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


NAME

     fdisk -- DOS partition maintenance program


SYNOPSIS

     fdisk [-ieu] [-f mbrname] [-c cylinders] [-h heads] [-s sectors]
           [-S size] [-b size] device


DESCRIPTION

     In order for the BIOS to boot the kernel, certain conventions must be
     adhered to.  Sector 0 of a bootable hard disk must contain boot code, an
     MBR partition table, and a magic number (0xAA55).  These MBR partitions
     (also known as BIOS partitions) can be used to break the disk up into
     several pieces.

     The BIOS loads sector 0 of the boot disk into memory, verifies the magic
     number, and begins executing the code at the first byte.  The normal DOS
     MBR boot code searches the MBR partition table for an ``active'' parti-
     tion (indicated by a `*' in the first column), and if one is found, the
     boot block from that partition is loaded and executed in place of the
     original (MBR) boot block.

     The options are as follows:

     -i      Initialize the MBR sector.

     -a style
             Specify an automatic partitioning style.

     -e      Edit existing MBR sectors.

     -f mbrname
             Specifies an alternate MBR template file.

     -u      Update MBR code, preserving existing partition table.

     -y      Do not ask for confirmation before writing.

     -d      Dump partition table in a format readable by the -r option.

     -r      Read a partition table from the standard input.

     -t      Test if the disk is partitioned.

     -c cylinders, -h heads, -s sectors
             Specifies an alternate BIOS geometry for fdisk to use.

     -S size
             Specify the disk size in blocks.

     -b size
             Specify the number of bytes per disk block.

     The DOS fdisk program can be used to divide space on the disk into parti-
     tions and set one active.  This fdisk program serves a similar purpose to
     the DOS program.  When called with no special flags, it prints the MBR
     partition table of the specified device, i.e.,

         # fdisk fd0
         Disk: fd0       geometry: 80/2/18 [2880 sectors]
         Offset: 0       Signature: 0xAA55
                  Starting        Ending
          #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
         ----------------------------------------------------------------------
         *1: A6    0   0   1 -   79   1  18 [         0 -       2880] OpenBSD
          2: 00    0   0   0 -    0   0   0 [         0 -          0] unused
          3: A7    0   0   2 -   79   1  18 [         1 -       2879] NEXTSTEP
          4: 00    0   0   0 -    0   0   0 [         0 -          0] unused

     The geometry displayed is a synthetic geometry unless another geometry
     has been selected using the -c, -h, -s, -S, and -b options.  In the
     future, fdisk will read the BIOS geometry from the IOKit registry.

     In this example, the disk is divided into two partitions that happen to
     fill the disk.  The first partition overlaps the third partition.  (Used
     for debugging purposes.)

     #           Number of partition table entry.  A ``*'' denotes the
                 bootable partition.

     id          System identifier.  OpenBSD reserves the magic number 166
                 decimal (A6 in hex).  If no 166 partition is found, it will
                 use an older FreeBSD partition (with a magic number of 165 or
                 A5 in hex).

     cyl/hd/sec  These fields provide the starting and ending address of the
                 partition in BIOS geometry

     start/size  These fields provide the starting sector and size in sectors
                 of the partition in linear block addresses.

     NOTE: The sectors field is ``1 based'', and the start field is ``0
     based''.  The CHS values may need to be in the BIOS's geometry for older
     systems to be able to boot and use the drive correctly; most modern sys-
     tems prefer the starting sector and size in preference to the CHS values.

     The -i flag is used to indicate that the partition data is to be initial-
     ized.  In this mode, fdisk will completely overwrite the primary MBR and
     partition table, either using the default MBR template, or the one speci-
     fied by the -f flag.

     In the default template, partition number 1 will be configured as a Dar-
     win boot partition spanning from cylinder 0, head 1, sector 1, and
     extending for 8 megabytes.  Partition number 2 will be configured as a
     Darwin HFS partition spanning the rest of the disk.  This mode is
     designed to initialize an MBR the very first time, or when it has been
     corrupted beyond repair.

