sbin/fdisk/fdisk.8

.\"	$NetBSD: fdisk.8,v 1.59 2008/01/20 15:04:09 dsl Exp $
.\"
.Dd January 20, 2008
.Dt FDISK 8
.Os
.Sh NAME
.Nm fdisk
.Nd MS-DOS partition maintenance program
.Sh SYNOPSIS
.Nm
.Op Fl afiuvBFS
.Bk -words
.Op Fl 0 | 1 | 2 | 3
.Ek
.Bk -words
.Op Fl t Ar disktab
.Ek
.Bk -words
.Op Fl T Ar disktype
.Ek
.Bk -words
.Op Fl E Ar number
.Ek
.Bk -words
.Op Fl b Ar cylinders/heads/sectors
.Ek
.Bk -words
.Op Fl s Ar id/start/size Ns Bq Ar /bootmenu
.Ek
.Bk -words
.Op Fl c Ar bootcode
.Ek
.Bk -words
.Op Fl r|w Ar file
.Ek
.Bk -words
.Op Ar device
.Ek
.Nm
.Fl l
.Sh DESCRIPTION
The
.Nm
program is used to display or update the
.Em "master boot record"
or
.Em MBR
in the first sector (sector 0)
of a disk that uses the MBR style of partitioning.
The following
.Nx
ports use this style of disk partitioning:
amd64, arc, bebox, cobalt, hpcarm, hpcmips, hpcsh, i386, macppc,
mvmeppc, netwinder, ofppc, playstation2, and prep.
.Pp
The MBR contains bootable code, a partition table,
an indication of which partition is
.Sq active ,
and (optionally, depending on the boot code) a menu
for selecting a partition to be booted.
There can be at most 4 partitions defined in sector 0,
one of which can be an extended
partition which can be split into any number of sub-partitions.
.Pp
The boot code in the MBR is usually invoked by the BIOS or firmware,
and the MBR passes control to the next stage boot code
stored in the first sector of the partition to be booted
(the
.Em "partition boot record"
or
.Em PBR ) .
.Pp
After booting,
.Nx
does not use the partitioning done by
.Nm ,
instead it uses a
.Nx
disklabel saved in sector 1 of the
.Nx
partition.
See
.Xr mbrlabel 8
for a way of using information from the MBR
to construct a
.Nx
disklabel.
.Pp
The standard MBR boot code will only boot the
.Sq active
partition.
However
.Nx
contains additional boot programs which allow the user to
interactively select which of the partitions to boot.
The
.Sq mbr_ext
code will also boot
.Nx
from an extended partition but will not work on old systems that do not
support LBA reads, the
.Sq mbr_com0
and
.Sq mbr_com0_9600
will read and write from a serial port.
At the start the
.Nm
program will determine whether the disk sector 0 is valid as a boot sector.
(This is determined by checking the magic number.)
If not,
.Nm
will initialise the boot code as well as the partition table.
During this, all four partitions will be marked empty.
.Pp
The flags
.Fl a ,
.Fl i
or
.Fl u
are used to indicate that the partition data is to be updated.
The
.Nm
program will enter an interactive conversational mode.
This mode is designed not to change any data unless you explicitly tell it to;
.Nm
selects defaults for its questions to guarantee that behaviour.
.Pp
.Nm
will calculate the correct
.Em cylinder ,
.Em head ,
and
.Em sector
values for any partition you edit.
If you specify
.Fl v
you will be asked whether you want to specify them yourself.
.Pp
Finally, when all the data for the first sector has been accumulated,
.Nm
will ask if you really want to write the new partition table.
Only if you reply affirmatively to this question will
.Nm
write anything to the disk.
.Pp
Available options:
.Pp
.Bl -tag -width Ds
.It Fl 0
Update partition slot 0.
.It Fl 1
Update partition slot 1.
.It Fl 2
Update partition slot 2.
.It Fl 3
Update partition slot 3.
.It Fl a
Change the active partition.
In interactive mode this question will be asked after the partitions
have been processed.
.It Fl b Ar cylinders/heads/sectors
Specify the BIOS parameters for
.Ar cylinders ,
.Ar heads ,
and
.Ar sectors .
It is used only in conjunction with the
.Fl u
flag.
.It Fl B
On an i386 or amd64 system, interactively update the boot selector settings.
(The boot selector permits the user to interactively select the boot
partition, and thus which operating system is run, at system boot time.
See
.Xr mbr 8
for more information.)
.It Fl c Ar bootcode
Specify the filename that
.Nm
should read the bootcode from.
If the name of a directory is specified, then
.Nm
will look for files with the default names in that directory.
The default is to read from
.Pa /usr/mdec/mbr ,
.Pa /usr/mdec/mbr_bootsel
or
.Pa /usr/mdec/mbr_ext
depending on whether
.Ar bootmenu
was specified for any partitions
on an i386 machine, and leave the bootcode empty for other
machines.
.It Fl E Ar number
Update extended partition
.Ar number .
If the specified extended partition doesn't exist an additional extended
partition will be created.
