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FDISK(8)		FreeBSD	System Manager's Manual		      FDISK(8)

NAME
     fdisk -- PC slice table maintenance program

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

PROLOGUE
     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 num-
     ber.  The sector 0	boot code then searches	the slice table	to determine
     which slice is marked active.  This boot code then	brings in the boot-
     strap 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 program
     can be used to divide space on the	disk into slices and set one active.

DESCRIPTION
     The FreeBSD program, fdisk, serves	a similar purpose to the DOS program.
     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:
     Change the	active slice only.  Ignored if -f is given.
     Get the boot code from the	file bootcode.	Default	is /boot/mbr.
     Reinitialize the boot code	contained in sector 0 of the disk.  Ignored if
     -f	is given.
     Set slice values using the	file configfile.  The configfile always	modi-
     fies 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 stdin 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!
     Initialize	sector 0 of the	disk.  This implies -u,	unless -f is given.
     Initialize	the contents of	sector 0 for one FreeBSD slice covering	the
     entire disk.
     Print summary information and exit.
     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.
     Is	used for updating (editing) sector 0 of	the disk.  Ignored if -f is
     given.
     Be	verbose.  When -f is used, fdisk prints	out the	slice table that is
     written to	the disk.
     Operate on	a single fdisk 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 fully qualified device node under /dev.  If omitted,	the disks
   `ad0' and `da0' are searched	in that	order, until one is found to respond.

   When	called without options,	fdisk 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:
	   <UNUSED>
	   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.  FreeBSD 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 understand-
			 ing 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 conversa-
   tional mode.	 In this mode, no changes will be written to disk unless you
   explicitly tell fdisk to.

   Fdisk 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	dis-
   play	it and ask you whether it is correct.  Fdisk 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 accumu-
   lated, 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 FreeBSD and make it active.

NOTES
     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	ini-
     tially gives you an opportunity to	change them.  This allows you to cre-
     ate 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 FreeBSD
     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.

CONFIGURATION FILE
     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:

	     cnum    Set the number of cylinders to num.

	     hnum    Set the number of heads to	num.

	     snum    Set the number of sectors/track to	num.

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

	     This line must occur before any lines that	specify	slice informa-
	     tion.

	     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 FreeBSD slices (the "/" filesystem) must lie	com-
	     pletely within the	first 1024 cylinders; if this is not true,
	     booting may fail.	Non-bootable slices do not have	this restric-
	     tion.

	     Example (all of these are equivalent), for	a disk with 1019
	     cylinders,	39 heads, and 63 sectors:

		   g	   c1019   h39	   s63
		   g	   h39	   c1019   s63
		   g	   s63	   h39	   c1019

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

	     Only those	slices explicitly mentioned by these lines are modi-
	     fied; any slice not referenced by a "p" line will not be modi-
	     fied.  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 FreeBSD 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 FreeBSD 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	   165	   1	   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

FILES
     /boot/mbr	The default boot code

SEE ALSO
     boot0cfg(8), disklabel(8),	newfs(8)

BUGS
     The default boot code will	not necessarily	handle all slice types cor-
     rectly, in	particular those introduced since MS-DOS 6.x.

     The entire	program	should be made more user-friendly.

     Most users	new to FreeBSD do not understand the difference	between	slice
     and partition causing difficulty to adjust.

     You cannot	use this command to completely dedicate	a disk to FreeBSD.
     The disklabel(8) command must be used for this.

FreeBSD	10.1			October	4, 1996			  FreeBSD 10.1

NAME | SYNOPSIS | PROLOGUE | DESCRIPTION | NOTES | CONFIGURATION FILE | FILES | SEE ALSO | BUGS

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