biosboot man page on OpenBSD

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BIOSBOOT(8)	    OpenBSD System Manager's Manual (i386)	   BIOSBOOT(8)

     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

     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

     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-

     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

     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
     /bsd.sp		   OpenBSD kernel for single processor machines
     /		   OpenBSD kernel for multiprocessor machines
     /bsd.rd		   OpenBSD kernel for installation/recovery

     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

     biosboot should perform and verify a checksum across the entire loaded
     boot(8) image, rather than just checking the magic number in the first

     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.

OpenBSD 4.9			August 10, 2010			   OpenBSD 4.9

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