The FreeBSD Bootstrap

http://www.freebsd.org/handbook/boot.html

     The boot blocks are responsible for finding (usually) the loader, and running it, and thus need to understand how to find that program on the filesystem, how to run the program, and also allow minor configuration of how they work.

boot0
     There is actually a preceding bootblock, named boot0, which lives on the Master Boot Record, the special part of the disk that the system bootstrap looks for and runs, and it simply shows a list of possible slices to boot from. boot0 is very simple, since the program in the MBR can only be 512 bytes in size.

boot1
     boot1 is 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. 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
     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.

Loader: Bootstrap Stage Three

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

Note: While /boot/boot0, /boot/boot1, and /boot/boot2 are files there, they are not the actual copies in the MBR, the boot sector, or the disklabel respectively.      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 then the interpreter is started, and the easy-to-use commands are explained to it. 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.

The easy-to-use command set for "loader" comprises of:

autoboot seconds
	Proceeds to boot the kernel if not interrupted within the time span
	given, in seconds. It displays a countdown, and the default timespan
	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
	Display the files specified, with a pause at each LINES displayed.

reboot
	Immediately reboots the system.

set variable, set variable=value
	Set loader's environment variables.

unload
	Removes all loaded modules.
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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've 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'd normally do in the kernel boot-time configurator):
	load -t userconfig_script
	/boot/kernel.conf

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Kernel Boot Flags
Here are the more common boot flags:

-a
	during kernel initialization, ask for the device to mount as as the
	root file system.

-C
	boot from CDROM.

-c
	run UserConfig, the boot-time kernel configurator

-s
	boot into single-user mode

-v
	be more verbose during kernel startup

Note: There are other boot flags, read boot(8) for more information on them.