FDISK(8) BSD System Manager's Manual FDISK(8)
fdisk - MBR partition maintenance program
fdisk [-ieu] [-c cylinders -h heads -s sectors] [-f mbrfile] device
On the i386 and other architectures, sector 0 of a bootable hard disc
must contain MBR bootcode, the MBR partition table containing 4 slots,
and a specific magic number (0xAA55). The 4 slots in the MBR partition
table allow a disk drive to be sub-divided up into chunks known as MBR
partitions.
On the i386, the BIOS loads sector 0 of the boot disk into memory, veri-
fies the magic number, and begins executing the MBR bootcode at the first
byte. The MBR bootcode then searches the MBR partition table for an
"active" MBR partition (indicated by a '*' in the first column), and if
one is found, the boot block from that MBR partition is loaded and exe-
cuted in place of the original (MBR) boot block.
Some other architectures (like the zaurus), consider sector 0 of the disk
to contain the MBR partition table, but do not use the MBR bootcode at
all.
When called with no special flags, fdisk prints the MBR partition table
of the specified device, i.e.,
# fdisk wd0
Disk: wd0 geometry: 5168/240/63 [78140160 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
------------------------------------------------------------------------
0: 04 0 1 1 - 170 0 63 [ 63: 2570462 ] DOS FAT-16
1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
*3: A6 170 1 1 - 5167 239 63 [ 2570463: 75569697 ] OpenBSD
It should be noted that the partition table shown above is invalid and a
bad idea in general. This 78140160 sector (38154MB) disk drive is divided
into two MBR partitions that span the whole disk. The first MBR partition
is a 1225MB FAT-16 DOS partition (in this case containing a Phoenix BIOS
Laptop Suspend-to-Disk file), the second is a 36GB OpenBSD MBR partition
using the remainder of the disk. The fields of the output are thus:
# Number identifying each MBR partition table entry. There are
a total of four slots. '*' denotes the MBR partition which is
declared bootable.
id MBR partition type identifier. MirBSD reserves the magic
number hexadecimal 27 (39 decimal). If no MBR partition of
type 27 is found, MirBSD can use an older OpenBSD, NetBSD or
FreeBSD MBR partition (with type 0xA6, 0xA9 and 0xA5, respec-
tively).
C/H/S These fields provide the starting and ending address of the
MBR partition in BIOS geometry.
start/size These fields provide the starting sector and size in sectors
of the MBR partition in linear block addresses.
NOTE: The BIOS geometry sectors field is "1 based", but 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. These values
must be kept correctly synchronized or a variety of problems develop
which are very difficult to diagnose.
The OpenBSD MBR partition shown above is subdivided further using the
functionality provided by disklabel(8), which provides MirBSD slices.
# Inside MBR partition 3: type A6 start 2570463 size 75569697
# /dev/rwd0c:
type: ESDI
disk: ESDI/IDE disk
label: HITACHI_DK13FA-4
flags:
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 16
sectors/cylinder: 1008
cylinders: 16383
total sectors: 78140160
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0 # microseconds
track-to-track seek: 0 # microseconds
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
a: 1740753 2570463 4.2BSD 2048 16384 328 # Cyl 2550*- 4276
b: 819504 4311216 swap # Cyl 4277 - 5089
c: 78140160 0 unused 0 0 # Cyl 0 - 77519
d: 2097648 5130720 4.2BSD 2048 16384 328 # Cyl 5090 - 7170
e: 614880 7228368 4.2BSD 2048 16384 328 # Cyl 7171 - 7780
g: 2097648 7843248 4.2BSD 2048 16384 328 # Cyl 7781 - 9861
h: 921312 9940896 4.2BSD 2048 16384 328 # Cyl 9862 - 10775
i: 2570462 63 MSDOS # Cyl 0*- 2549
j: 3355632 10862208 4.2BSD 2048 16384 328 # Cyl 10776 - 14104
k: 2097648 14217840 4.2BSD 2048 16384 328 # Cyl 14105 - 16185
l: 2097648 16315488 4.2BSD 2048 16384 328 # Cyl 16186 - 18266
m: 59727024 18413136 4.2BSD 2048 16384 328 # Cyl 18267 - 77519
These MirBSD filesystems are then mounted as follows using /etc/fstab:
/dev/wd0a / ffs rw,softdep 1 1
/dev/wd0m /home ffs rw,softdep,nodev,nosuid 1 2
/dev/wd0d /tmp ffs rw,softdep,nodev,nosuid 1 2
/dev/wd0g /usr ffs rw,softdep,nodev 1 2
/dev/wd0h /usr/X11R6 ffs rw,softdep,nodev 1 2
/dev/wd0j /usr/local ffs rw,softdep,nodev 1 2
/dev/wd0k /usr/obj ffs rw,softdep,nodev,nosuid 1 2
/dev/wd0l /usr/src ffs rw,softdep,nodev,nosuid 1 2
/dev/wd0e /var ffs rw,softdep,nodev,nosuid 1 2
The options are as follows:
-c cylinders, -h heads, -s sectors
Specifies an alternate BIOS geometry for fdisk to use. By de-
fault, an automatic calculation of disk size will be built using
heuristics. These figures are taken from the in-core disklabel
(see disklabel(8)), or values that /boot has passed to the ker-
nel.
