boot.conf man page on OpenBSD

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

     boot, boot.conf - i386-specific second-stage bootstrap

     The main purpose of this program is to load the system kernel while
     dealing with the downfalls of the PC BIOS architecture.

     As described in boot_i386(8), this program is loaded by the biosboot(8)
     primary bootstrap loader and provides a convenient way to load the
     kernel.  This program acts as an enhanced boot monitor for PC systems,
     providing a common interface for the kernel to start from.

     Basic operations include:

     o	 Detecting and switching between multiple consoles.
     o	 Loading kernels from any device supported by your system BIOS.
     o	 Loading kernels compressed by gzip(1).
     o	 Passing system parameters queried from the BIOS to the kernel.
     o	 Providing an interactive command line.

     The sequence of its operation is as follows: initialization, parsing the
     configuration file, then an interactive command line.  While at the
     command line you have 5 seconds to type any commands, if needed.  If time
     expires, the kernel will be loaded according to the current variable
     settings (see the set command).  Each time a kernel load fails, the
     timeout is increased by one second.  The sequence of boot operations is
     as follows:

     1.	  Set up a protected mode environment which catches and reports
	  processor exceptions and provides a simple protected-mode BIOS

     2.	  Probe for console devices, which includes the (default) PC
	  VGA+Keyboard console (pc0) and up to four serial consoles (com0
	  through com3) connected to the serial ports.	Display messages to
	  the default console about the devices found.

     3.	  Detect memory.  Conventional memory is detected by querying the
	  BIOS.	 Extended memory is detected by probing page-by-page through
	  the address space, rather than asking the BIOS; many BIOS's cannot
	  report larger than 64M of memory.  All memory found is reported to
	  the default console device.

     4.	  Probe for APM support in the BIOS.  Display a message if support is

     5.	  If the file /etc/boot.conf exists on the filesystem boot was loaded
	  from, open and parse it.  This file may contain any commands boot
	  accepts at the interactive prompt.  Though default settings usually
	  suffice, they can be changed here.

	  boot.conf processing can be skipped, and the automatic boot
	  cancelled, by holding down either Control key as boot starts.

     6.	  The header line

		>> OpenBSD/i386 BOOT [x.xx]

	  is displayed to the active console, where x.xx is the version number
	  of the boot program, followed by the


	  prompt, which means you are in interactive mode and may enter
	  commands.  If you do not, boot will proceed to load the kernel with
	  the current parameters after the timeout period has expired.

     By default, boot attempts to load the kernel executable /bsd.  If it
     fails to find the kernel and no alternative kernel image has been
     specified, the system will be unable to boot.

     The following commands are accepted at the boot prompt:

     boot [image [-acds]]
	      Boots the kernel image specified by image with any options
	      given.  Image specification consists of a pair device:filename;
	      either or both can be omitted (`:' is not needed if both are
	      omitted), in which case values from boot variables will be used.

	      When selecting the device to boot from, boot makes no
	      distinction between SCSI and IDE type drives; they are detected
	      as `hd' devices.	Therefore, to boot kernel /bsd from slice `a'
	      on the first hard drive (irrespective of device type), specify
	      ``boot hd0a:/bsd''.

	      -a   Causes the kernel to ask for the root device to use.

	      -c   Causes the kernel to go into boot_config(8) before
		   performing autoconf(4) procedures.

	      -d   Causes the kernel to drop into ddb(4) at the earliest
		   convenient point.

	      -s   Causes the kernel to boot single-user.

     echo [args]
	      Displays args on the console device.

     help     Prints a list of available commands and machine dependent
	      commands, if any.

     machine [command]
	      Issues machine-dependent commands.  These are defined for i386

	      diskinfo	Prints a list of hard disks installed on your system
			including: BIOS device number, and the BIOS geometry.

	      memory	If used without any arguments, this command will print
			out the memory configuration as determined through
			BIOS routines.	Otherwise the arguments specify how to
			modify the memory configuration.  They take the form


			Meaning to add(+), exempt(-) or limit(=) the amount of
			memory specified by <size> at the location specified
			by <address>.  Both size and base address can be
			specified as octal, decimal, or hexadecimal numbers,
			as accepted by the strtoul(3) routine.

