path: root/sbin/fdisk/fdisk.8

.\"	$OpenBSD: fdisk.8,v 1.39 2003/01/31 21:54:40 jmc Exp $
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.\" Copyright (c) 1997 Tobias Weingartner
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.Dd January 3, 2002
.Dt FDISK 8
.Os
.Sh NAME
.Nm fdisk
.Nd DOS partition maintenance program
.Sh SYNOPSIS
.Nm fdisk
.Op Fl ieu
.Op Fl f Ar mbrname
.Op Fl c Ar cylinders
.Op Fl h Ar heads
.Op Fl s Ar sectors
.Ar device
.Sh 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.
.Pp
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
.Dq active
partition (indicated by a
.Ql \&*
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.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl i
Initialize the MBR sector.
.It Fl e
Edit existing MBR sectors.
.It Fl f Ar mbrname
Specifies an alternate MBR template file.
.It Fl u
Update MBR code, preserving existing partition table.
.It Xo Fl c Ar cylinders ,
.Fl h Ar heads ,
.Fl s Ar sectors
.Xc
Specifies an alternate BIOS geometry for
.Nm
to use.
.El
.Pp
The DOS
.Nm
program can be used to divide space on the disk into partitions and set
one active.
This
.Nm
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.,
.Bd -literal
    # 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]
    ----------------------------------------------------------------------
    *0: A6    0   0   1 -   79   1  18 [         0 -       2880] OpenBSD
     1: 00    0   0   0 -    0   0   0 [         0 -          0] unused
     2: A7    0   0   2 -   79   1  18 [         1 -       2879] NEXTSTEP
     3: 00    0   0   0 -    0   0   0 [         0 -          0] unused
.Ed
.Pp
The geometry displayed is the BIOS geometry unless another geometry
has been selected using the
.Fl c ,
.Fl h ,
and
.Fl s
options.
.Pp
This disk is divided into two partitions that happen to fill the disk.
The first partition overlaps the third partition.
(Used for debugging purposes.)
.Bl -tag -width "start/size"
.It Em "#"
Number of partition table entry.
A
.Dq \&*
denotes the bootable partition.
.It Em "id"
System identifier.
.Ox
reserves the
magic number 166 decimal (A6 in hex).
If no 166 partition is found, it will use an older
.Fx
partition (with a magic number of 165 or A5 in hex).
.It Em "cyl/hd/sec"
These fields provide the starting and ending address of the partition
in BIOS geometry.
.It Em "start/size"
These fields provide the starting sector and size in sectors of the
partition in linear block addresses.
.El
.Pp
.Em NOTE :
The sectors field is
.Dq 1 based ,
and 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.
.Pp
The
.Fl i
flag is used to indicate that the partition data is to be initialized.
In this mode,
.Nm
will completely overwrite the primary MBR and partition table, either
using the default MBR template, or the one specified by the
.Fl f
flag. In the default template, partition number 3 will be configured as an
.Ox
partition spanning from cylinder 0, head 1, sector 1, and extending
to the end of the disk.
This mode is designed to initialize an MBR the very first time,
or when it has been corrupted beyond repair.
.Pp
The
.Fl 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
.Fl i
flag, except the existing partition table is preserved. This
is useful for writing new MBR code 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.
.Pp
The
.Fl e
flag is used to modify a partition table using an 
interactive edit mode of the
.Nm
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
.Em write
command, or answer in the negative (the default) when
.Nm
asks you about writing out changes.
.Sh COMMAND MODE
When you first enter this mode, you are presented with a prompt, that looks
like so:
.Em "fdisk: 0>" .
This prompt has two important pieces of information 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:
.Em "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 other than zero when
you are editing extended partitions.
The list of commands and their explanations are given below.
.Bl -tag -width "update"
.It Em help
Display a list of commands that
.Nm
understands in the interactive edit mode.
.It Em manual
Display this manual page.
.It Em reinit
Initialize the currently selected, in-memory copy of the
boot block.
.It Em disk
Display the current drive geometry that
.Nm
has
probed.
You are given a chance to edit it if you wish.
.It Em 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.
.It Em setpid
Change the partition
identifier of the given partition table entry.
This command is particularly useful for reassigning
an existing partition to OpenBSD.
.It Em 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.
.It Em 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.
.It Em select
Select and load into memory the boot block pointed
to by the extended partition table entry in the current boot block.
.It Em print
Print the currently selected in-memory copy of the boot
block and its MBR table to the terminal.
.It Em write
Write the in-memory copy of the boot block to disk.
You will be asked to confirm this operation.
.It Em 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 Em 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 Em abort
Quit program without saving current changes.
.El
.Sh 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
.Em /boot
has passed to the kernel, but
.Nm
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.
.Pp
If you hand craft your disk layout,
please make sure that the
.Ox
partition starts on a cylinder boundary.
(This restriction may be changed in the future.)
.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
.Dq write
questions in the
negative.
.Sh FILES
.Bl -tag -width /usr/mdec/mbr -compact
.It Pa /usr/mdec/mbr
default MBR template
.El
.Sh SEE ALSO
.Xr boot_i386 8 ,
.Xr disklabel 8
.Sh BUGS
There are subtleties
.Nm
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.