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FDISK(8) NetBSD System Manager's Manual FDISK(8)
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fdisk -- MS-DOS partition maintenance program
fdisk [-aBFfIiSuv] [-0 | -1 | -2 | -3 | -E number
[-s [id][/[start][/[size][/[bootmenu]]]]]] [-r file | -w file]
[-A ptn_alignment[/ptn_0_offset]] [-b cylinders/heads/sectors]
[-c bootcode] [-T disktype] [-t disktab] [-z sectorsize] [device]
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 parti-
tioning: 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 par-
titions 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 parti-
tion. 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. How-
ever, 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 Parti-
tion Tables, fdisk will remove both primary and backup GPT headers from
the disk. See gpt(8) for information on how to manipulate GUID Partition
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
-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.
Specify the alignment for all partitions and optionally the off-
set for the first partition of the disk and of logical parti-
tions. 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 align-
ment 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 selec-
tor 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 mbr(8) for more
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.
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.
Specify logical partition number to be printed or updated. If
the specified logical partition does not exist on updating parti-
tion 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 pro-
vided 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 automati-
cally computed using the BIOS geometry.
-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 (simi-
lar to what fdisk /mbr does under MS-DOS), even if the magic num-
ber in the first sector is ok. The partition table is left alone
by this (but see above).
-l Lists known sysid values and exit.
Read the boot record from file file 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.
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).
Use the disklabel disktype instead of the disklabel on device.
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 ask-
ing for a new one. The start and size can be specified in blocks
(NN), cylinders (NNc or NNcyl), megabytes (NNm or NNMB), or giga-
bytes (NNg or NNGB), values in megabytes and gigabytes will be
rounded to the nearest cylinder boundary. The size may be speci-
fied 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
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
Write the modified partition table to file file instead of the
Specify a sector size other than 512, for devices that only sup-
port 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
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
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)
start 63, size 4208967 (2055 MB, Cyls 0-262)
2: NetBSD (sysid 169)
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)
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)
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.
are the start address and size of the partition in sectors.
MB is the size of the partition in megabytes.
are the bounds of this partition displayed as cylin-
der/head/sector. If the partition starts (or ends) on a cylin-
der 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'' fol-
lowed 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 for-
mats. Note that, even if no errors are reported, an attempt to boot from
the partition might fail. NetBSD partitions may be made bootable using
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 file to write the output to a
file and later specify -r file to read back the updated information.
This can be done without having write access to the disk volume.
/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), mbr(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
<email@example.com>. 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 9.1 April 9, 2019 NetBSD 9.1