Contributed by &a.phk;. v1.1, April 26th. Booting FreeBSD is essentially a three step: Load the kernel, determine the root filesystem and initialize user-land things. This leads to some interesting possibilities shown below. Loading a kernel

We presently have three basic mechanisms for loading the kernel as described below: They all pass some information to the kernel to help the kernel decide what to do next. Biosboot Biosboot is our ``bootblocks'', it consists of two files, which will be installed in the first 8Kbytes of the floppy or hard-disk slice to be booted from. Biosboot can load a kernel from a FreeBSD filesystem. Dosboot Dosboot was written by DI. Christian Gusenbauer, and is unfortunately at this time one of the few pieces of code that will not compile under FreeBSD itself because it is written for Microsoft compilers. Dosboot will boot the kernel from a MS-DOS file or from a FreeBSD filesystem partition on the disk. It attempts to negotiate with the various and strange kinds of memory manglers that lurk in high memory on MS/DOS systems and usually wins them for its case. Netboot Netboot will try to find a supported Ethernet card, and use BOOTP, TFTP and NFS to find a kernel file to boot. Determine the root filesystem

Once the kernel is loaded and the boot-code jumps to it, the kernel will initialize itself, trying to determine what hardware is present and so on, and then it needs to find a root filesystem. Presently we support the following types of root filesystems: UFS This is the most normal type of root filesystem. It can reside on a floppy or on hard disk. MSDOS While this is technically possible, it is not particular useful, because of ``FAT'' filesystems inability to make links, device nodes and such ``UNIXisms''. MFS This is actually a UFS filesystem which has been compiled into the kernel. That means that the kernel does not really need any disks/floppies or other HW to function. CD9660 This is for using a CD-ROM as root filesystem. NFS This is for using a fileserver as root filesystem, basically making it a diskless machine. Initialize user-land things

To get the user-land going, when the kernel has finished initialization, it will create a process with ``/sbin/init''. You can substitute any program for /sbin/init, as long as you keep in mind that: there is no stdin/out/err unless you open it yourself, if you exit, the machine panics, signal handling is special for ``/stand/sysinstall'' program on the installation floppy. Interesting combinations

Boot a kernel with a MFS in it with a special /sbin/init which... mounts your /C: Attaches C:/freebsd.fs on /dev/vn0 mounts /dev/vn0 as /rootfs makes symlinks /rootfs/bin -> /bin /rootfs/etc -> /etc /rootfs/sbin -> /sbin (etc...) Now you run FreeBSD without repartitioning your hard disk... server:˜you/FreeBSD as /nfs, chroots to /nfs and executes /sbin/init there Now you run FreeBSD diskless, even though you do not control the NFS server... /dev/rwd0 and writes it to a remote tape station or fileserver. Now you finally got that backup you should have made a year ago... E -- Acts as a firewall/web-server/what do I know... This is particular interesting since you can boot from a write- protected floppy, but still write to your root filesystem...