.It Fl f
Run
.Nm
in a non-interactive mode.
In this mode, you can only change the disk parameters by using the
.Fl b
flag.
This is provided only so scripts or other programs may use
.Nm
as part of an automatic installation process.
.Pp
Using the
.Fl f
flag with
.Fl u
makes it impossible to specify the starting and ending
.Ar cylinder ,
.Ar head ,
and
.Ar sector
fields.
They will be automatically computed using the BIOS geometry.
.Pp
If
.Fl u
and
.Fl s
are specified then the details of the specified partition will be changed.
Any other partitions which overlap the requested part of the disk will be
silently deleted.
.It Fl F
Indicate that
.Ar device
is a regular file.
Unless the geometry of
.Ar device
is told to
.Nm
by
.Fl T Ar disktype ,
.Nm
will count the 512-byte sectors in
.Ar device
and produce a fake geometry.
.It Fl i
Explicitly request initialisation of the master boot code
(similar to what
.Ic fdisk /mbr
does under
.Tn MS-DOS ) ,
even if the magic number in the first sector is ok.
The partition table is left alone by this (but see above).
.It Fl l
Lists known
.Em sysid
values and exit.
.It Fl r Ar file
Read the boot record from file
.Ar file
instead of the specified disk.
The geometry information used is still that of the disk volume.
Any changes are written back to the file.
.It Fl s Ar id/start/size Ns Bq Ar /bootmenu
Specify the partition
.Ar id ,
.Ar start ,
.Ar size ,
and optionally
.Ar bootmenu .
This flag requires the use of a partition selection flag
.Po
.Fl 0 ,
.Fl 1 ,
.Fl 2 ,
.Fl 3 ,
or
.Fl E Ar number
.Pc
.It Fl S
When used with no other flags print a series of
.Pa /bin/sh
commands for setting variables to the partition information.
This could be used by installation scripts.
.It Fl t Ar disktab
Read
.Ar disktype
from the named
.Xr disktab 5
file instead of from
.Pa /etc/disktab .
.It Fl T Ar disktype
Use the disklabel
.Ar disktype
instead of the disklabel on
.Ar device .
.It Fl u
Display the partitions and interactively ask which one you want to edit.
.Nm
will step through each field showing the old value and asking for a new one.
The
.Em start
and
.Em size
can be specified in blocks (nn), cylinders (nnc), megabytes (nnm),
or gigabytes (nng), values in megabytes and gigabytes
will be rounded to the nearest cylinder boundary.
The
.Em size
may be specified as
.Em $
in which case the partition will extend to the end of the available free space.
.Pp
.Nm
will not allow you to create partitions which overlap.
.Pp
If
.Em bootmenu
is specified for any partition
.Nm
will determine whether the installed boot code supports the bootselect code,
if it doesn't you will be asked whether you want to install the required
boot code.
To remove a
.Em bootmenu
label, simply press
.Aq space
followed by
.Aq return .
.It Fl v
Be more verbose, specifying
.Fl v
more than once may increase the amount of output.
.Pp
Using
.Fl v
with
.Fl u
allows the user to change more parameters than normally permitted.
.It Fl w Ar file
Write the modified partition table to file
.Ar file
instead of the disk.
.El
.Pp
When called with no arguments, it prints the partition table.
An example follows:
.Bd -literal
    Disk: /dev/rwd0d
    NetBSD disklabel disk geometry:
    cylinders: 16383, heads: 16, sectors/track: 63 (1008 sectors/cylinder)
    total sectors: 40032696

    BIOS disk geometry:
    cylinders: 1023, heads: 255, sectors/track: 63 (16065 sectors/cylinder)
    total sectors: 40032696

    Partition table:
    0: NetBSD (sysid 169)
	bootmenu: net 1.5.
	start 4209030, size 8289540 (4048 MB, Cyls 262-778), Active
    1: Primary DOS with 32 bit FAT (sysid 11)
	bootmenu: win98
	start 63, size 4208967 (2055 MB, Cyls 0-262)
    2: NetBSD (sysid 169)
	bootmenu: current
	start 32515560, size 7517136 (3670 MB, Cyls 2024-2491/234/40)
    3: Ext. partition - LBA (sysid 15)
	start 12498570, size 20016990 (9774 MB, Cyls 778-2024)
    Extended partition table:
    E0: NetBSD (sysid 169)
	bootmenu: test
	start 12498633, size 12305727 (6009 MB, Cyls 778-1544)
    E1: Primary DOS with 32 bit FAT (sysid 11)
	start 24804423, size 4096512 (2000 MB, Cyls 1544-1799)
    E2: Primary DOS with 32 bit FAT (sysid 11)
	start 28900998, size 3614562 (1765 MB, Cyls 1799-2024)
    Bootselector enabled, infinite timeout.
    First active partition: 0
.Ed
.Pp
This example disk is divided into four partitions the last of which is
an extended partition.
The sub-partitions of the extended partition are also shown.
In this case there is no free space in either the disk or in the extended
partition.
