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

boot, boot.confi386-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:

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 interface.
  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 present.
  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”.

Causes the kernel to ask for the root device to use.
Causes the kernel to go into boot_config(8) before performing autoconf(4) procedures.
Causes the kernel to drop into ddb(4) at the earliest convenient point.
Causes the kernel to boot single-user.
echo [args]
Displays args on the console device.
Prints a list of available commands and machine dependent commands, if any.
machine [command]
Issues machine-dependent commands. These are defined for i386 architecture:
Prints a list of hard disks installed on your system including: BIOS device number, and the BIOS geometry.
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 of:


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
Prints contents of processor registers if compiled with .
ls [directory]
Prints contents of the specified directory in long format including: attributes and file type, owner, group, size, filename.
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:

Address at which to load the kernel.
Debug flag if boot was compiled with DEBUG defined.
Boot device name (e.g., fd0a, hd0a).
Options to pass to the loaded kernel.
File name containing the kernel image.
Number of seconds boot will wait for human intervention before booting the default kernel image.
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 for the active console, the baudrate is set for the time you switch to a serial console. The baudrate value is not used for the pc0 console.

The default baudrate is 9600bps.

Displays system time and date.

first stage bootstrap
PXE bootstrap
system bootstrap
system bootstrap's startup file
kernel image
kernel image for single processor machines
kernel image for multiprocessor machines
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.

August 10, 2010 OpenBSD-5.1