			The limit(=) option simply ignores any memory above
			the given memory limit.	 This is useful for testing
			kernels in an artificially constrained memory
			situation.  For example, the following limits the
			kernel to using only memory below 64M:

			      machine mem =64M

			Memory segments are not required to be adjacent to
			each other; the only requirement is that there is real
			physical memory under the range added.	The following
			example adds 32M of memory right after the first 16M:

			      machine mem +0x2000000@0x1000000

			Another useful command is to withdraw a range of
			memory from OS usage (it may have been wrongfully
			reported as useful by the BIOS).  This example
			effectively excludes the 15-16M range from the map of
			useful memory:

			      machine mem -0x100000@0xf00000

	      regs	Prints contents of processor registers if compiled
			with DEBUG.

     ls [directory]
	      Prints contents of the specified directory in long format
	      including: attributes and file type, owner, group, size,

     reboot   Reboots the machine by initiating a warm boot procedure.

     set [varname [value]]
	      If invoked without arguments, prints a list of variables and
	      their values.  If only varname is specified, displays contents
	      of that variable.	 If varname and value are both specified, sets
	      that variable to the given value.	 Variables include:

	      addr     Address at which to load the kernel.
	      debug    Debug flag if boot was compiled with DEBUG defined.
	      device   Boot device name (e.g., fd0a, hd0a).
	      howto    Options to pass to the loaded kernel.
	      image    File name containing the kernel image.
	      timeout  Number of seconds boot will wait for human intervention
		       before booting the default kernel image.
	      tty      Active console device name (e.g., com0, com1, pc0).

     stty [device [speed]]
	      Displays or sets the speed for a console device.	If changing
	      the baudrate for the currently active console, boot offers you
	      five seconds of grace time before committing the change to allow
	      you to change your terminal's speed to match.  If changing speed
	      not for the active console, the baudrate is set for the next
	      time you switch to a serial console.  The baudrate value is not
	      used for the pc0 console.

	      The default baudrate is 9600bps.

     time     Displays system time and date.

     /usr/mdec/biosboot	   first stage bootstrap
     /usr/mdec/pxeboot	   PXE bootstrap
     /boot		   system bootstrap
     /etc/boot.conf	   system bootstrap's startup file
     /bsd		   kernel image
     /bsd.sp		   kernel image for single processor machines
     /		   kernel image for multiprocessor machines
     /bsd.rd		   kernel image for installation/recovery

     Boot the default kernel:

	   boot> boot

     Remove the 5 second pause at boot-time permanently, causing boot to load
     the kernel immediately without prompting:

	   # echo "boot" > /etc/boot.conf

     Use serial console.  A null modem cable should connect the specified
     serial port to a terminal.	 Useful for debugging.

	   boot> set tty com0

     Invoke the serial console at every boot:

	   # echo "set tty com0" > /etc/boot.conf

     Boot the kernel named /bsd from the second hard disk in ``User Kernel
     Configuration'' mode (see boot_config(8)).	 This mechanism allows for the
     explicit enabling and disabling of devices during the current boot
     sequence, as well as the modification of device parameters.  Once booted,
     such changes can be made permanent by using config(8)'s -e option.

	   boot> boot hd1a:/bsd -c

     gzip(1), autoconf(4), ddb(4), biosboot(8), boot_config(8), boot_i386(8),
     fdisk(8), installboot(8), pxeboot(8), reboot(8)

     RFC 1950 describes the zlib library interface.

     The official home page for the version of zlib used in this operating
     system is at

     This program was written by Michael Shalayeff for OpenBSD 2.1.

OpenBSD 4.9			August 10, 2010			   OpenBSD 4.9

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