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

     fdisk - PC partition table maintenance program

     fdisk [-BIaistu] [-b bootcode] [-1234] [disk]
           fdisk -f configfile [-itv] [disk]

     In order for the BIOS to boot the kernel, certain conventions must be
     adhered to.  Sector 0 of the disk must contain boot code, a partition
     table, and a magic number.  BIOS partitions can be used to break the disk
     up into several pieces.  The BIOS brings in sector 0 and verifies the
     magic number.  The sector 0 boot code then searches the partition table
     to determine which partition is marked active.  This boot code then
     brings in the bootstrap from the active partition and, if marked
     bootable, runs it.  Under DOS, you can have one or more partitions with
     one active.  The DOS fdisk program can be used to divide space on the
     disk into partitions and set one active.

     The FreeBSD program fdisk serves a similar purpose to the DOS program.
     The first form is used to display partition information or to
     interactively edit the partition table.  The second is used to write a
     partition table using a configfile and is designed to be used by other

     Options are:
     Change the active partition only.  Ignored if -f is given.
     Get the boot code from the file bootcode.  Default is /boot/mbr.
     Reinitialize the boot code contained in sector 0 of the disk.  Ignored if
     -f is given.
     Set partition values using the file configfile.  The configfile always
     modifies existing partitions, unless -i is also given, in which case all
     existing partitions are deleted (marked as "unused") before the
     configfile is read.  The configfile can be "-", in which case stdin is
     read.  See CONFIGURATION FILE, below, for file syntax.

     WARNING: when -f is used, you are not asked if you really want to write
     the partition table (as you are in the interactive mode).  Use with
     Initialize sector 0 of the disk.  This implies -u, unless -f is given.
     Initialize the contents of sector 0 with one FreeBSD slice covering the
     entire disk.
     Print summary information and exit.
     Test mode; do not write partition values.  Generally used with the -f
     option to see what would be written to the partition table.  Implies -v.
     Is used for updating (editing) sector 0 of the disk.  Ignored if -f is
     Be verbose.  When -f is used, fdisk prints out the partition table that
     is written to the disk.
     Operate on a single fdisk entry only.  Ignored if -f is given.

     The final disk name can be provided as a `bare' disk name only, e.g.
     `da0', or as a fully qualified device node under /dev.  If omitted, the
     disks `ad0', and `da0' are being searched in that order, until one is
     being found responding.

     When called with no arguments, it prints the sector 0 partition table.
     An example follows:

             ******* Working on device /dev/ad0 *******
             parameters extracted from in-core disklabel are:
             cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

             parameters to be used for BIOS calculations are:
             cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

             Warning: BIOS sector numbering starts with sector 1
             Information from DOS bootblock is:
             The data for partition 1 is:
             sysid 165,(FreeBSD/NetBSD/386BSD)
                 start 495, size 380160 (185 Meg), flag 0
                     beg: cyl 1/ sector 1/ head 0;
                     end: cyl 768/ sector 33/ head 14
             The data for partition 2 is:
             sysid 164,(unknown)
                 start 378180, size 2475 (1 Meg), flag 0
                     beg: cyl 764/ sector 1/ head 0;
                     end: cyl 768/ sector 33/ head 14
             The data for partition 3 is:
             The data for partition 4 is:
             sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
                 start 380656, size 224234 (109 Meg), flag 80
                     beg: cyl 769/ sector 2/ head 0;
                     end: cyl 197/ sector 33/ head 14

     The disk is divided into three partitions that happen to fill the disk.
     The second partition overlaps the end of the first.  (Used for debugging

     sysid                 is used to label the partition.  FreeBSD reserves
                           the magic number 165 decimal (A5 in hex).

     start and size        fields provide the start address and size of a
                           partition in sectors.

     flag 80               specifies that this is the active partition.

     cyl, sector and head  fields are used to specify the beginning address
                           and end address for the partition.

     Note:                 these numbers are calculated using BIOS's
                           understanding of the disk geometry and saved in the

     The flags -i or -u are used to indicate that the partition data is to be
     updated, unless the -f option is used.  If the -f option is not used, the
     fdisk program will enter a 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 the above behavior.

     It displays each partition and ask if you want to edit it.  If you say
     yes, it will step through each field showing the old value and asking for
     a new one.  When you are done with a partition, fdisk will display it and
     ask if it is correct.  Fdisk will then proceed to the next entry.

