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biosboot(8/amd64) amd64-specific first-stage system bootstrap
biosboot(8/i386) i386-specific first-stage system bootstrap

BIOSBOOT(8) System Manager's Manual (amd64) BIOSBOOT(8)


biosbootamd64-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 softraid(4) volume, 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.
When 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 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 CD-ROM.
biosboot may fail with any of the following error messages:
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.)
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.
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.
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.


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


softraid(4), boot(8), boot_amd64(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.
September 10, 2015 OpenBSD-current