i386-specific first-stage system bootstrap
This small program (roughly 512 bytes of code) is responsible for loading the
program (typically /boot), which in turn will load the kernel.
must be installed by
As part of the installation,
with information about the
location of boot(8)
disk. Specifically, it writes the filesystem block number of
's inode, the
offset within this block of the inode, and various filesystem parameters
(taken from the superblock) required to convert filesystem blocks to disk
is loaded from
the root filesystem of the boot disk. If the boot disk is a
arranges for a copy of
to be loaded
from a dedicated single-inode filesystem located within the volume's meta data
You must re-run
changed, as its inode may change. While it should not be necessary, it may
also be advisable to re-run
if you move your disk between machines and/or controllers.
receives control from either
the BIOS or the master boot record (MBR) it will print the message:
followed by a dot for every filesystem block it attempts to load. If /boot is
will put the
cursor on the next line just before transferring control to the newly-loaded
will read disk sectors
using calls detailed in the Phoenix Enhanced Disk Drive Specification (EDD,
sometimes known as LBA, reads). It will fall back to CHS reads only if EDD
calls are not available. However, to allow users to boot on hardware that
claims LBA capability, but which requires CHS reads in order to boot, the user
may hold down either Shift key during boot. If
detects this, it will force itself
to use CHS calls, ignoring any LBA capability. This will of course prevent
booting if /boot lies above the 8 GB CHS limit. There is an exported symbol
“force_chs” of type u_int8_t which may be set to 1 to force CHS
reads always. (However, no tool is currently provided to set this flag.)
prints a ‘!’ before
the “Loading” message if it is being forced to use CHS rather
than LBA reads (by the user holding down either Shift key during boot, or
having set the “force_chs” flag in the boot sector).
prints a ‘;’ after the
“Loading” message if it is going to use CHS reads for any
reason. For example, when booting from floppy or CD-ROM.
may fail with any of the following
- Too many indirect blocks.
capable of reading the direct blocks in
(the location of which is patched into
and the first indirect block, but it is not capable of reading further
indirect blocks. This error indicates that further such indirect blocks
were found. The system will not be able to boot.
This is unlikely to ever happen in practice, as
boot(8) has to be
quite large for this to be an issue. The smallest possible filesystem
block size is 512 bytes (one sector per filesystem block). On such a
system, there are 140 filesystem blocks that
biosboot can read, so
boot(8) can be up
to 70 KB.
However, even on floppy disks the filesystem block size is 1024 bytes. This
allows boot(8) to
occupy up to 268 disk blocks, i.e. to be 268 KB. On hard disks (default
filesystem block size 16 KB) 4,108 disk blocks are available, to allow
boot(8) to be over
64 MB in size! (Only direct blocks are required for
boot(8)s of up to
- Bad magic. The ELF “magic number” \7fELF in
was not found. This indicates that the first block of
boot(8) was not
read correctly. This could be due to disk corruption, failing to run
giving an invalid
boot(8) program as
the boot argument to
or incorrect geometry translation.
- Read error. The BIOS returned an error indication when
biosboot attempted to read a disk
sector. This might be any media error, including bad sectors (common on
floppy disks), and invalid sectors (can occur with bad geometry
If this error occurs during an LBA boot (no ‘;’ after
“Loading”), then a CHS boot may succeed. To do this, you
should reboot, then hold down either Shift key before
biosboot starts. You should see a
‘!’ before “Loading” as confirmation that your
override was accepted.
- Can't boot. Issued when trying to read sectors in CHS mode, but the BIOS
call get drive parameters
failed or gave a value of 0 for the number of sectors per track. In either
case, it is not possible for
to calculate the (cylinder, head, sector) values required to read any
as the MBR, as has been done
in the past, is not recommended, and is not supported. Instead, create a
partition that spans the entire disk.
Despite the support for
over the 8 GB
partitioning practices should still be followed.
- Master Boot Record block
- primary bootstrap
- secondary bootstrap
- PXE bootstrap
- OpenBSD kernel
- OpenBSD kernel for single processor machines
- OpenBSD kernel for multiprocessor machines
- OpenBSD kernel for installation/recovery
was originally written by Michael
Shalayeff for OpenBSD 2.1
. However it was based on
bootstrap code from older versions of this operating system, other operating
systems, other programs, and other people's work.
It was significantly revised in December 2003 by Tom Cosgrove, in order to
support LBA disk access (via the Phoenix Enhanced Disk Drive Specification
API). At that time the internal table of disk blocks was removed, and
modified to read filesystem block
numbers from the inode.
should perform and verify a checksum
across the entire loaded
than just checking the magic number in the first block.
There is no BIOS error number reported nor is the location of the error
You can pick your motherboard, and you can pick your BIOS, but you can't pick
your motherboard's BIOS.