255 lines
6.7 KiB
Groff
255 lines
6.7 KiB
Groff
.\" $NetBSD: fdisk.8,v 1.16 1998/10/15 15:23:23 ws Exp $
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.\"
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.Dd April 4, 1993
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.Dt FDISK 8
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.\" .Os BSD 4
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.Sh NAME
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.Nm fdisk
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.Nd DOS partition maintenance program
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.Sh SYNOPSIS
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.Nm
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.Op Fl aiufS
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.Op Fl 0 | 1 | 2 | 3
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.Op Fl b Ar cylinders/heads/sectors
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.Op Fl s Ar id/start/size
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.Op Fl c Ar bootcode
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.Op Ar device
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.Sh PROLOGUE
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In order for the BIOS to boot the kernel, certain conventions must be
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adhered to.
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Sector 0 of the disk must contain boot code, a partition table, and a
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magic number.
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Newer BIOS programs want the entire first track reserved.
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BIOS partitions can be used to break the disk up into several pieces.
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The BIOS brings in sector 0, verifies the magic number, and begins
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executing the code at the first byte.
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This code in turn searches the DOS partition table for an `active'
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partition.
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If one is found, the boot block from that partition is loaded and replaces
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the original boot block.
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Under DOS, you could have one or more partitions with one active.
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The DOS
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.Nm
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program can be used to divide space on the disk into partitions and set
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one active.
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.Pp
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The
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.Nx
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program
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.Nm
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serves a similar purpose to the DOS program.
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When called with no arguments, it prints the sector 0 partition table.
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An example follows:
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.Bd -literal
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******* Working on device /dev/rwd0d *******
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Warning: BIOS sector numbering starts with sector 1
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parameters extracted from in-core disklabel are:
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cylinders=769 heads=15 sectors/track=33 (495 sectors/cylinder)
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parameters to be used for BIOS calculations are:
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cylinders=769 heads=15 sectors/track=33 (495 sectors/cylinder)
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Information from DOS bootblock is:
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The data for partition 0 is:
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0: sysid 169 (NetBSD)
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start 495, size 380160 (185 MB), flag 0
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beg: cylinder 1, head 0, sector 1
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end: cylinder 768, head 14, sector 33
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1: sysid 164 (reserved)
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start 378180, size 2475 (1 MB), flag 0
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beg: cylinder 764, head 0, sector 1
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end: cylinder 768, head 14, sector 33
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2: <UNUSED>
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3: sysid 99 (GNU HURD or Mach or Sys V/386 (such as ISC UNIX))
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start 380656, size 224234 (109 MB), flag 80
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beg: cylinder 769, head 0, sector 2
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end: cylinder 197, head 14, sector 33
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.Ed
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.Pp
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This example disk is divided into three partitions that happen to fill the
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disk. The second partition overlaps the end of the first.
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(Used for debugging purposes.)
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.Pp
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The various fields in each partition are:
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.Bl -tag -width "cylinder"
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.It Em "sysid"
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is used to label the partition.
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.Nx
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reserves the magic number 169 decimal (A9 in hex).
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.It Xo
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.Em start ,
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.Em size
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.Xc
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start address and size of a partition in sectors.
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.It Em "flag 80"
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specifies that this is the active partition.
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.It Xo
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.Em "cylinder" ,
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.Em "head" ,
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.Em "sector"
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.Xc
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the beginning or ending address of a partition.
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.El
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.Pp
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.Em "Note:"
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these numbers are calculated using BIOS's understanding of the disk geometry
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and saved in the bootblock.
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.Pp
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The flags
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.Fl a ,
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.Fl i
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or
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.Fl u
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are used to indicate that the partition data is to be updated.
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The
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.Nm
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program will enter a conversational mode.
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This mode is designed not to change any data unless you explicitly tell it to;
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.Nm
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selects defaults for its questions to guarantee that behavior.
