| FDISK(8) | System Manager's Manual | FDISK(8) | 
fdisk —
| fdisk | [ -aBFfgIiSuv] [-0|-1|-2|-3|-Enumber [-s[id][/[start][/[size][/[bootmenu]]]]]]
      [-rbootfile |-wbootfile]
      [-Aptn_alignment[/ptn_0_offset]]
      [-bcylinders/heads/sectors]
      [-cbootcode]
      [-Tdisktype]
      [-tdisktab]
      [-zsectorsize]
      [device] | 
| fdisk | -l | 
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 partitions).
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 behaviour.
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:
-0-1-2-3-A
    ptn_alignment[/ptn_0_offset]-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-B-b
    cylinders/heads/sectors-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
    bootcodefdisk 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-Ffdisk 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.-ffdisk 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-I-I flag ignores overlapping error checks and does
      not fix them, allowing these incorrect configurations to be used.-ifdisk /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-r
    bootfile-S-s
    [id][/[start][/[size][/[bootmenu]]]]-0, -1,
      -2, -3,
      or -E
      number).-T
    disktype-t
    disktab-u-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-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-z
    sectorsizeWhen called with no arguments, it prints the partition table. An example follows:
    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:
fdisk -l to list the known
      partition types.-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.If the -v flag is specified, the beginning
    and end of each partition are also displayed as follows:
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).
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 entered.
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 volume.
fdisk -u
  /dev/rwd0dChange active MBR partition of /dev/rwd0d in interactive mode:
fdisk -a
  /dev/rwd0dInstall MBR bootcode /usr/mdec/mbr_bootsel into /dev/rwd0d:
fdisk -c
  /usr/mdec/mbr_bootsel /dev/rwd0dSet MBR partition data for slot 0 of /dev/rwd0d specifying values without prompt:
fdisk -f -u -0 -s
  169/63/2097089 /dev/rwd0dMake partition slot 0 of /dev/rwd0d active without prompt:
fdisk -f -a -0
  /dev/rwd0dInitialize 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 diskimgCreate 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 diskimgfdisk 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
  <julian@tfs.com>.
There are subtleties that the program detects that are not explained in this manual page.
| May 24, 2020 | NetBSD 10.1 |