Turning on a computer and starting the operating system poses an interesting dilemma. By definition, the computer does not know how to do anything until the operating system is started. This includes running programs from the disk. If the computer can not run a program from the disk without the operating system, and the operating system programs are on the disk, how is the operating system started?
This problem parallels one in the book The Adventures of Baron Munchausen. A character had fallen part way down a manhole, and pulled himself out by grabbing his bootstraps, and lifting. In the early days of computing the term bootstrap was applied to the mechanism used to load the operating system, which has become shortened to “booting”.
On x86 hardware the Basic Input/Output System (BIOS) is responsible for loading the operating system. To do this, the BIOS looks on the hard disk for the Master Boot Record (MBR), which must be located on a specific place on the disk. The BIOS has enough knowledge to load and run the MBR, and assumes that the MBR can then carry out the rest of the tasks involved in loading the operating system, possibly with the help of the BIOS.
The code within the MBR is usually referred to as a boot manager, especially when it interacts with the user. In this case the boot manager usually has more code in the first track of the disk or within some OS's file system. (A boot manager is sometimes also called a boot loader, but FreeBSD uses that term for a later stage of booting.) Popular boot managers include boot0 (aka Boot Easy, the standard FreeBSD boot manager), Grub, GAG, and LILO. (Only boot0 fits within the MBR.)
If only one operating system is installed, a standard PC MBR will suffice. This MBR searches for the first bootable (active) slice on the disk, and then runs the code on that slice to load the remainder of the operating system. By default, the MBR installed by fdisk(8) is such an MBR and is based on /boot/mbr.
If multiple operating systems are present, a different boot manager can be installed which displays the list of operating systems so that the user can choose which one to boot from. Two boot managers are discussed in the next subsection.
The remainder of the FreeBSD bootstrap system is divided into three stages. The first stage is run by the MBR, which 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 PC standards put 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 it 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 init(8), which then makes sure the disks are in a usable state. init(8) then 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 on a FreeBSD system at startup.