BOOT0CFG(8) MidnightBSD System Manager’s Manual BOOT0CFG(8)


boot0cfg — boot manager installation/configuration utility


boot0cfg [−Bv] [−b boot0] [−d drive] [−f file] [−m mask] [−o options] [−s slice] [−t ticks] disk


The MidnightBSD ‘boot0’ boot manager permits the operator to select from which disk and slice an i386 machine (PC) is booted.

Note that what are referred to here as ‘‘slices’’ are typically called ‘‘partitions’’ in non-BSD documentation relating to the PC. Typically, only non-removable disks are sliced.

The boot0cfg utility optionally installs the ‘boot0’ boot manager on the specified disk; and allows various operational parameters to be configured.

On PCs, a boot manager typically occupies sector 0 of a disk, which is known as the Master Boot Record (MBR). The MBR contains both code (to which control is passed by the PC BIOS) and data (an embedded table of defined slices).

The options are:


Install the ‘boot0’ boot manager. This option causes MBR code to be replaced, without affecting the embedded slice table.

−b boot0

Specify which ‘boot0’ image to use. The default is /boot/boot0 which will use the video card as output, alternatively /boot/boot0sio can be used for output to the COM1 port. (Be aware that nothing will be output to the COM1 port unless the modem signals DSR and CTS are active.)

−d drive

Specify the drive number used by the PC BIOS in referencing the drive which contains the specified disk. Typically this will be 0x80 for the first hard drive, 0x81 for the second hard drive, and so on; however any integer between 0 and 0xff is acceptable here.

−f file

Specify that a backup copy of the preexisting MBR should be written to file. This file is created if it does not exist, and replaced if it does.

−m mask

Specify slices to be enabled/disabled, where mask is an integer between 0 (no slices enabled) and 0xf (all four slices enabled).

−o options

A comma-separated string of any of the following options may be specified (with ‘‘no’’ prepended as necessary):


Use the disk packet (BIOS INT 0x13 extensions) interface, as opposed to the legacy (CHS) interface, when doing disk I/O. This allows booting above cylinder 1023, but requires specific BIOS support. The default is ‘packet’.


Forces the drive containing the disk to be referenced using drive number definable by means of the -d option. The default is ‘nosetdrv’.


Allow the MBR to be updated by the boot manager. (The MBR may be updated to flag slices as ‘active’, and to save slice selection information.) This is the default; a ‘noupdate’ option causes the MBR to be treated as read-only.

−s slice

Set the default boot selection to slice. Values between 1 and 4 refer to slices; a value of 5 refers to the option of booting from a second disk.

−t ticks

Set the timeout value to ticks. (There are approximately 18.2 ticks per second.)


Verbose: display information about the slices defined, etc.


The default ‘boot0’ image


Image for serial consoles (COM1,9600,8,N,1,MODEM)


The boot0cfg utility exits 0 on success, and >0 if an error occurs.


To boot slice 2 on the next boot:

boot0cfg -s 2 ad0

To enable just slices 1 and 2 in the menu:

boot0cfg -m 0x3 ad0

To go back to non-interactive booting, use fdisk(8) to install the default MBR:

fdisk -B ad0


geom(4), boot(8), fdisk(8)


Robert Nordier 〈〉.


Protection mechanisms in the geom(4) subsystem might prevent boot0cfg from being able to update the MBR on a mounted disk. Instructions for temporarily disabling these protection mechanisms can be found in the geom(4) manpage.

Use of the ‘packet’ option may cause ‘boot0’ to fail, depending on the nature of BIOS support.

Use of the ‘setdrv’ option with an incorrect -d operand may cause the MBR to be written to the wrong disk. Be careful!

MidnightBSD 0.3 June 7, 2007 MidnightBSD 0.3