The code in the MBR or boot manager is sometimes referred to as stage zero of the boot process. This subsection discusses two of the boot managers previously mentioned: boot0 and LILO.
The boot0 Boot Manager: The MBR installed by FreeBSD's installer or boot0cfg(8), by default, is based on /boot/boot0. (The boot0 program is very simple, since the program in the MBR can only be 446 bytes long because of the slice table and 0x55AA identifier at the end of the MBR.) If you have installed boot0 and multiple operating systems on your hard disks, then you will see a display similar to this one at boot time:
Other operating systems, in particular Windows®, 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. Or, if you want a custom configuration of the MBR, use boot0cfg(8).
The LILO Boot Manager: To install this boot manager so it will also boot FreeBSD, first start Linux and add the following to your existing /etc/lilo.conf configuration file:
other=/dev/hdXY table=/dev/hdX loader=/boot/chain.b label=FreeBSD
In the above, specify FreeBSD's primary partition and drive using Linux specifiers,
replacing X with the Linux drive letter and Y with the Linux primary partition number. If you are
using a SCSI drive, you will need to change
/dev/hd to read something similar to /dev/sd. The loader=/boot/chain.b line can be omitted if you have both operating
systems on the same drive. Now run /sbin/lilo -v to commit
your new changes to the system; this should be verified by checking its screen
messages.
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 copied from the combined file /boot/boot by the installer or bsdlabel (see below).
They 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 number of sectors used is easily determined from the size of /boot/boot.
boot1 is very simple, since it can only be 512 bytes in size, and knows just enough about the FreeBSD bsdlabel, which stores information about the slice, to find and execute boot2.
boot2 is slightly more sophisticated, and understands the FreeBSD file system 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.
If you ever need to replace the installed boot1 and boot2 use bsdlabel(8):
# bsdlabel -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 bsdlabel(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 bsdlabel(8) command before you press Return.
The loader is the final stage of the three-stage bootstrap, and is located on the file system, 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.
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.
These are the most commonly used loader commands. For a complete discussion of all available commands, please see loader(8).
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.
Immediately proceeds to boot the kernel, with the given options, if any, and with the kernel name given, if it is. Providing a kernel name on the command-line is only applicable after an unload command has been issued, otherwise the previously-loaded kernel will be used.
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.
Shows help messages read from /boot/loader.help. If the topic given is index, then the list of available topics is given.
Processes the file with the given filename. The file is read in, and interpreted line by line. An error immediately stops the include command.
-t type] filenameLoads the kernel, kernel module, or file of the type given, with the filename given. Any arguments after filename are passed to the file.
-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.
-v]Lists all of the devices from which it may be possible to load modules. If -v is specified, more details are printed.
-v]Displays loaded modules. If -v is specified, more details
are shown.
Displays the files specified, with a pause at each LINES
displayed.
Immediately reboots the system.
Sets the loader's environment variables.
Removes all loaded modules.
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
The splash screen creates a more visually appealing boot screen compared to the original boot messages. This screen will be displayed until a console login prompt or an X display manager offers a login prompt.
There are two basic environments available in FreeBSD. The first is the default legacy virtual console command line environment. After the system finishes booting, a console login prompt is presented. The second environment is the X11 Desktop graphical environment. After X11 and one of the graphical desktop environments, such as GNOME, KDE, or XFce are installed, the X11 desktop can be launched by using the startx command.
Some users prefer the X11 graphical login screen over the traditional text based login prompt. Display managers like XDM for Xorg, gdm for GNOME, and kdm for KDE (and any other from the Ports Collection) basically provide a graphical login screen in place of the console login prompt. After a successful login, they present the user with a graphical desktop.
In the command line environment, the splash screen would hide all the boot probe messages and task startup messages before displaying the login prompt. In X11 environment, the users would get a visually clearer system start up experience resembling something closer to what a (Microsoft® Windows or non-unix type system) user would experience.
The splash screen function only supports 256-color bitmap (.bmp) or ZSoft PCX (.pcx) files. In addition, the splash image files must have a resolution of 320 by 200 pixels or less to work on standard VGA adapters.
To use larger images, up to the maximum resolution of 1024 by 768 pixels, activate the VESA support included in FreeBSD. This can be enabled by loading the VESA module during system boot, or adding a VESA kernel configuration option and building a custom kernel (see Chapter 8). The VESA support gives users the ability to display a splash screen image that fills the whole display screen.
While the splash screen is being displayed during the booting process, it can be turned off any time by hitting any key on the keyboard.
The splash screen also defaults to being a screen saver outside of X11. After a time period of non-use the screen will change to the splash screen and cycle through steps of changing intensity of the image, from bright to a very dark and over again. This default splash screen (screen saver) behavior could be overridden by adding a saver= line to /etc/rc.conf. Option saver= has several built-in screen savers to choose from, the full list can be found in the splash(4) manual page. The default screen saver is called “warp”. Note that the saver= option specified in /etc/rc.conf only applies to virtual consoles. It has no effect on X11 display managers.
A few boot loader messages, including the boot options menu and a timed wait count down prompt are displayed at boot time, even when the splash screen is enabled.
Sample splash screen files can be downloaded from the gallery at http://artwork.freebsdgr.org. By installing the sysutils/bsd-splash-changer port, splash images can be chosen from a collection randomly at each boot.
The splash screen (.bmp) or (.pcx) file has to be placed on the root partition, for example in the /boot directory.
For default boot display resolution (256-color, 320 by 200 pixels, or less), edit /boot/loader.conf, so it contains the following:
splash_bmp_load="YES" bitmap_load="YES" bitmap_name="/boot/splash.bmp"
For larger video resolutions up to the maximum of 1024 by 768 pixels, edit /boot/loader.conf, so it contains the following:
vesa_load="YES" splash_bmp_load="YES" bitmap_load="YES" bitmap_name="/boot/splash.bmp"
The above assumes that /boot/splash.bmp is used for splash screen. When a PCX file is desired, use the following statements, plus the vesa_load="YES" line depending on the resolution.
splash_pcx_load="YES" bitmap_load="YES" bitmap_name="/boot/splash.pcx"
The file name is not restricted to “splash” as shown in the above example. It can be anything as long as it has type of BMP or PCX, such as splash_640x400.bmp or blue_wave.pcx.
Some other interesting loader.conf options:
This will stop the boot options menu from being displayed, but the timed wait count down prompt will still be present. Even with the display of the boot options menu disabled, entering an option selection at the timed wait count down prompt will enact the corresponding boot option.
This will replace the default words “FreeBSD”, which are displayed to the right of the boot options menu with the colored beastie logo like releases in the past had.
For more information, please see the splash(4), loader.conf(5), and vga(4) manual pages.
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.