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Manual Pages  — BOOT0CFG


boot0cfg – boot manager installation/configuration utility



boot0cfg [-Bv] [-b boot0] [-d drive] [-e bell character] [-f file] [-i volume-id] [-m mask] [-o options] [-s slice] [-t ticks] disk


The FreeBSD '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-Bx 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.
-e bell character
  Set the character to be printed in case of input error.
-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.
-i volume-id
  Specifies a volume-id (in the form XXXX-XXXX) to be saved at location 0x1b8 in the MBR. This information is sometimes used by NT, XP and Vista to identify the disk drive. The option is only compatible with version 2.00 of the 512-byte boot block.
-m mask
  Specify slices to be enabled/disabled, where mask is an integer between 0 (no slices enabled) and 0xf (all four slices enabled). Each mask bit enables corresponding slice if set to 1. The least significant bit of the mask corresponds to slice 1, the most significant bit of the mask corresponds to slice 4.
-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. The special string "PXE" or a value of 6 can be used to boot via PXE.
-t ticks
  Set the timeout value to ticks. (There are approximately 18.2 ticks per second.)
  Verbose: display information about the slices defined, etc.


/boot/boot0 The default 'boot0' image
  Image for serial consoles (COM1,9600,8,N,1,MODEM)


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


To boot slice 2 on the next boot:

    boot0cfg -s 2 ada0

To enable just slices 1 and 3 in the menu:

    boot0cfg -m 0x5 ada0

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

    gpart bootcode -b /boot/mbr ada0


geom(4), boot(8), gpart(8)


Robert Nordier <Mt rnordier@FreeBSD.org>


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 boot0 code to write the MBR to the wrong disk, thus trashing its previous content. Be careful.

BOOT0CFG (8) October 1, 2013

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