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

     fdisk - PC slice 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 slice table,
     and a magic number.  BIOS slices 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 slice table to
     determine which slice is marked active.  This boot code then brings in
     the bootstrap from the active slice and, if marked bootable, runs it.
     Under DOS, you can have one or more slices with one active.  The DOS
     fdisk program can be used to divide space on the disk into slices and set
     one active.

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

     Options are:
     Change the active slice 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 slice values using the file configfile.  The configfile always
     modifies existing slices, unless -i is also given, in which case all
     existing slices 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 slices table (as you are in the interactive mode).  Use with caution!
     Initialize sector 0 of the disk.  This implies -u, unless -f is given.
     Initialize the contents of sector 0 for one FreeBSD slice covering the
     entire disk.
     Print summary information and exit.
     Test mode; do not write slice values.  Generally used with the -f option
     to see what would be written to the slice 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 slice 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 searched in that order, until one is found to

     When called without options, fdisk prints the sector 0 slice 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 slices that happen to fill the disk.  The
     second slice overlaps the end of the first.  (Used for debugging

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

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

     flag 80               specifies that this is the active slice.

     cyl, sector and head  fields are used to specify the beginning and end
                           addresses of the slice.

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

     The -i and -u flags are used to indicate that the slice data is to be
     updated.  Unless the -f option is also given, fdisk will enter a
     conversational mode.  In this mode, no changes will be written to disk
     unless you explicitly tell fdisk to.

     Fdisk will display each slice and ask whether you want to edit it.  If
     you say yes, fdisk will step through each field, show you the old value,
     and ask you for a new one.  When you are done with the slice, fdisk will
     display it and ask you whether 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
     choose to though.

     After all the slices are processed, you are given the option to change
     the active slice.  Finally, when all the new data for sector 0 has been
     accumulated, you are asked to confirm whether you really want to rewrite

     The difference between the -u and -i flags is that the -u flag just edits
     (updates) the fields as they appear on the disk, while the -i flag is
     used to "initialize" sector 0; it will set up the last BIOS slice 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 the geometry of the drive is.  These
     figures are taken from the in-core disklabel by default, but fdisk
     initially gives you an opportunity to change them.  This allows you 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
     slice starts on a cylinder boundary.  A number of decisions made later
     may assume this.  (This might not be necessary later.)

     Editing an existing slice will most likely result in the loss of all data
     in that slice.

     You should run fdisk 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 fdisk detects that are not fully
     explained in this manual page.

     When the -f option is given, a disk's slice 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, as follows:

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

     g spec1 spec2 spec3
             Set the BIOS geometry used in slice 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 slice

             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 printed.  Note
             that bootable FreeBSD slices (the "/" filesystem) must lie
             completely within the first 1024 cylinders; if this is not true,
             booting may fail.  Non-bootable slices 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 slice type start length
             Set the slice given by slice (1-4) to type type, starting at
             sector start for length sectors.

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

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

             The type is 165 for FreeBSD slices.  Specifying a slice type of
             zero is the same as clearing the slice 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 slice 4 and mark it as unused:

                   p       4       0       0       0

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

                   p       1       165     1       2503871

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

             Example: to make slice 1 the active slice:

                   a       1

     /boot/mbr  The default boot code

     boot0cfg(8), disklabel(8), newfs(8)

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

     The entire program should be made more user-friendly.

     Most users new to FreeBSD do not understand the difference between slice
     and partition causing difficulty to adjust.

     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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