     <rvb@cs.cmu.edu>.  It was ported to 386BSD by Julian Elischer

     The word `partition' is used to mean both an MBR partition and a NetBSD
     partition, sometimes in the same sentence.

     There are subtleties that the program detects that are not explained in
     this manual page.

NetBSD 9.99                      May 24, 2020                      NetBSD 9.99