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The remainder of the FreeBSD bootstrap system is divided into three stages. The first stage knows just enough to get the computer into a specific state and run the second stage. The second stage can do a little bit more, before running the third stage. The third stage finishes the task of loading the operating system. The work is split into three stages because the MBR puts limits on the size of the programs that can be run at stages one and two. Chaining the tasks together allows FreeBSD to provide a more flexible loader.
The kernel is then started and begins to probe for devices and initialize them for use. Once the kernel boot process is finished, the kernel passes control to the user process man:init[8], which makes sure the disks are in a usable state, starts the user-level resource configuration which mounts file systems, sets up network cards to communicate on the network, and starts the processes which have been configured to run at startup.
This section describes these stages in more detail and demonstrates how to interact with the FreeBSD boot process.
The Boot Manager
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 [.filename]#/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:
[.filename]#boot0# Screenshot
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 [.filename]#ad0# for the first IDE disk, [.filename]#ad2# for the first IDE disk on a second IDE controller, or [.filename]#da0# for the first SCSI disk. To create a custom configuration of the MBR, refer to man:boot0cfg[8].
Stage One and Stage Two
Conceptually, the first and second stages are part of the same program on the same area of the disk. Due to space constraints, they have been split into two, but are always installed together. They are copied from the combined [.filename]#/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, [.filename]#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 [.filename]#boot2#.
Stage two, [.filename]#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:
[.filename]#boot2# Screenshot
>> FreeBSD/i386 BOOT
Default: 0:ad(0,a)/boot/loader
To replace the installed [.filename]#boot1# and [.filename]#boot2#, use `bsdlabel`, where _diskslice_ is the disk and slice to boot from, such as [.filename]#ad0s1# for the first slice on the first IDE disk: