fdisk - DOS partition maintenance program
fdisk [-ieu] [-f mbrname] [-c cylinders] [-h heads] [-s
sectors] device
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.
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 ``active'' partition
(indicated by a `*' 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.
The options are as follows:
-i Initialize the MBR sector.
-e Edit existing MBR sectors.
-f mbrname
Specifies an alternate MBR template file.
-u Update MBR code, preserving existing partition
table.
-c cylinders, -h heads, -s sectors
Specifies an alternate BIOS geometry for fdisk to
use.
The DOS fdisk program can be used to divide space on the
disk into partitions
and set one active. This fdisk 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.,
# 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
The geometry displayed is the BIOS geometry unless another
geometry has
been selected using the -c, -h, and -s options.
This disk is divided into two partitions that happen to fill
the disk.
The first partition overlaps the third partition. (Used for
debugging
purposes.)
# Number of partition table entry. A ``*'' denotes the
bootable partition.
id System identifier. OpenBSD reserves the magic
number 166
decimal (A6 in hex). If no 166 partition is
found, it will
use an older FreeBSD partition (with a magic
number of 165 or
A5 in hex).
cyl/hd/sec These fields provide the starting and ending address of the
partition in BIOS geometry.
start/size These fields provide the starting sector and
size in sectors
of the partition in linear block addresses.
NOTE: The sectors field is ``1 based'', and 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.
The -i flag is used to indicate that the partition data is
to be initialized.
In this mode, fdisk will completely overwrite the
primary MBR and
partition table, either using the default MBR template, or
the one specified
by the -f flag. In the default template, partition
number 3 will be
configured as an OpenBSD 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.
The -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 -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 ``FDISK
/MBR''.
Note that this option will overwrite the NT disk signature,
if present.
The -e flag is used to modify a partition table using an interactive edit
mode of the fdisk 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
write command, or answer in the negative (the default) when
fdisk asks
you about writing out changes.
When you first enter this mode, you are presented with a
prompt, that
looks like so: 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: 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.
help Display a list of commands that fdisk understands in
the interactive
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 has
probed. You
are given a chance to edit it if you wish.
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.
setpid Change the partition identifier of the given partition table entry.
This command is particularly useful for reassigning an existing
partition to OpenBSD.
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.
update Update the machine code and 0xAA55 signature in the
memory copy
of the currently selected boot block. Note that
this option will
overwrite the NT disk signature, if present.
select Select and load into memory the boot block pointed
to by the extended
partition table entry in the current boot
block.
print Print the currently selected in-memory copy of the
boot block and
its MBR table to the terminal.
write Write the in-memory copy of the boot block to disk.
You will be
asked to confirm this operation.
exit Exit the current level of fdisk, either returning to
the previously
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 previously
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.
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
/boot has passed to the kernel, but fdisk 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.
If you hand craft your disk layout, please make sure that
the OpenBSD
partition starts on a cylinder boundary. (This restriction
may be
changed in the future.)
Editing an existing partition is risky, and may cause you to
lose all the
data in that partition.
You should run this program interactively once or twice to
see how it
works. This is completely safe as long as you answer the
``write'' questions
in the negative.
/usr/mdec/mbr default MBR template
boot_i386(8), disklabel(8)
There are subtleties fdisk 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.
OpenBSD 3.6 January 3, 2002
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