|BIOSBOOT(8)||System Manager's Manual (amd64)||BIOSBOOT(8)|
amd64-specific first-stage system bootstrap
This small program (roughly 512 bytes of code) is responsible for loading the second-stage boot(8) program (typically /boot), which in turn will load the kernel.
biosboot must be installed by
installboot(8). As part of the
installation, installboot(8) patches
biosboot with information about the location of
boot(8) on disk. Specifically, it writes
the filesystem block number of boot(8)'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 sectors. Usually, boot(8) is loaded
from the root filesystem of the boot disk. If the boot disk is a
installboot(8) arranges for a copy
of boot(8) to be loaded from a dedicated
single-inode filesystem located within the volume's meta data area.
You must re-run installboot(8) whenever boot(8) is changed, as its inode may change. While it should not be necessary, it may also be advisable to re-run installboot(8) if you move your disk between machines and/or controllers.
biosboot 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 loaded successfully,
biosboot will put
the cursor on the next line just before transferring control to the
biosboot 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
biosboot 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
biosboot 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).
biosboot 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.
biosboot may fail with any of the
following error messages:
biosbootis capable of reading the direct blocks in boot(8)'s inode (the location of which is patched into
biosbootby installboot(8)) 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 192 KB.)
biosbootattempted 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 translations).
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.
biosbootto calculate the (cylinder, head, sector) values required to read any sectors.
biosboot as the MBR, as has been
done in the past, is not recommended, and is not supported. Instead, create
a single fdisk(8) partition that spans the
biosboot 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
biosboot modified to read filesystem
block numbers from the inode.
biosboot should perform and verify a
checksum across the entire loaded boot(8)
image, rather than just checking the magic number in the first block.
There is no BIOS error number reported nor is the location of the error reported.
You can pick your motherboard, and you can pick your BIOS, but you can't pick your motherboard's BIOS.
|September 10, 2015||OpenBSD-current|