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fdisk - PC slice table maintenance utility


Configuration File
See Also


fdisk [-BIaistu] [-b bootcode] [-1234] [disk] fdisk -f configfile [-itv] [disk]


In order for the BIOS to boot the kernel, certain conventions must be adhered to. Sector 0 of the disk must contain boot code, a slice table, and a magic number. BIOS slices can be used to break the disk up into several pieces. The BIOS brings in sector 0 and verifies the magic number. The sector 0 boot code then searches the slice table to determine which slice is marked "active". This boot code then brings in the bootstrap from the active slice and, if marked bootable, runs it. Under DOS, you can have one or more slices with one active. The DOS fdisk utility can be used to divide space on the disk into slices and set one active.


.Fx utility, fdisk, serves a similar purpose to the DOS utility. The first form is used to display slice information or to interactively edit the slice table. The second is used to write a slice table using a configfile, and is designed to be used by other scripts/programs.

Options are:

-a Change the active slice only. Ignored if -f is given.
-b bootcode
Get the boot code from the file bootcode. Default is /boot/mbr.
-B Reinitialize the boot code contained in sector 0 of the disk. Ignored if -f is given.
-f configfile
Set slice values using the file configfile. The configfile always modifies existing slices, unless -i is also given, in which case all existing slices are deleted (marked as "unused") before the configfile is read. The configfile can be '', in which case standard input is read. See CONFIGURATION FILE, below, for file syntax.

WARNING: when -f is used, you are not asked if you really want to write the slices table (as you are in the interactive mode). Use with caution!

-i Initialize sector 0 of the disk. This implies -u , unless -f is given.
-I Initialize the contents of sector 0 for one
.Fx slice covering the entire disk.
-s Print summary information and exit.
-t Test mode; do not write slice values. Generally used with the -f option to see what would be written to the slice table. Implies -v .
-u Is used for updating (editing) sector 0 of the disk. Ignored if -f is given.
-v Be verbose. When -f is used, fdisk prints out the slice table that is written to the disk.
-1234 Operate on a single slice table entry only. Ignored if -f is given.

The final disk name can be provided as a "bare" disk name only, e.g. da0, or as a full pathname. If omitted, fdisk tries to figure out the default disk device name from the mounted root device.

When called with no arguments, it prints the sector 0 slice table. An example follows:
******* Working on device /dev/ad0 *******
parameters extracted from in-core disklabel are:
cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

parameters to be used for BIOS calculations are:
cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

Warning: BIOS sector numbering starts with sector 1
Information from DOS bootblock is:
The data for partition 1 is:
sysid 165,(FreeBSD/NetBSD/386BSD)
start 495, size 380160 (185 Meg), flag 0
beg: cyl 1/ sector 1/ head 0;
end: cyl 768/ sector 33/ head 14
The data for partition 2 is:
sysid 164,(unknown)
start 378180, size 2475 (1 Meg), flag 0
beg: cyl 764/ sector 1/ head 0;
end: cyl 768/ sector 33/ head 14
The data for partition 3 is:
The data for partition 4 is:
sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
start 380656, size 224234 (109 Meg), flag 80
beg: cyl 769/ sector 2/ head 0;
end: cyl 197/ sector 33/ head 14

The disk is divided into three slices that happen to fill the disk. The second slice overlaps the end of the first. (Used for debugging purposes.)

sysid is used to label the slice.
.Fx reserves the magic number 165 decimal (A5 in hex).
start and size
fields provide the start address and size of a slice in sectors.
"flag 80" specifies that this is the active slice.
cyl, sector and head
fields are used to specify the beginning and end addresses of the slice.

Note: these numbers are calculated using BIOS’s understanding of the disk geometry and saved in the bootblock.

The -i and -u flags are used to indicate that the slice data is to be updated. Unless the -f option is also given, fdisk will enter a conversational mode. In this mode, no changes will be written to disk unless you explicitly tell fdisk to.

