biosboot - i386-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.
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.
When biosboot receives control from either the BIOS or the
master boot
record (MBR) it will print the message:
Loading
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 newly-loaded
program.
If possible, 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 flag.)
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 CDROM.
biosboot may fail with any of the following error messages:
ERR I Too many indirect blocks. biosboot is capable of
reading the
direct blocks in boot(8)'s inode (the location of
which is
patched into biosboot by 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.)
ERR M Bad magic. The ELF ``magic number'' 7fELF in
boot(8)'s header
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 installboot(8), giving an invalid
boot(8) program
as the boot argument to installboot(8), or incorrect geometry
translation.
ERR R 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 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.
ERR X 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 biosboot to calculate the (cylinder,
head, sector)
values required to read any sectors.
Using 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 entire disk.
Despite the support for boot(8) over the 8 GB boundary, good
disklabel(8)
partitioning practices should still be followed.
/usr/mdec/mbr Master Boot Record block
/usr/mdec/biosboot primary bootstrap
/boot secondary bootstrap
/usr/mdec/pxeboot PXE bootstrap
/bsd OpenBSD kernel
boot(8), boot_i386(8), disklabel(8), fdisk(8), installboot(8), pxeboot(8)
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 removed,
and 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.
OpenBSD 3.6 December 23, 2003
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