The boot manager code in the MBR is sometimes referred to as stage zero of the boot process. By default, FreeBSD uses the boot0 boot manager.

The MBR installed by the FreeBSD installer is based on /boot/boot0. The size and capability of boot0 is restricted to 446 bytes due to the slice table and 0x55AA identifier at the end of the MBR. If boot0 and multiple operating systems are installed, a message similar to this example will be displayed at boot time:

F1 Win
F2 FreeBSD

Default: F2

Other operating systems will overwrite an existing MBR if they are installed after FreeBSD. If this happens, or to replace the existing MBR with the FreeBSD MBR, use the following command:

# fdisk -B -b /boot/boot0 device

where device is the boot disk, such as ad0 for the first IDE disk, ad2 for the first IDE disk on a second IDE controller, or da0 for the first SCSI disk. To create a custom configuration of the MBR, refer to boot0cfg(8).

Conceptually, the first and second stages are part of the same program on the same area of the disk. Because of space constraints, they have been split into two, but are always installed together. They are copied from the combined /boot/boot by the FreeBSD installer or bsdlabel.

These two stages are located outside file systems, in the first track of the boot slice, starting with the first sector. This is where boot0, or any other boot manager, expects to find a program to run which will continue the boot process.

The first stage, boot1, is very simple, since it can only be 512 bytes in size. It knows just enough about the FreeBSD bsdlabel, which stores information about the slice, to find and execute boot2.

Stage two, boot2, is slightly more sophisticated, and understands the FreeBSD file system enough to find files. It can provide a simple interface to choose the kernel or loader to run. It runs loader, which is much more sophisticated and provides a boot configuration file. If the boot process is interrupted at stage two, the following interactive screen is displayed:

>> FreeBSD/i386 BOOT
Default: 0:ad(0,a)/boot/loader
boot:

To replace the installed boot1 and boot2, use bsdlabel, where diskslice is the disk and slice to boot from, such as ad0s1 for the first slice on the first IDE disk:

# bsdlabel -B diskslice

The loader is the final stage of the three-stage bootstrap process. It is located on the file system, usually as /boot/loader.

The loader is intended as an interactive method for configuration, using a built-in command set, backed up by a more powerful interpreter which has a more complex command set.

During initialization, loader will probe for a console and for disks, and figure out which disk it is booting from. It will set variables accordingly, and an interpreter is started where user commands can be passed from a script or interactively.

The loader will then read /boot/loader.rc, which by default reads in /boot/defaults/loader.conf which sets reasonable defaults for variables and reads /boot/loader.conf for local changes to those variables. loader.rc then acts on these variables, loading whichever modules and kernel are selected.

Finally, by default, loader issues a 10 second wait for key presses, and boots the kernel if it is not interrupted. If interrupted, the user is presented with a prompt which understands the command set, where the user may adjust variables, unload all modules, load modules, and then finally boot or reboot. Table 12.1, “Loader Built-In Commands” lists the most commonly used loader commands. For a complete discussion of all available commands, refer to loader(8).

Here are some practical examples of loader usage. To boot the usual kernel in single-user mode :

boot -s

To unload the usual kernel and modules and then load the previous or another, specified kernel:

unload
load kernel.old

Use kernel.GENERIC to refer to the default kernel that comes with an installation, or kernel.old, to refer to the previously installed kernel before a system upgrade or before configuring a custom kernel.

Use the following to load the usual modules with another kernel:

unload
set kernel="kernel.old"
boot-conf

To load an automated kernel configuration script:

load -t userconfig_script /boot/kernel.conf

Once the kernel is loaded by either loader or by boot2, which bypasses loader, it examines any boot flags and adjusts its behavior as necessary. Table 12.2, “Kernel Interaction During Boot” lists the commonly used boot flags. Refer to boot(8) for more information on the other boot flags.

Once the kernel has finished booting, it passes control to the user process init(8), which is located at /sbin/init, or the program path specified in the init_path variable in loader. This is the last stage of the boot process.

The boot sequence makes sure that the file systems available on the system are consistent. If a UFS file system is not, and fsck cannot fix the inconsistencies, init drops the system into single-user mode so that the system administrator can resolve the problem directly. Otherwise, the system boots into multi-user mode.

Enter full pathname of shell or RETURN for /bin/sh:

# name  getty                           type    status          comments
#
# If console is marked "insecure", then init will ask for the root password
# when going to single-user mode.
console none                            unknown off insecure