MirBSD manpage: fdisk(8)

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


     fdisk - MBR partition maintenance program


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


     On the i386 and other architectures, sector 0 of a bootable hard disc
     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

     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


     When called with no special flags, fdisk prints the MBR partition table
     of the specified device, i.e.,

      # 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 [     2570463:    75569697 ] OpenBSD

     It should be noted that the partition table shown above is invalid and a
     bad idea in general. This 78140160 sector (38154MB) disk drive is divided
     into two MBR partitions that span the whole disk. The first MBR partition
     is a 1225MB FAT-16 DOS partition (in this case containing a Phoenix BIOS
     Laptop Suspend-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. MirBSD reserves the magic
                 number hexadecimal 27 (39 decimal). If no MBR partition of
                 type 27 is found, MirBSD can use an older OpenBSD, NetBSD or
                 FreeBSD MBR partition (with type 0xA6, 0xA9 and 0xA5, respec-

     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 is "1 based", but the 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 MirBSD slices.

      # Inside MBR partition 3: type A6 start 2570463 size 75569697
      # /dev/rwd0c:
      type: ESDI
      disk: ESDI/IDE disk
      label: HITACHI_DK13FA-4
      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 MirBSD filesystems 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


     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-

     -e      Use the interactive edit of fdisk to modify a MBR partition
             table. The editor permits configuration of the MBR partition, as
             well as extended MBR partitions.

     -f mbrfile
             Specifies an alternate MBR template file. The default file is

     -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 MirBSD 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 op-
             tion will overwrite the NT disk signature, if present. If you
             would like to preserve the NT disk signature, you can use the
             umin command in the interactive command mode. Just be damn sure
             you use a boot code which supports this.


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


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

     0 is the disk offset of the currently selected boot block being edited.
     This number could be something other than zero when extended MBR parti-
     tions are being edited (using the select subcommand).

     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

     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.

     fdef #  Make the given partition table entry bootable. This does only
             work with the MirOS boot blocks. This is not the same as the flag
             command, which is commonly used to mark the default entry, too.
             Only one entry can be marked bootable. Attention: This command
             should only be used by people who know what they are doing. This
             command can destroy third party bootblocks. To disable the func-
             tionality after it has been enabled, use the number 9 as parame-

     flag #  Make the given partition table entry active. Only one entry can
             be marked active. If no default entry has been selected with
             fdef, or an older, for example from MirBSD #7, or non-MirOS boot-
             block is used, this entry will be the default booted entry as

     update  Update the machine MBR bootcode and 0xAA55 signature in the
             memory copy of the currently selected boot block. Note that this
             option will overwrite an NT disk signature, if present. This op-
             tion will also revert the fdef choice to unset, if you use recent
             MirBSD boot code.

     umin    This option performs basically the same operation as the update
             command, but leaving the last part of the machine boot code,
             which contains the Microsoft NT disk signature and, shortly be-
             fore, the fdef flag, untouched. If you do not use recent MirBSD
             boot code which supports this, your hard disc will be left un-
             bootable because this option cuts off the end of the machine
             code, possibly containing instructions.

     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 MirBSD.

     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, kibibytes, me-
             bibytes, or gibibytes. Otherwise the number of sectors is print-

     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.


     /usr/mdec/mbrmgr  default MBR template


     fstab(5), boot_i386(8), disklabel(8)


     Hand crafted disk layouts are highly error prone. MBR partitions should
     start on a cylinder boundary (head 0, sector 1), except when starting on
     track 0, (these should begin at head 1, sector 1). MBR partitions should
     also end at cylinder boundaries.

MirBSD #10-current             January 6, 2022                               3

Generated on 2022-12-24 01:00:14 by $MirOS: src/scripts/roff2htm,v 1.113 2022/12/21 23:14:31 tg Exp $ — This product includes material provided by mirabilos.

These manual pages and other documentation are copyrighted by their respective writers; their sources are available at the project’s CVSweb, AnonCVS and other mirrors. The rest is Copyright © 2002–2022 MirBSD.

This manual page’s HTML representation is supposed to be valid XHTML/1.1; if not, please send a bug report — diffs preferred.

Kontakt / Impressum & Datenschutzerklärung