path: root/sbin/reboot/boot_i386.8

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.\" Almost completely rewritten for FreeBSD 2.1 by Joerg Wunsch.
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.\" Substantially revised for FreeBSD 3.1 by Robert Nordier.
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.\"     @(#)boot_i386.8	8.2 (Berkeley) 4/19/94
.\"
.\" $Id: boot_i386.8,v 1.17 1999/03/16 13:16:11 rnordier Exp $
.\"
.Dd April 19, 1994
.Dt BOOT 8 i386
.Os
.Sh NAME
.Nm boot
.Nd
system bootstrapping procedures
.Sh DESCRIPTION
.Sy Power fail and crash recovery.
Normally, the system will reboot itself at power-up or after crashes.
An automatic consistency check of the file systems will be performed,
and unless this fails, the system will resume multi-user operations.
.Pp
.Sy Cold starts.
Most i386 PCs attempt to boot first from floppy disk drive 0 (sometimes
known as drive A:) and, failing that, from hard disk drive 0 (sometimes
known as drive C:, or as drive 0x80 to the BIOS).  Some BIOSes allow
you to change this default sequence, and may also include a CD-ROM
drive as a boot device.
.Pp
By default, a three-stage bootstrap is employed, and control is
automatically passed from the boot blocks (bootstrap stages one and
two) to a separate third-stage bootstrap program,
.Pa loader .
This third stage provides more sophisticated control over the booting
process than it is possible to achieve in the boot blocks, which are
constrained by occupying limited fixed space on a given disk or slice.
.Pp
However, it is possible to dispense with the third stage altogether,
either by specifying a kernel name in the boot block parameter
file,
.Pa /boot.config ,
or by hitting a key during a brief pause (while one of the characters
.Dv - ,
.Dv \e ,
.Dv \&| ,
or
.Dv /
is displayed) before
.Pa loader
is invoked.  Booting will also be attempted at stage two, if the
third stage cannot be loaded.
.Pp
The remainder of this subsection deals only with the boot blocks.  The
.Pa loader
program is documented separately.
.Pp
After the boot blocks have been loaded,
you should see a prompt similar to the following:
.Bd -literal
>> FreeBSD/i386 BOOT
Default: 0:wd(0,a)/kernel
boot:
.Ed
.Pp
The automatic boot will attempt to load
.Pa /kernel
from partition
.Ql a
of either the floppy or the hard disk.
This boot may be aborted by typing any character on the keyboard
at the
.Ql boot:
prompt.  At this time, the following input will be accepted:
.Bl -tag -width 10x
.It \&?
Give a short listing of the files in the root directory of the default
boot device, as a hint about available boot files.  (A
.Dv \&?
may also be specified as the last segment of a path, in which case
the listing will be of the relevant subdirectory.)
.Pp
.It bios_drive:interface(unit,part) filename Op Fl aCcDdghPrsv
Specify boot file and flags.
.Bl -tag -width 10x -compact
.It bios_drive
The drive number as recognized by the BIOS. 
0 for the first drive, 1 for the second drive, etc.
.It interface
The type of controller to boot from.  Note that the controller is required
to have BIOS support since the BIOS services are used to load the
boot file image.
.Pp
The supported interfaces are:
.Bl -tag -width "wdXX" -compact
.It wd
ST506, IDE, ESDI, RLL disks on a WD100[2367] or lookalike
controller
.It fd
5 1/4" or 3 1/2" High density floppies
.It da
SCSI disk on any supported SCSI controller
.\".It cd
.\"boot from CDROM
.El
.It unit
The unit number of the drive on the interface being used.
0 for the first drive, 1 for the second drive, etc.
.It part
The partition letter inside the BSD portion of the disk.  See
.Xr disklabel 8 .
By convention, only partition
.Ql a
contains a bootable image.  If sliced disks are used
.Pq Dq fdisk partitions ,
any slice can be booted from, with the default being the active slice
or, otherwise, the first FreeBSD slice.
.It filename
The pathname of the file to boot (relative to the root directory
on the specified partition).  Defaults to
.Pa /kernel .