The fdisk utility will display each slice and ask whether you want to edit it. If you say yes, fdisk will step through each field, show you the old value, and ask you for a new one. When you are done with the slice, fdisk will display it and ask you whether it is correct. It will then proceed to the next entry.

Getting the cyl, sector, and head fields correct is tricky, so by default, they will be calculated for you; you can specify them if you choose to though.

After all the slices are processed, you are given the option to change the "active" slice. Finally, when all the new data for sector 0 has been accumulated, you are asked to confirm whether you really want to rewrite it.

The difference between the -u and -i flags is that the -u flag just edits (updates) the fields as they appear on the disk, while the -i flag is used to "initialize" sector 0; it will set up the last BIOS slice to use the whole disk for
.Fx and make it active.


The automatic calculation of starting cylinder etc. uses a set of figures that represent what the BIOS thinks the geometry of the drive is. These figures are taken from the in-core disklabel by default, but fdisk initially gives you an opportunity to change them. This allows you to create a bootblock that can work with drives that use geometry translation under the BIOS.

If you hand craft your disk layout, please make sure that the
.Fx slice starts on a cylinder boundary. A number of decisions made later may assume this. (This might not be necessary later.)

Editing an existing slice will most likely result in the loss of all data in that slice.

You should run fdisk interactively once or twice to see how it works. This is completely safe as long as you answer the last question in the negative. There are subtleties that fdisk detects that are not fully explained in this manual page.


When the -f option is given, a disk’s slice table can be written using values from a configfile. The syntax of this file is very simple; each line is either a comment or a specification, as follows:
# comment ...
Lines beginning with a # are comments and are ignored.
g spec1 spec2 spec3
Set the BIOS geometry used in slice calculations. There must be three values specified, with a letter preceding each number:
c num Set the number of cylinders to num.
h num Set the number of heads to num.
s num Set the number of sectors/track to num.

These specs can occur in any order, as the leading letter determines which value is which; however, all three must be specified.

This line must occur before any lines that specify slice information.

It is an error if the following is not true:
1 <= number of cylinders
1 <= number of heads <= 256
1 <= number of sectors/track < 64

The number of cylinders should be less than or equal to 1024, but this is not enforced, although a warning will be printed. Note that bootable
.Fx slices (the "/" file system) must lie completely within the first 1024 cylinders; if this is not true, booting may fail. Non-bootable slices do not have this restriction.

Example (all of these are equivalent), for a disk with 1019 cylinders, 39 heads, and 63 sectors:
g c1019 h39s63
g h39c1019 s63
g s63h39c1019

p slice type start length
Set the slice given by slice (1-4) to type type, starting at sector start for length sectors.

Only those slices explicitly mentioned by these lines are modified; any slice not referenced by a p line will not be modified. However, if an invalid slice table is present, or the -i option is specified, all existing slice entries will be cleared (marked as unused), and these p lines will have to be used to explicitly set slice information. If multiple slices need to be set, multiple p lines must be specified; one for each slice.

These slice lines must occur after any geometry specification lines, if one is present.

The type is 165 for
.Fx slices. Specifying a slice type of zero is the same as clearing the slice and marking it as unused; however, dummy values (such as "0") must still be specified for start and length.

Note: the start offset will be rounded upwards to a head boundary if necessary, and the end offset will be rounded downwards to a cylinder boundary if necessary.

Example: to clear slice 4 and mark it as unused:

"p 4 0 0 0"

Example: to set slice 1 to a
.Fx slice, starting at sector 1 for 2503871 sectors (note: these numbers will be rounded upwards and downwards to correspond to head and cylinder boundaries):

"p 1 1651 2503871"

a slice
Make slice the active slice. Can occur anywhere in the config file, but only one must be present.

Example: to make slice 1 the active slice:

"a 1"


/boot/mbr The default boot code.


boot0cfg(8), bsdlabel(8), newfs(8)



Created by Blin Media, 2008-2013