system bootstrapping procedures
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.
. 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.
Some newer PCs boot using UEFI firmware, not BIOS. That process is described in
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,
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.
However, it is possible to dispense with the third stage altogether, either by
specifying a kernel name in the boot block parameter file,
, or, unless option
is set, by hitting a key during a brief
pause (while one of the characters -
is displayed) before
is invoked. Booting will also be attempted at stage two, if the third stage
cannot be loaded.
The remainder of this subsection deals only with the boot blocks. The
program is documented separately.
After the boot blocks have been loaded, you should see a prompt similar to the
>> FreeBSD/i386 BOOT
The automatic boot will attempt to load
’ of either the floppy or the hard
disk. This boot may be aborted by typing any character on the keyboard at the
’ prompt. At this time, the
following input will be accepted:
- Give a short listing of the files in the root directory of the default
boot device, as a hint about available boot files. (A
? may also be specified as the last
segment of a path, in which case the listing will be of the relevant
- Specify boot file and flags.
- The drive number as recognized by the BIOS. 0 for the first drive, 1
for the second drive, etc.
- 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.
The supported interfaces are:
- ST506, IDE, ESDI, RLL disks on a WD100 or lookalike
- 5 1/4" or 3 1/2" High density floppies
- SCSI disk on any supported SCSI controller
- The unit number of the drive on the interface being used. 0 for the
first drive, 1 for the second drive, etc.
- The partition letter inside the BSD portion of
the disk. See
By convention, only partition
a’ contains a bootable image.
If sliced disks are used (“fdisk partitions”), any
slice (1 for the first slice, 2 for
the second slice, etc.) can be booted from, with the default (if not
specified) being the active slice or, otherwise, the first
FreeBSD slice. If
slice is specified as 0, the first
FreeBSD slice (also known as
“compatibility” slice) is booted from.
- The pathname of the file to boot (relative to the root directory on
the specified partition). Defaults to
/boot/kernel/kernel. Symbolic links
are not supported (hard links are).
- Boot flags:
- during kernel initialization, ask for the device to mount as the
root file system.
- try to mount root file system from a CD-ROM.
- this flag is currently a no-op.
- boot with the dual console configuration. In the single
configuration, the console will be either the internal display or
the serial port, depending on the state of the
-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
- enter the DDB kernel debugger (see
as early as possible in kernel initialization.
- use the GDB remote debugging protocol.
- force the serial console. For instance, if you boot from the
internal console, you can use the
-h option to force the kernel
to use the serial port as its console device. The serial port
has a flag (0x20) to override this option. If that flag is set,
the serial port will always be used as the console, regardless of
-h option described
- mute the console to suppress all kernel console input and output
during the boot.
- ignore key press to interrupt boot before
- probe the keyboard. If no keyboard is found, the
-h options are automatically
- pause after each attached device during the device probing
- be quiet, do not write anything to the console unless automatic
boot fails or is disabled. This option only affects second-stage
bootstrap, to prevent next stages from writing to the console use
in combination with the
- use the statically configured default for the device containing
the root file system (see
Normally, the root file system is on the device that the kernel
was loaded from.
- boot into single-user mode; if the console is marked as
the root password must be entered.
- set the speed of the serial console to
speed. The default is 9600 unless
it has been overridden by setting
and recompiling and reinstalling the boot blocks.
- be verbose during device probing (and later).
Use the /boot.config
file to set the default
configuration options for the boot block code. See
for more information about the /boot.config
- parameters for the boot blocks (optional)
- first stage bootstrap file
- second stage bootstrap file
- third stage bootstrap
- default kernel
- typical non-default kernel (optional)
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
- Invalid argument
- Address mark not found
- Sector not found
- DMA overrun
- DMA attempt across 64K boundary
- Invalid media
- Uncorrectable CRC/ECC error
- Controller failure
- Seek failed
: On older machines, or otherwise where EDD
support (disk packet interface support) is not available, 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 “Disk error 0x1” is reported
by the second-stage bootstrap, it generally means that this requirement has
not been adhered to.
The bsdlabel format used by this version of BSD
different from that of other architectures.
Due to space constraints, the keyboard probe initiated by the
option is simply a test that the BIOS
has detected an “extended” keyboard. If an “XT/AT”
keyboard (with no F11 and F12 keys, etc.) is attached, the probe will