.Pp
The various fields in each partition entry are:
.br
.in +4
.Em ptn_number : id_name
(sysid
.Em id_number )
.br
.in +4
bootmenu:
.Em bootmenu
.br
start
.Em start ,
size
.Em size ( MB
MB, Cyls
.Em first Ns No - Ns Em next )
.Op , Active
.in -4
.in -4
.Bl -tag -width "bootmenu"
.It Em ptn_number
is the number of the partition.
.It Em id_name
is the name of the filesystem type or operating system that uses this partition.
.It Em id_number
is the number that identifies the partition type.
169 decimal is used for
.Nx
partitions,
15 decimal to create an extended partition
and 0 to mark a partition as unused.
Use
.Nm
.Fl l
to list the known partition types.
.It Em bootmenu
is the menu prompt output by the interactive boot code for this partition.
This line is omitted if the prompt is not defined.
.It Xo
.Em start ,
.Em size
.Xc
are the start address and size of the partition in sectors.
.It Em MB
is the size of the partition in megabytes.
.It Xo
.Em first ,
.Em next
.Xc
are the bounds of this partition displayed as cylinder/head/sector.
If the partition starts (or ends) on a cylinder boundary the head and
sector values are omitted.
If
.Fl v
is not specified the start of extended partitions and the first partition
on the disk are rounded down to include the mandatory red tape in the
preceding track.
.It Active
is output if this is the active partition.
.El
.Pp
If the
.Fl v
flag is specified, the beginning and end of each partition are also
displayed as follows:
.in +4
beg: cylinder
.Em cylinder ,
head
.Em head ,
sector
.Em sector
.br
end: cylinder
.Em cylinder ,
head
.Em head ,
sector
.Em sector
.in -4
.Bl -tag -width "bootmenu"
.It Xo
.Em "cylinder" ,
.Em "head" ,
.Em "sector"
.Xc
are the beginning or ending address of a partition.
.Pp
.Em "Note:"
these numbers are read from the bootblock, so are the values calculated
by a previous run of
.Nm .
.El
.Pp
.Nm
attempts to check whether each partition is bootable,
by checking the magic number and some other characteristics
of the first sector of each partition (the PBR).
If the partition does not apear to be bootable,
.Nm
will print a line containing
.Dq "PBR is not bootable"
followed by an error message.
If the partition is bootable, and if the
.Fl v
flag is specified,
.Nm
will print
.Dq "PBR appears to be bootable" .
If the
.Fl v
flag is specified more than once,
.Nm
will print the heading
.Dq "Information from PBR:"
followed by one or more lines of information gleaned from the PBR;
this additional information may be incorrect or misleading,
because different operating systems use different PBR formats.
Note that, even if no errors are reported, an attempt to boot
from the partition might fail.
.Nx
partitions may be made bootable using
.Xr installboot 8 .
.Sh NOTES
This program is only available (and useful) on systems with PC-platform-style
MBR partitioning.
.Pp
Traditionally the partition boundaries should be on cylinder boundaries
using the BIOS geometry, with the exception of the first partition,
which traditionally begins in the second track of the first cylinder
(cylinder 0, head 1, sector 1).
Although the BIOS geometry is typically different from the geometry
reported by the drive, neither will match the actual physical geometry
for modern disks (the actual geometry will vary across the disk).
Keeping the partition boundaries on cylinder boundaries makes partitioning
a driver easier as only relatively small numbers need be entered.
.Pp
The automatic calculation of the starting cylinder and
other parameters uses
a set of figures that represent what the BIOS thinks is the
geometry of the drive.
The default values should be correct for the system on which
.Nm
is run, however if you move the disk to a different system the
BIOS of that system might use a different geometry translation.
.Pp
If you run the equivalent of
.Nm
on a different operating system then the
.Ar bootmenu
strings associated with extended partitions may be lost.
.Pp
Editing an existing partition is risky, and may cause you to
lose all the data in that partition.
.Pp
You should run this program interactively once or twice to see how it works.
This is completely safe as long as you answer the last question in the negative.
You can also specify
.Fl w Ar file
to write the output to a file and later specify
.Fl r Ar file
to read back the updated information.
This can be done without having write access to the disk volume.
.Sh FILES
.Bl -tag -width /usr/mdec/mbrxxxxxxxx -compact
.It Pa /usr/mdec/mbr
Default location of i386 bootcode
.It Pa /usr/mdec/mbr_bootsel
Default location of i386 bootselect code
.It Pa /usr/mdec/mbr_ext
Default location of i386 bootselect for extended partitions
.El
.Sh SEE ALSO
.Xr disktab 5 ,
.Xr boot 8 ,
.Xr disklabel 8 ,
.Xr installboot 8 ,
.Xr mbr 8 ,
.Xr mbrlabel 8
.Sh BUGS
The word
.Sq partition
is used to mean both an MBR partition and a
.Nx
partition, sometimes in the same sentence.
.Pp
There are subtleties that the program detects that are not explained in
this manual page.