The MBR, and Boot Stages One, Two, and Three

FreeBSD Handbook
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7.3.1 MBR, `/boot/boot0`

The FreeBSD MBR is located in /boot/boot0. This is a copy of the MBR, as the real MBR must be placed on a special part of the disk, outside the FreeBSD area.

boot0 is very simple, since the program in the MBR can only be 512 bytes in size. If you have installed the FreeBSD MBR and have installed multiple operating systems on your hard disks then you will see a display similar to this one at boot time:

Example 7-1. boot0 Screenshot

    F1 DOS
    F2 FreeBSD
    F3 Linux
    F4 ??
    F5 Drive 1
    
    Default: F2

Other operating systems, in particular Windows 95, have been known to overwrite an existing MBR with their own. If this happens to you, or you want to replace your existing MBR with the FreeBSD MBR then use the following command:

    # fdisk -B -b /boot/boot0 device

Where device is the device that you boot from, such as ad0 for the first IDE disk, ad2 for the first IDE disk on a second IDE controller, da0 for the first SCSI disk, and so on.

If you are a Linux user, however, and prefer that LILO control the boot process, you can edit the /etc/lilo.conf file for FreeBSD, or select Leave The Master Boot Record Untouched during the FreeBSD installation process. If you have installed the FreeBSD boot manager, you can boot back into Linux and modify the LILO configuration file /etc/lilo.conf and add the following option:

    other=/dev/hdXY
    table=/dev/hdb
    loader=/boot/chain.b
    label=FreeBSD

which will permit the booting of FreeBSD and Linux via LILO. In our example, we use XY to determine drive number and partition. If you are using a SCSI drive, you will want to change /dev/hdXY to read something similar to /dev/sdXY, which again uses the XY syntax. The loader=/boot/chain.b can be omitted if you have both operating systems on the same drive. You can now run /sbin/lilo -v to commit your new changes to the system, this should be verified with screen messages.

7.3.2 Stage One, `/boot/boot1`, and Stage Two, `/boot/boot2`

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 you would always install them together.

They are found on the boot sector of the boot slice, which is where boot0, or any other program on the MBR expects to find the program to run to continue the boot process. The files in the /boot directory are copies of the real files, which are stored outside of the FreeBSD filesystem.

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

boot2 is slightly more sophisticated, and understands the FreeBSD filesystem enough to find files on it, and can provide a simple interface to choose the kernel or loader to run.

Since the loader is much more sophisticated, and provides a nice easy-to-use boot configuration, boot2 usually runs it, but previously it was tasked to run the kernel directly.

Example 7-2. boot2 Screenshot

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

If you ever need to replace the installed boot1 and boot2 use disklabel(8).

    # disklabel -B diskslice

Where diskslice is the disk and slice you boot from, such as ad0s1 for the first slice on the first IDE disk.

Dangerously Dedicated Mode: If you use just the disk name, such as ad0, in the disklabel(8) command you will create a dangerously dedicated disk, without slices. This is almost certainly not what you want to do, so make sure you double check the disklabel(8) command before you press Return.

7.3.3 Stage Three, `/boot/loader`

The loader is the final stage of the three-stage bootstrap, and is located on the filesystem, usually as /boot/loader.

The loader is intended as a user-friendly method for configuration, using an easy-to-use built-in command set, backed up by a more powerful interpreter, with a more complex command set.

7.3.3.1 Loader Program Flow

During initialization, the loader will probe for a console and for disks, and figure out what 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, the 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 easy-to-use command set, where the user may adjust variables, unload all modules, load modules, and then finally boot or reboot.

7.3.3.2 Loader Built-In Commands

These are the most commonly used loader commands. For a complete discussion of all available commands, please see loader(8).

autoboot seconds: Proceeds to boot the kernel if not interrupted within the time span given, in seconds. It displays a countdown, and the default time span is 10 seconds.
boot [-options] [kernelname]: Immediately proceeds to boot the kernel, with the given options, if any, and with the kernel name given, if it is.
boot-conf: Goes through the same automatic configuration of modules based on variables as what happens at boot. This only makes sense if you use unload first, and change some variables, most commonly kernel.
help [topic]: Shows help messages read from /boot/loader.help. If the topic given is index, then the list of available topics is given.
include filename ...: Processes the file with the given filename. The file is read in, and interpreted line by line. An error immediately stops the include command.
load [-t type] filename: Loads the kernel, kernel module, or file of the type given, with the filename given. Any arguments after filename are passed to the file.
ls [-l] [path]: Displays a listing of files in the given path, or the root directory, if the path is not specified. If -l is specified, file sizes will be shown too.
lsdev [-v]: Lists all of the devices from which it may be possible to load modules. If -v is specified, more details are printed.
lsmod [-v]: Displays loaded modules. If -v is specified, more details are shown.
more filename: Displays the files specified, with a pause at each LINES displayed.
reboot: Immediately reboots the system.
set variable, set variable=value: Sets the loader's environment variables.
unload: Removes all loaded modules.

7.3.3.3 Loader Examples

Here are some practical examples of loader usage:

To simply boot your usual kernel, but in single-user mode:
```
    boot -s
```
To unload your usual kernel and modules, and then load just your old (or another) kernel:
```
    unload
    load kernel.old
```
You can use kernel.GENERIC to refer to the generic kernel that comes on the install disk, or kernel.old to refer to your previously installed kernel (when you have upgraded or configured your own kernel, for example).
Note: Use the following to load your usual modules with another kernel:
```
    unload
    set kernel="kernel.old"
    boot-conf
```
To load a kernel configuration script (an automated script which does the things you would normally do in the kernel boot-time configurator):
```
    load -t userconfig_script /boot/kernel.conf
```

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The Booting Problem	Up	Kernel Interaction During Boot

7.3 The MBR, and Boot Stages One, Two, and Three

7.3.1 MBR, /boot/boot0

7.3.2 Stage One, /boot/boot1, and Stage Two, /boot/boot2

7.3.3 Stage Three, /boot/loader