Multibooting is having several operating systems on one computer, and some means of selecting the which OS is to boot. It is not a trivial task! If you don't understand what you are doing, you may end up deleting large amounts of data from your computer. New OpenBSD users are strongly encouraged to start with a blank hard drive on a dedicated machine, and then practice your desired configuration on a non-production system before attempting a multiboot configuration on a production machine. FAQ 14 has more information about the OpenBSD boot process.
Here are several options to multibooting:
Setting active partitions
This is probably the most overlooked, and yet, sometimes the best solution for multibooting. Simply set the active partition in whatever OS you are currently using to be the one you want to boot by default when you next boot. Virtually every OS offers a program to do this; OpenBSD's is fdisk(8), similar named programs are in Windows 9x and DOS, and many other operating systems. This can be highly desirable for OSs or systems which take a long time to shut down and reboot -- you can set it and start the reboot process, then walk away, grab a cup of coffee, and come back to the system booted the way you want it -- no waiting for the Magic Moment to select the next OS.
Boot floppy
If you have a system that is used to boot OpenBSD infrequently (or don't wish other users of the computer to note anything has changed), consider using a boot floppy. Simply use one of the standard OpenBSD install floppies, and create an /etc/boot.conf file (yes, you will also have to create an /etc directory on the floppy) with the contents:
boot hd0a:/bsd
to cause the system to boot from hard drive 0, OpenBSD partition 'a', kernel file /bsd. Note you can also boot from other drives with a line like: "boot hd2a:/bsd" to boot off the third hard drive on your system. To boot from OpenBSD, slip your floppy in, reboot. To boot from the other OS, eject the floppy, reboot.
In this case, the boot(8) program is loaded from the floppy, looks for and reads /etc/boot.conf. The "boot hd0a:/bsd" line instructs boot(8) where to load the kernel from -- in this case, the first HD the BIOS sees. Keep in mind, only a small file (/boot) is loaded from the floppy -- the system loads the entire kernel off the hard disk, so this only adds about five seconds to the boot process.
Windows NT/2000/XP NTLDR
To multiboot OpenBSD and Windows NT/2000/XP, you can use NTLDR, the boot loader that NT uses. To multi-boot with NT, you need a copy of your OpenBSD Partition Boot Record (PBR). After running installboot, you can copy it to a file using dd(1), following a process similar to:
# dd if=/dev/rsd0a of=openbsd.pbr bs=512 count=1
Note: this is a really good time to remind you that blindly typing commands in you don't understand is a really bad idea. This line will not work directly on most computers. It is left to the reader to adapt it to their machine.
Now boot NT and put openbsd.pbr in C:. Add a line like this to the end of C:\BOOT.INI:
c:\openbsd.pbr="OpenBSD"
When you reboot, you should be able to select OpenBSD from the NT loader menu. There is much more information available about NTLDR at the NTLDR Hacking Guide.
On Windows XP you can also edit the boot information using the GUI; see the XP Boot.ini HOWTO.
Programs that do much of this for you are available, for example, BootPart. This program can be run from Windows NT/2000/XP, and will fetch the OpenBSD PBR, place it on your NT/2000/XP partition, and will add it to C:\BOOT.INI
Note: The Windows NT/2000/XP boot loader is only capable of booting OSs from the primary hard drive. You can not use it to load OpenBSD from the second drive on a system.
Other boot loaders
Some other bootloaders OpenBSD users have used successfully include GAG, OS-BS, The Ranish Partition Manager and GRUB.
OpenBSD and Linux (i386)
Please refer to INSTALL.linux, which gives in depth instructions on getting OpenBSD working with Linux.
Time zone issues
OpenBSD expects the computer's real-time clock to be set to UTC (Universal Coordinated Time). Some other OSs expect the real-time clock to be set to local time. Obviously, this can create a bit of a problem if you are using both OSs on the same computer. One or the other is most likely going to have to be adapted. More info on doing this is in FAQ 8 - Why is my clock off by several hours?
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