.\" $OpenBSD: fdisk.8,v 1.66 2008/05/13 13:28:15 jmc Exp $ .\" .\" Copyright (c) 1997 Tobias Weingartner .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: May 13 2008 $ .Dt FDISK 8 .Os .Sh NAME .Nm fdisk .Nd MBR partition maintenance program .Sh SYNOPSIS .Nm fdisk .Op Fl eiuy .Oo .Fl c Ar cylinders .Fl h Ar heads .Fl s Ar sectors .Oc .Op Fl f Ar mbrfile .Ar device .Sh 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. .Pp On the i386, the BIOS loads sector 0 of the boot disk into memory, verifies the magic number, and begins executing the MBR bootcode at the first byte. The MBR bootcode then searches the MBR partition table for an .Dq active MBR partition (indicated by a .Ql \&* in the first column), and if one is found, the boot block from that MBR partition is loaded and executed in place of the original (MBR) boot block. .Pp 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. .Pp Upon first access to a disk, the partition information is retrieved, typically in .Xr 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 .Xr disklabel 8 . This spoofing mechanism is useful for permitting partition access for devices which would not normally have a .Xr disklabel 5 sector. .Pp The options are as follows: .Bl -tag -width Ds .It Xo .Fl c Ar cylinders .Fl h Ar heads .Fl s Ar sectors .Xc Specifies an alternate BIOS geometry for .Nm to use. By default, an automatic calculation of disk size will be built using heuristics. These figures are taken from the in-core disklabel (see .Xr disklabel 8 ) , or values that .Em /boot has passed to the kernel. .It Fl e Use the interactive edit of .Nm to modify an MBR partition table. The editor permits configuration of the MBR partition, as well as extended MBR partitions. See .Sx COMMAND MODE , below, for more information. .It Fl f Ar mbrfile Specifies an alternate MBR template file. The default file is .Pa /usr/mdec/mbr . .It Fl i Requests that the MBR partition data be re-initialized. In this mode, .Nm will completely overwrite the primary MBR bootcode and MBR partition table using the default MBR template .Pa /usr/mdec/mbr (or the one optionally specified by the .Fl f flag). In the default template, MBR partition number 3 will be configured as an .Ox 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. .It Fl 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 .Fl 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 .Dq FDISK /MBR . Note that this option will overwrite the NT disk signature, if present. .It Fl y Avoid asking yes/no questions when not desirable. .El .Sh TYPICAL LAYOUT When called with no special flags, .Nm prints the MBR partition table of the specified device: .Bd -literal -offset 1n # 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 .Ed .Pp 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 .Ox MBR partition using the remainder of the disk. The fields of the output are thus: .Bl -tag -width "start/size" .It Em "#" Number identifying each MBR partition table entry. There are a total of four slots. .Sq * denotes the MBR partition which is declared bootable. .It Em "id" MBR partition type identifier. .Ox reserves the magic number hexadecimal A6 (166 decimal). .It Em "C/H/S" These fields provide the starting and ending address of the MBR partition in BIOS geometry. .It Em "start/size" These fields provide the starting sector and size in sectors of the MBR partition in linear block addresses. .El .Pp .Em NOTE : The BIOS geometry sectors field is .Dq 1 based , but the start field is .Dq 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. .Pp The .Ox MBR partition shown above is subdivided further using the functionality provided by .Xr disklabel 8 , which provides .Ox partitions. .Bd -literal -offset 1n # 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 .Ed .Pp These .Ox partitions are then mounted as follows using .Pa /etc/fstab : .Bd -literal -offset indent /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 .Ed .Sh COMMAND MODE The .Fl e flag causes .Nm to enter an interactive command mode. The prompt contains information about the state of the edit process. .Pp .Dl fdisk:*0> .Pp .Sq * means that the in-memory copy of the boot block has been modified, but not yet written to disk. .Pp 0 is the disk offset of the currently selected boot block being edited. This number could be something other than zero when extended MBR partitions are being edited (using the .Em select subcommand). .Pp The list of commands and their explanations are given below. .Bl -tag -width Ds .It help Display a list of commands that .Nm understands in the interactive edit mode. .It manual Display this manual page. .It reinit Initialize the currently selected, in-memory copy of the boot block. .It disk Display the current drive geometry that .Nm probed using kernel provided information and various heuristics. The disk geometry may be changed at this point. .It edit Ar # 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. .It flag Ar # Make the given MBR partition table entry bootable. Only one entry can be marked bootable. .\" If you wish to boot from an extended .\" MBR partition, you will need to mark the MBR partition table entry for the .\" extended MBR partition as bootable. .\" If an optional value is given, the MBR partition is marked with the given .\" value, and other MBR partitions are not touched. .It 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. .It select Ar # Select and load into memory the boot block pointed to by the extended MBR partition table entry in the current boot block. .It setpid Ar # Change the MBR partition identifier of the given MBR partition table entry. This command is particularly useful for reassigning an existing MBR partition to .Ox . .It swap Ar # Ar # Swap two MBR entries. .It print Op unit Print the currently selected in-memory copy of the boot block and its MBR table to the terminal. If .Ar unit is specified as .Sq b , .Sq k , .Sq m , or .Sq g , the MBR is printed in units of bytes, kilobytes, megabytes, or gigabytes. Otherwise the number of sectors is printed. .It write Write the in-memory copy of the boot block to disk. .It exit Exit the current level of .Nm fdisk , either returning to the previously selected in-memory copy of a boot block, or exiting the program if there is none. .It quit Exit the current level of .Nm fdisk , either returning to the previously selected in-memory copy of a boot block, or exiting the program if there is none. Unlike .Em exit it does write the modified block out. .It abort Quit program without saving current changes. .El .Sh FILES .Bl -tag -width /usr/mdec/mbr -compact .It Pa /usr/mdec/mbr default MBR template .El .Sh SEE ALSO .Xr fstab 5 , .Xr boot_amd64 8 , .Xr boot_armish 8 , .Xr boot_i386 8 , .Xr boot_landisk 8 , .Xr boot_macppc 8 , .Xr boot_zaurus 8 , .Xr disklabel 8 .Sh CAVEATS 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.