     Getting the cyl, sector, and head fields correct is tricky.  So by
     default, they will be calculated for you; you can specify them if you

     After all the partitions are processed, you are given the option to
     change the active partition.  Finally, when the all the data for the
     first sector has been accumulated, you are asked if you really want to
     rewrite sector 0.  Only if you answer yes, will the data be written to

     The difference between the -u flag and -i flag is that the -u flag just
     edits the fields as they appear on the disk.  While the -i flag is used
     to "initialize" sector 0; it will setup the last BIOS partition to use
     the whole disk for FreeBSD; and make it active.

     The automatic calculation of starting cylinder etc. uses a set of figures
     that represent what the BIOS thinks is the geometry of the drive.  These
     figures are by default taken from the incore disklabel, but the program
     initially gives you an opportunity to change them.  This allows the user
     to create a bootblock that can work with drives that use geometry
     translation under the BIOS.

     If you hand craft your disk layout, please make sure that the FreeBSD
     partition starts on a cylinder boundary.  A number of decisions made
     later may assume this.  (This might not be necessary later.)

     Editing an existing partition will most likely 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.  There are subtleties that the program detects that are
     not fully explained in this manual page.

     When the -f option is given, a disk's partition table can be written
     using values from a configfile.  The syntax of this file is very simple.
     Each line is either a comment or a specification, and whitespace (except
     for newlines) are ignored:

     # comment ...
             Lines beginning with a "#" are comments and are ignored.

     g spec1 spec2 spec3
             Set the BIOS geometry used in partition calculations.  There must
             be three values specified, with a letter preceding each number:

             cnum    Set the number of cylinders to num.

             hnum    Set the number of heads to num.

             snum    Set the number of sectors/track to num.

             These specs can occur in any order, as the leading letter
             determines which value is which; however, all three must be

             This line must occur before any lines that specify partition

             It is an error if the following is not true:

                   1 <= number of cylinders
                   1 <= number of heads <= 256
                   1 <= number of sectors/track < 64

             The number of cylinders should be less than or equal to 1024, but
             this is not enforced, although a warning will be output.  Note
             that bootable FreeBSD partitions (the "/" filesystem) must lie
             completely within the first 1024 cylinders; if this is not true,
             booting may fail.  Non-bootable partitions do not have this

             Example (all of these are equivalent), for a disk with 1019
             cylinders, 39 heads, and 63 sectors:

                   g       c1019   h39     s63
                   g       h39     c1019   s63
                   g       s63     h39     c1019

     p partition type start length
             Set the partition given by partition (1-4) to type type, starting
             at sector start for length sectors.

             Only those partitions explicitly mentioned by these lines are
             modified; any partition not referenced by a "p" line will not be
             modified.  However, if an invalid partition table is present, or
             the -i option is specified, all existing partition entries will
             be cleared (marked as unused), and these "p" lines will have to
             be used to explicitly set partition information.  If multiple
             partitions need to be set, multiple "p" lines must be specified;
             one for each partition.

             These partition lines must occur after any geometry specification
             lines, if one is present.

             The type is 165 for FreeBSD partitions.  Specifying a partition
             type of zero is the same as clearing the partition and marking it
             as unused; however, dummy values (such as "0") must still be
             specified for start and length.

             Note: the start offset will be rounded upwards to a head boundary
             if necessary, and the end offset will be rounded downwards to a
             cylinder boundary if necessary.

             Example: to clear partition 4 and mark it as unused:

                   p       4       0       0       0

             Example: to set partition 1 to a FreeBSD partition, starting at
             sector 1 for 2503871 sectors (note: these numbers will be rounded
             upwards and downwards to correspond to head and cylinder

                   p       1       165     1       2503871

     a partition
             Make partition the active partition.  Can occur anywhere in the
             config file, but only one must be present.

             Example: to make partition 1 the active partition:

                   a       1

     /boot/mbr  The default boot code

     boot0cfg(8), disklabel(8)

     The default boot code will not necessarily handle all partition types
     correctly, in particular those introduced since MS-DOS 6.x.

     The entire program should be made more user-friendly.

     Throughout this man page, the term `partition' is used where it should
     actually be `slice', in order to conform with the terms used elsewhere.

     You cannot use this command to completely dedicate a disk to FreeBSD.
     The disklabel(8) command must be used for this.

FreeBSD 11.0-PRERELEASE         October 4, 1996        FreeBSD 11.0-PRERELEASE


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