— amd64-specific second-stage
The main purpose of this program is to load the system kernel while dealing with
the pitfalls of the PC BIOS architecture.
As described in boot_amd64(8)
this program is loaded by the
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:
- Detecting and switching between multiple consoles.
- Loading kernels from any device supported by your system
- Loading kernels compressed by
- Passing system parameters queried from the BIOS to the
- 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
command). Each time a kernel load fails, the
timeout is increased by one second. The sequence of
operations is as follows:
- Set up a protected mode environment which catches and
reports processor exceptions and provides a simple protected-mode BIOS
- Probe for console devices, which includes the (default) PC
VGA+Keyboard console (
pc0) and up to four serial
com3) connected to the serial ports. Display
messages to the default console about the devices found.
- 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
- Probe for APM support in the BIOS. Display a message if
support is present.
- If the file /etc/boot.conf
exists on the filesystem boot was loaded
from, open and parse it. Lines beginning with the ‘#’
character, as well as whitespace at the beginning of lines, are ignored.
The 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
- The header line
is displayed to the active console, where
x.xx is the version number of the
boot program, followed by the
>> OpenBSD/amd64 BOOT
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
By default, boot
attempts to load the kernel
. If it fails to find the kernel
and no alternative kernel image has been specified, the system will be unable
supports booting from
RAID 1 and CRYPTO
The following commands are accepted at the boot
- Boots the specified kernel image with any options given. If
image are omitted, 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
- Causes the kernel to drop into
ddb(4) at the earliest
- Causes the kernel to boot single-user.
- Displays args on the
- Prints a list of available commands and machine dependent
commands, if any.
- Prints contents of the specified
directory in long format including:
attributes and file type, owner, group, size, filename.
- Issues machine-dependent commands. These are defined for
- Boots the specified partition boot block in place of
the original (MBR) boot block:
Where a is the first MBR partition
table entry, and d the last.
- Set the I/O base address for the serial port to be used
as serial console.
- 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
Memory size may be suffixed by K or k, for kilobytes; M or m, for
megabytes; and G or g, for gigabytes.
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 64MB:
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 32MB of memory right after the first
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 15–16MB range from the map of
machine mem +0x2000000@0x1000000
machine mem -0x100000@0xf00000
- Prints contents of processor registers if compiled with
- On EFI systems, sets the video resolution to
mode is not given, a list of
available resolutions is shown.
- Reboots the machine by initiating a warm boot
- If invoked without arguments, prints a list of variables
and their values. If only varname is
specified, displays contents of that variable. If
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.,
- 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.,
- 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
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:
Remove the 5 second pause at boot-time permanently, causing
to load the kernel immediately without
# echo "boot" >
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" >
Boot the kernel named /bsd
from the second hard
disk in “User Kernel Configuration” mode (see
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
boot> boot hd1a:/bsd -c
This program was written by Michael Shalayeff for OpenBSD
on the i386 platform, and was later ported to the amd64