     You can specify other default partition styles with the -a flag.  The
     available styles are:

     boothfs     Creates an 8Mb boot partition (type AB hex) and makes the
                 rest of the disk a Darwin HFS partition (type AF hex).

     hfs         Makes the entire disk one HFS+ partition (type AF hex).

     dos         Makes the entire disk one DOS partition (type 0C hex).

     raid        Makes the entire disk one type AC hex partition.

     The -u flag is used to update the MBR code on a given drive.  The MBR
     code extends from offset 0x000 to the start of the partition table at
     offset 0x1BE.  It is similar to the -i flag, except the existing parti-
     tion table is preserved. This is useful for writing new MBR code onto an
     existing drive, and is equivalent to the DOS command ``FDISK /MBR''.
     Note that this option will overwrite the NT disk signature, if present.
     The -u and -i flags may not be specified together.

     The flag -e is used to modify a partition table using a interactive edit
     mode of the fdisk program.  This mode is designed to allow you to change
     any partition on the drive you choose, including extended partitions.  It
     is a very powerful mode, but is safe as long as you do not execute the
     write command, or answer in the negative (the default) when fdisk asks
     you about writing out changes.


COMMAND MODE

     When you first enter this mode, you are presented with a prompt, that
     looks like so: fdisk: 0>.  This prompt has two important pieces of infor-
     mation for you.  It will tell you if the in-memory copy of the boot block
     has been modified or not.  If it has been modified, the prompt will
     change to look like: fdisk:*0>.  The second piece of information pertains
     to the number given in the prompt.  This number specifies the disk offset
     of the currently selected boot block you are editing.  This number could
     be something different that zero when you are editing extended parti-
     tions.  The list of commands and their explanations are given below.

     help    Display a list of commands that fdisk understands in the interac-
             tive edit mode.

     manual  Display this manual page.

     reinit  Initialize the currently selected, in-memory copy of the boot
             block.

     auto    Partition the disk with one of the automatic partition styles.

     disk    Display the current drive geometry that fdisk has probed.  You
             are given a chance to edit it if you wish.

     edit    Edit a given table entry in the memory copy of the current boot
             block.  You may edit either in BIOS geometry mode, or in sector
             offsets and sizes.

     setpid  Change the partition identifier of the given partition table
             entry.  This command is particularly useful for reassigning an
             existing partition to OpenBSD.

     flag    Make the given partition table entry bootable.  Only one entry
             can be marked bootable.  If you wish to boot from an extended
             partition, you will need to mark the partition table entry for
             the extended partition as bootable.

     update  Update the machine code in the memory copy of the currently
             selected boot block.  Note that this option will overwrite the NT
             disk signature, if present.

     select  Select and load into memory the boot block pointed to by the
             extended partition table entry in the current boot block.

     print   Print the currently selected in-memory copy of the boot block and
             its MBR table to the terminal.

     write   Write the in-memory copy of the boot block to disk.  You will be
             asked to confirm this operation.

     exit    Exit the current level of fdisk, either returning to the previ-
             ously selected in-memory copy of a boot block, or exiting the
             program if there is none.

     quit    Exit the current level of fdisk, either returning to the previ-
             ously selected in-memory copy of a boot block, or exiting the
             program if there is none.  Unlike exit it does write the modified
             block out.

     abort   Quit program without saving current changes.


NOTES

     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 in-core disklabel, or values that
     /boot has passed to the kernel, but fdisk gives you an opportunity to
     change them if there is a need to.  This allows the user to create a
     bootblock that can work with drives that use geometry translation under a
     potentially different BIOS.

     If you hand craft your disk layout, please make sure that the OpenBSD
     partition starts on a cylinder boundary.  (This restriction may be
     changed in the future.)

     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 ``write'' ques-
     tions in the negative.


FILES

     /usr/mdec/mbr  default MBR template


SEE ALSO

     gpt(8), pdisk(8)


BUGS

     There are subtleties fdisk detects that are not explained in this manual
     page.  As well, chances are that some of the subtleties it should detect
     are being steamrolled.  Caveat Emptor.

BSD                             January 3, 2002                            BSD

Mac OS X 10.8 - Generated Mon Sep 3 18:53:41 CDT 2012
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