OpenBSD i386 first-stage system bootstrap

biosboot - i386-specific first-stage system bootstrap

DESCRIPTION
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 installa-
tion, 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 vari-
ous 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 avail-
able.  However, to allow users to boot on hardware that claims LBA capa-
bility, 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.)

DIAGNOSTICS
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:

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 sec-
tors (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.

NOTES
Using biosboot as the MBR, as has been done in the past, is not recom-
mended, and is not supported.  Instead, create a single fdisk(8) parti-
tion 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.

FILES
/usr/mdec/mbr         Master Boot Record block
/usr/mdec/biosboot    primary bootstrap
/boot                 secondary bootstrap
/usr/mdec/pxeboot     PXE bootstrap
/bsd                  OpenBSD kernel
/bsd.mp               OpenBSD kernel for multi-processor machines
/bsd.rd               OpenBSD kernel for installation/recovery