| FDISK(8) | System Manager's Manual | FDISK(8) |
fdisk — DOS
partition maintenance program
fdisk |
[-ieu] [-f
mbrname] [-c
cylinders] [-h
heads] [-s
sectors] [-S
size] [-b
size] 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-a
style-e-f
mbrname-u-y-d-r-t-c
cylinders, -h
heads, -s
sectorsfdisk to
use.-S
size-b
sizeThe 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]
----------------------------------------------------------------------
*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, -s,
-S, and -b 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.)
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 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 1 will be configured as a Darwin 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:
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
-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 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 different that zero when you are editing extended partitions. The list of commands and their explanations are given below.
fdisk understands
in the interactive edit mode.fdisk has
probed. You are given a chance to edit it if you wish.fdisk, either returning
to the previously selected in-memory copy of a boot block, or exiting the
program if there is none.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.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.
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.
| January 3, 2002 | macOS 15.2 |