-e Use the interactive edit of fdisk to modify a MBR partition
table. The editor permits configuration of the MBR partition, as
well as extended MBR partitions.
-f mbrfile
Specifies an alternate MBR template file. The default file is
/usr/mdec/mbr.
-i Requests that the MBR partition data be re-initialized. In this
mode, fdisk will completely overwrite the primary MBR bootcode
and MBR partition table using the default MBR template
/usr/mdec/mbr (or the one optionally specified by the -f flag).
In the default template, MBR partition number 3 will be config-
ured as an MirBSD MBR partition spanning from cylinder 0, head 1,
sector 1, and extending to the end of the disk. This mode is
designed to initialize the MBR the very first time.
-u Update MBR bootcode, preserving existing MBR partition table. The
MBR bootcode extends from offset 0x000 to the start of the MBR
partition table at offset 0x1BE. It is similar to the -i flag,
except the existing MBR partition table is preserved. This is
useful for writing new MBR bootcode onto an existing drive, and
is equivalent to the DOS command "FDISK /MBR". Note that this op-
tion will overwrite the NT disk signature, if present. If you
would like to preserve the NT disk signature, you can use the
umin command in the interactive command mode. Just be damn sure
you use a boot code which supports this.
The -e flag causes fdisk to enter an interactive command mode. The prompt
contains information about the state of the edit process.
fdisk:*0>
'*' means that the in-memory copy of the boot block has been modified,
but not yet written to disk.
0 is the disk offset of the currently selected boot block being edited.
This number could be something other than zero when extended MBR parti-
tions are being edited (using the select subcommand).
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
block.
disk Display the current drive geometry that fdisk probed using kernel
provided information and various heuristics. The disk geometry
may be changed at this point.
edit # Edit a given table entry in the memory copy of the current boot
block. Sizes may be adjusted using either in BIOS geometry mode
or sector offsets and sizes.
fdef # Make the given partition table entry bootable. This does only
work with the MirOS boot blocks. This is not the same as the flag
command, which is commonly used to mark the default entry, too.
Only one entry can be marked bootable. Attention: This command
should only be used by people who know what they are doing. This
command can destroy third party bootblocks. To disable the func-
tionality after it has been enabled, use the number 9 as parame-
ter.
flag # Make the given partition table entry active. Only one entry can
be marked active. If no default entry has been selected with
fdef, or an older, for example from MirBSD #7, or non-MirOS boot-
block is used, this entry will be the default booted entry as
well.
update Update the machine MBR bootcode and 0xAA55 signature in the
memory copy of the currently selected boot block. Note that this
option will overwrite an NT disk signature, if present. This op-
tion will also revert the fdef choice to unset, if you use recent
MirBSD boot code.
umin This option performs basically the same operation as the update
command, but leaving the last part of the machine boot code,
which contains the Microsoft NT disk signature and, shortly be-
fore, the fdef flag, untouched. If you do not use recent MirBSD
boot code which supports this, your hard disc will be left un-
bootable because this option cuts off the end of the machine
code, possibly containing instructions.
select #
Select and load into memory the boot block pointed to by the ex-
tended MBR partition table entry in the current boot block.
setpid #
Change the MBR partition identifier of the given MBR partition
table entry. This command is particularly useful for reassigning
an existing MBR partition to MirBSD.
swap # #
Swap two MBR entries.
print [unit]
Print the currently selected in-memory copy of the boot block and
its MBR table to the terminal. If unit is specified as 'b', 'k',
'm', or 'g', the MBR is printed in units of bytes, kibibytes, me-
bibytes, or gibibytes. Otherwise the number of sectors is print-
ed.
write Write the in-memory copy of the boot block to disk.
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
block out.
abort Quit program without saving current changes.
/usr/mdec/mbrmgr default MBR template
fstab(5), boot_i386(8), disklabel(8)
Hand crafted disk layouts are highly error prone. MBR partitions should
start on a cylinder boundary (head 0, sector 1), except when starting on
track 0, (these should begin at head 1, sector 1). MBR partitions should
also end at cylinder boundaries.
MirBSD #10-current January 6, 2022 3