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.Pp
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At the start the
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.Nm
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program will determine whether the disk sector 0 is valid as a boot sector.
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(This is determined by checking the magic number.)
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If not,
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.Nm
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will initialize the boot code as well as the partition table.
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During this, all four partitions will be marked empty.
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.Pp
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If the
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.Fl u
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flag is used, it displays each partition and asks if you want to edit it.
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If you reply affirmatively,
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it will step through each field showing the old value
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and asking for a new one.
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When you are done with a partition,
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.Nm
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will display the information again and ask if it is correct.
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.Nm
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will then procede to the next entry.
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.Pp
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Getting the
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.Em cylinder ,
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.Em head ,
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and
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.Em sector
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fields correct is tricky.
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So by default,
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they will be calculated for you;
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you can specify them if you choose.
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.Pp
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After all the partitions are processed,
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if the
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.Fl a
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flag was given, you are given the option to change the
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.Em active
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partition.
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To change only the
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.Em active
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partition, you can use only the
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.Fl a .
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.Pp
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Finally,
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when all the data for the first sector has been accumulated,
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.Nm
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will ask if you really want to rewrite sector 0.
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Only if you reply affirmatively to this question will
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.Nm
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write anything to the disk.
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.Pp
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The
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.Fl i
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explicitly requests initialization of the master boot code,
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even if the magic number in the first sector is ok.
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The partition table is left alone by this (but see above).
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.Pp
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You can use
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.Fl c
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to specify the filename that
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.Nm
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should read the bootcode from. The default is to read from
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.Em /usr/mdec/mbr ,
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if run on an i386, end leave the bootcode empty for other
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machines.
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.Pp
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The flags
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.Fl 0 ,
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.Fl 1 ,
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.Fl 2
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and
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.Fl 3
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allow the user to selectively update or set to active a specific
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partition.
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.Pp
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The
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.Fl s
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flag allows the user to specify the partition type, start and
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size from the command line. This flag requires the use of the
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partition selection flag.
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.Pp
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The
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.Fl b
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flag allows the user to specify the BIOS parameters for
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cylinders, heads and sectors. It is used only in conjunction with the
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.Fl u
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flag.
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.Pp
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The
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.Fl f
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flag makes
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.Nm
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work in a non-interactive mode. In this mode, you can only change the
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disk parameters by using the
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.Fl b
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flag. This is provided only so scripts or other programs may
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use fdisk as part of an automatic installation process. Using the
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.Fl f
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flag with
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.Fl u
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makes it impossible to specify the starting and ending
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.Em cylinder ,
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.Em head
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and
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.Em sector
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fields. They will be automatically computed using the BIOS geometry.
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.Pp
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The
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.Fl S
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flag used with no other flags prints a series of
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.Nm /bin/sh
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commands for setting variables to the partition information. This
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is used for the install script.
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.Sh NOTES
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The automatic calculation of the starting cylinder and
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other parameters uses
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a set of figures that represent what the BIOS thinks is the
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geometry of the drive.
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These figures are by default taken from the incore disklabel, but
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.Nm
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gives you an opportunity to change them.
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This allows the user to create a bootblock that can work with drives
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that use geometry translation under the BIOS.
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.Pp
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If you hand craft your disk layout,
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please make sure that the
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.Nx
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partition starts on a cylinder boundary.
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(This restriction may be changed in the future.)
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.Pp
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Editing an existing partition is risky, and may cause you to
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lose all the data in that partition.
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.Pp
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You should run this program interactively once or twice to see how it works.
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This is completely safe as long as you answer the last question in the negative.
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.Sh FILES
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.Bl -tag -width /usr/mdec/mbrxx -compact
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.It Pa /usr/mdec/mbr
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Default location of i386 bootcode
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.El
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.Sh SEE ALSO
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.Xr disklabel 8
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.Sh BUGS
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There are subtleties that the program detects that are not explained in
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this manual page.
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