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

NAME
     fdisk - MBR partition maintenance program

SYNOPSIS
     fdisk [-eiuy] [-c cylinders -h heads -s sectors] [-f mbrfile] device

DESCRIPTION
     On the i386 and other architectures, sector 0 of a bootable hard disk
     must contain MBR bootcode, the MBR partition table containing 4 slots,
     and a specific magic number (0xAA55).  The 4 slots in the MBR partition
     table allow a disk drive to be sub-divided up into chunks known as MBR
     partitions.

     On the i386, the BIOS loads sector 0 of the boot disk into memory, veri-
     fies the magic number, and begins executing the MBR bootcode at the first
     byte.  The MBR bootcode then searches the MBR partition table for an
     ``active'' MBR partition (indicated by a `*' in the first column), and if
     one is found, the boot block from that MBR partition is loaded and exe-
     cuted in place of the original (MBR) boot block.

     Some other architectures (like the zaurus), consider sector 0 of the disk
     to contain the MBR partition table, but do not use the MBR bootcode at
     all.

     Upon first access to a disk, the partition information is retrieved, typ-
     ically in disklabel(5) format.  The location of the disklabel can vary
     from architecture to architecture, but if one is not found the existence
     of an MBR partition table will create a spoofed prototypical disklabel
     which can be viewed using disklabel(8).  This spoofing mechanism is use-
     ful for permitting partition access for devices which would not normally
     have a disklabel(5) sector.

     The options are as follows:

     -c cylinders -h heads -s sectors
             Specifies an alternate BIOS geometry for fdisk to use.  By de-
             fault, an automatic calculation of disk size will be built using
             heuristics.  These figures are taken from the in-core disklabel
             (see disklabel(8)), or values that /boot has passed to the ker-
             nel.

     -e      Use the interactive edit of fdisk to modify an MBR partition
             table.  The editor permits configuration of the MBR partition, as
             well as extended MBR partitions.  See COMMAND MODE, below, for
             more information.

     -f mbrfile
             Specifies an alternate MBR template file.  The default file is
             /usr/mdec/mbr.

     -i      Requests that the MBR partition data be re-initialized.  In this
             mode, fdisk will completely overwrite the primary MBR bootcode
             and MBR partition table using the default MBR template
             /usr/mdec/mbr (or the one optionally specified by the -f flag).
             In the default template, MBR partition number 3 will be config-
             ured as an OpenBSD MBR partition spanning from cylinder 0, head
             1, sector 1, and extending to the end of the disk.  This mode is
             designed to initialize the MBR the very first time.

     -u      Update MBR bootcode, preserving existing MBR partition table.
             The MBR bootcode extends from offset 0x000 to the start of the
             MBR partition table at offset 0x1BE.  It is similar to the -i
             flag, except the existing MBR partition table is preserved.  This
             is useful for writing new MBR bootcode 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.

     -y      Avoid asking yes/no questions when not desirable.

TYPICAL LAYOUT
     When called with no special flags, fdisk prints the MBR partition table
     of the specified device:

      # fdisk wd0
      Disk: wd0       geometry: 5168/240/63 [78140160 Sectors]
      Offset: 0       Signature: 0xAA55
                Starting      Ending        LBA Info:
       #: id    C   H  S -    C   H  S [       start:      size   ]
      -------------------------------------------------------------------------
       0: 04    0   1  1 -  170   0 63 [          63:     2570462 ] DOS FAT-16
       1: 00    0   0  0 -    0   0  0 [           0:           0 ] unused
       2: 00    0   0  0 -    0   0  0 [           0:           0 ] unused
      *3: A6  170   1  1 - 5167 239 63 [     2570525:    75569697 ] OpenBSD

     This 78140160 sector (38154MB) disk drive is divided into two MBR parti-
     tions that span the whole disk.  The first MBR partition is a 1225MB
     FAT-16 DOS partition (in this case containing a Phoenix BIOS Laptop Sus-
     pend-to-Disk file), the second is a 36GB OpenBSD MBR partition using the
     remainder of the disk.  The fields of the output are thus:

     #           Number identifying each MBR partition table entry.  There are
                 a total of four slots.  `*' denotes the MBR partition which
                 is declared bootable.

     id          MBR partition type identifier.  OpenBSD reserves the magic
                 number hexadecimal A6 (166 decimal).

     C/H/S       These fields provide the starting and ending address of the
                 MBR partition in BIOS geometry.

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

     NOTE: The BIOS geometry sectors field (C/H/S) is ``1 based'', but the LBA
     "start" field is ``0 based''.

     The CHS values will need to be in the BIOS's geometry for the system to
     be able to boot and use the drive correctly.  These values must be kept
     correctly synchronized or a variety of problems develop which are very
     difficult to diagnose.

     The OpenBSD MBR partition shown above is subdivided further using the
     functionality provided by disklabel(8), which provides OpenBSD parti-
     tions.