Symbolic links are not supported (hard links are).
.It Fl acCdDghPrsv
Boot flags:
.Bl -tag -width "-CXX" -compact
.It Fl a
during kernel initialization,
ask for the device to mount as as the root file system.
.It Fl C
boot from CDROM.
.It Fl c
run UserConfig to modify hardware parameters for the loaded
kernel.  If the kernel was built with the USERCONFIG_BOOT option,
remain in UserConfig regardless of any
.Ic quit
commands present in the script.
.It Fl D
toggle single and dual console configurations.  In the single
configuration the console will be either the internal display
or the serial port, depending on the state of the 
.Fl h
option below.  In the dual console configuration, 
both the internal display and the serial port will become the console 
at the same time, regardless of the state of the 
.Fl h
option.  However, the dual console configuration takes effect only during
the boot prompt.  Once the kernel is loaded, the console specified 
by the
.Fl h
option becomes the only console.
.It Fl d
enter the DDB kernel debugger
.Pq see Xr ddb 4
as early as possible in kernel initialization.
.It Fl g
use the GDB remote debugging protocol.
.It Fl h
toggle internal and serial consoles.  You can use this to switch 
console devices.  For instance, if you boot from the internal console, 
you can use the
.Fl h
option to force the kernel to use the serial port as its 
console device.  Alternatively, if you boot from the serial port, 
you can use this option to force the kernel to use the internal display 
as the console instead.  This option has no effect if the kernel was
compiled with
.Em options COMCONSOLE .
.It Fl P
probe the keyboard.  If no keyboard is found, the
.Fl D
and
.Fl h
options are automatically set.
.It Fl r
use the statically configured default for the device containing the
root file system
.Pq see Xr config 8 .
Normally, the root file system is on the device
that the kernel was loaded from.
.It Fl s
boot into single-user mode; if the console is marked as
.Dq insecure
.Pq see Xr ttys 5 ,
the root password must be entered.
.It Fl v
be verbose during device probing (and later).
.El
.El
.El
.Pp
You may put a BIOS drive number, a controller type, a unit number,
a partition, a kernel file name, and any valid option in
.Pa /boot.config
to set defaults.  Enter them in one line just as you type at the
.Ql boot:
prompt.
.Sh FILES
.Bl -tag -width /kernel.old.config -compact
.It Pa /boot.config
parameters for the boot blocks (optional)
.It Pa /boot/boot1
first stage bootstrap file
.It Pa /boot/boot2
second stage bootstrap file
.It Pa /boot/loader
third stage bootstrap
.It Pa /kernel
default kernel
.It Pa /kernel.old
typical non-default kernel (optional)
.El
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr ttys 5 ,
.Xr btxld 8 ,
.Xr config 8 ,
.Xr disklabel 8 ,
.Xr halt 8 ,
.Xr loader 8 ,
.Xr reboot 8 ,
.Xr shutdown 8
.Sh DIAGNOSTICS
When disk-related errors occur, these are reported by the second-stage
bootstrap using the same error codes returned by the BIOS, for example
``Disk error 0x1 (lba=0x12345678)''.  Here is a partial list of these
error codes:
.Bl -tag -width "0x80" -compat
.It 0x1
Invalid argument
.It 0x2
Address mark not found
.It 0x4
Sector not found
.It 0x8
DMA overrun
.It 0x9
DMA attempt across 64K boundary
.It 0xc
Invalid media
.It 0x10
Uncorrectable CRC/ECC error
.It 0x20
Controller failure
.It 0x40
Seek failed
.It 0x80
Timeout
.El
.Pp
IMPORTANT NOTE: Because of limitations imposed by the conventional
disk interface provided by the BIOS, all boot-related files and
structures (including the kernel) that need to be accessed during the
boot phase must reside on the disk at or below cylinder 1023 (as the
BIOS understands the geometry).  When a
.Dq Disk error 0x1
is reported by the second-stage bootstrap, it generally means that this
requirement has not been adhered to.
.Sh BUGS
The disklabel format used by this version of
.Bx
is quite
different from that of other architectures.
.Pp
Some features are not yet documented.