转贴boot的启动选项:

rainren

Boot time Options - Using the OpenBSD bootloader
When booting your OpenBSD system, you have probably noticed the boot prompt.
boot>
For most people, you won't have to do anything here. It will automatically boot if no commands are given. But sometimes problems arise, or special functions are needed. That's where these options will come in handy. To start off, you should read through the boot(8) man page. Here we will go over the most common used commands for the bootloader.
To start off, if no commands are issued, the bootloader will automatically try to boot /bsd. If that fails it will try /obsd, and if that fails, it will try /bsd.old. You can specify this by hand by typing:
boot> boot wd0a:/bsd
or
boot> b /bsd
This will work if device wd0a is configured as your root device.
Here is a brief list of options you can use with the OpenBSD kernel.
·        -a : This will allow you to specify an alternate root device after booting the kernel.
·        -c : This allows you to enter the boot time configuration. Check the Boot Time Config section of the faq.
·        -s : This is the option to boot into single user mode.
·        -d : This option is used to dump the kernel into ddb. Keep in mind that you must have DDB built into the kernel.
These are entered in the format of: boot [ image [-acds]]
For further reading you can read boot_i386(8) man page