      # Inside MBR partition 3: type A6 start 2570463 size 75569697
      # /dev/rwd0c:
      type: ESDI
      disk: ESDI/IDE disk
      label: HITACHI_DK13FA-4
      flags:
      bytes/sector: 512
      sectors/track: 63
      tracks/cylinder: 16
      sectors/cylinder: 1008
      cylinders: 16383
      total sectors: 78140160
      rpm: 3600
      interleave: 1
      trackskew: 0
      cylinderskew: 0
      headswitch: 0           # microseconds
      track-to-track seek: 0  # microseconds
      drivedata: 0

      16 partitions:
      #           size     offset  fstype [fsize bsize  cpg]
        a:     1740753    2570463  4.2BSD   2048 16384  328 # Cyl  2550*-  4276
        b:      819504    4311216    swap                   # Cyl  4277 -  5089
        c:    78140160          0  unused      0     0      # Cyl     0 - 77519
        d:     2097648    5130720  4.2BSD   2048 16384  328 # Cyl  5090 -  7170
        e:      614880    7228368  4.2BSD   2048 16384  328 # Cyl  7171 -  7780
        g:     2097648    7843248  4.2BSD   2048 16384  328 # Cyl  7781 -  9861
        h:      921312    9940896  4.2BSD   2048 16384  328 # Cyl  9862 - 10775
        i:     2570462         63   MSDOS                   # Cyl     0*-  2549
        j:     3355632   10862208  4.2BSD   2048 16384  328 # Cyl 10776 - 14104
        k:     2097648   14217840  4.2BSD   2048 16384  328 # Cyl 14105 - 16185
        l:     2097648   16315488  4.2BSD   2048 16384  328 # Cyl 16186 - 18266
        m:    59727024   18413136  4.2BSD   2048 16384  328 # Cyl 18267 - 77519

     These OpenBSD partitions are then mounted as follows using /etc/fstab:

           /dev/wd0a / ffs rw,softdep 1 1
           /dev/wd0m /home ffs rw,softdep,nodev,nosuid 1 2
           /dev/wd0d /tmp ffs rw,softdep,nodev,nosuid 1 2
           /dev/wd0g /usr ffs rw,softdep,nodev 1 2
           /dev/wd0h /usr/X11R6 ffs rw,softdep,nodev 1 2
           /dev/wd0j /usr/local ffs rw,softdep,nodev 1 2
           /dev/wd0k /usr/obj ffs rw,softdep,nodev,nosuid 1 2
           /dev/wd0l /usr/src ffs rw,softdep,nodev,nosuid 1 2
           /dev/wd0e /var ffs rw,softdep,nodev,nosuid 1 2

COMMAND MODE
     The -e flag causes fdisk to enter an interactive command mode.  The
     prompt contains information about the state of the edit process.

           fdisk:*1>

     `*' means that the in-memory copy of the boot block has been modified,
     but not yet written to disk.

     1 is the disk offset of the currently selected boot block being edited.
     This number will be 2 when editing an extended MBR partition, 3 when
     editing an extended MBR partition within an extended MBR partition, and
     so on.

     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.

     disk    Display the current drive geometry that fdisk probed using kernel
             provided information and various heuristics.  The disk geometry
             may be changed at this point.

     edit #  Edit a given table entry in the memory copy of the current boot
             block.  Sizes may be adjusted using either in BIOS geometry mode
             or sector offsets and sizes.

     flag #  Make the given MBR partition table entry bootable.  Only one en-
             try can be marked bootable.

     update  Update the machine MBR bootcode and 0xAA55 signature in the memo-
             ry copy of the currently selected boot block.  Note that this op-
             tion will overwrite an NT disk signature, if present.

     select #
             Select and load into memory the boot block pointed to by the ex-
             tended MBR partition table entry in the current boot block.

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

     swap # #
             Swap two MBR entries.

     print [unit]
             Print the currently selected in-memory copy of the boot block and
             its MBR table to the terminal.  If unit is specified as `b', `k',
             `m', or `g', the MBR is printed in units of bytes, kilobytes,
             megabytes, or gigabytes.  Otherwise the number of sectors is
             printed.

     write   Write the in-memory copy of the boot block to disk.

     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.

FILES
     /usr/mdec/mbr  default MBR template

SEE ALSO
     fstab(5), boot_amd64(8), boot_armish(8), boot_i386(8), boot_landisk(8),
     boot_macppc(8), boot_zaurus(8), disklabel(8)

CAVEATS
     Hand crafted disk layouts are highly error prone.  It is common practice,
     though by no means required, that MBR partitions start on a cylinder
     boundary (generally head 0, sector 1, but head 1, sector 1 for track 0),
     and that MBR partitions also end at cylinder boundaries.

OpenBSD 4.7                     March 30, 2009                               4
