FDISK(8) BSD System Manager's Manual FDISK(8)NAMEfdisk — DOS partition maintenance program
SYNOPSISfdisk [-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
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.
Specify an automatic partitioning style.
-e Edit existing MBR sectors.
Specifies an alternate MBR template file.
-u Update MBR code, preserving existing partition table.
-y Do not ask for confirmation before writing.
-d Dump partition table in a format readable by the -r option.
-r Read a partition table from the standard input.
-t Test if the disk is partitioned.
-c cylinders, -h heads, -s sectors
Specifies an alternate BIOS geometry for fdisk to use.
Specify the disk size in blocks.
The DOS fdisk program can be used to divide space on the disk into parti‐
tions 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
#: id cyl hd sec - cyl hd sec [ start - size]
*1: A6 0 0 1 - 79 1 18 [ 0 - 2880] OpenBSD
2: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
3: A7 0 0 2 - 79 1 18 [ 1 - 2879] NEXTSTEP
4: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
The geometry displayed is a synthetic geometry unless another geometry
has been selected using the -c, -h, and -s options. In the future, fdisk
will read the BIOS geometry from the IOKit registry.
In this example, the 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
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 may need to be in the BIOS's geometry for older systems to
be able to boot and use the drive correctly; most modern systems prefer
the starting sector and size in preference to the CHS values.
The -i flag is used to indicate that the partition data is to be initial‐
ized. In this mode, fdisk will completely overwrite the primary MBR and
partition table, either using the default MBR template, or the one speci‐
fied by the -f flag.
In the default template, partition number 1 will be configured as a Dar‐
win boot partition spanning from cylinder 0, head 1, sector 1, and
extending for 8 megabytes. Partition number 2 will be configured as a
Darwin HFS partition spanning the rest of the disk. This mode is
designed to initialize an MBR the very first time, or when it has been
corrupted beyond repair.
You can specify other default partition styles with the -a flag. The
available styles are:
boothfs Creates an 8Mb boot partition (type AB hex) and makes the
rest of the disk a Darwin HFS partition (type AF hex).
bootufs Creates an 8Mb boot partition (type AB hex) and makes the
rest of the disk a Darwin UFS partition (type A8 hex).
hfs Makes the entire disk one HFS+ partition (type AF hex).
ufs Makes the entire disk one Darwin UFS partition (type A8 hex).
dos Makes the entire disk one DOS partition (type 0C hex).
raid Makes the entire disk one type AC hex partition.
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 parti‐
tion 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
-u and -i flags may not be specified together.
The flag -e is used to modify a partition table using a 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 infor‐
mation 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 different that zero when you are editing extended parti‐
tions. 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
auto Partition the disk with one of the automatic partition styles.
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 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 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
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” ques‐
tions in the negative.
/usr/mdec/mbr default MBR template
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.
BSD January 3, 2002 BSD