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NetBSD 6.1.5 - man page for fdisk (netbsd section 8)

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

     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] [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, 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 con-
     trol 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 parti-
     tions 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.  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 ques-
     tion 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 isn't
	     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 isn't 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.  (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 informa-

     -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 simultaenous equations from
	     the existing partition information.  If that fails then either the geometry from the
	     disklabel or 63 sectors and 16 heads is used.

     -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 doesn't 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.

     -I      Ignore errors from overlapping partitions.  Some devices (cameras CHDK) require
	     overlapping partitions to support bigger than 4GB cards.  The -I flag ignores over-
	     lapping 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 file
	     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 vari-
	     ables to the partition information.  This could be used by installation scripts.

     -s id/start/size[/bootmenu]
	     Specify the partition id, start, size, and optionally bootmenu.  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 /etc/disktab.

     -u      Update partition data, including id, start, and size.  Unless -f option
	     (non-interactive mode) is specified, fdisk will display the partitions and interac-
	     tively 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 speci-
	     fied 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 spec-
	     ified 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 doesn't 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 file
	     Write the modified partition table to file file instead of the disk.

     When 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 parti-
     tion.  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 parti-
	       tion starts (or ends) on a cylinder boundary the head and sector values are omit-
	       ted.  If -v is not specified the start of logical partitions and the first parti-
	       tion on the disk are rounded down to include the mandatory red tape in the preced-
	       ing 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
	 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 parti-
     tion 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 partition-

     Traditionally the partition boundaries should be on cylinder boundaries using the BIOS geom-
     etry, 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 fig-
     ures 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 par-

     You should run this program interactively once or twice to see how it works.  This is com-
     pletely 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 prompt:

	   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), mbrlabel(8)

     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.

BSD					 December 1, 2011				      BSD

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