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


     boot, boot.conf - amd64-specific second-stage bootstrap


     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 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.  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 boot starts.

     6.   The header line

                >> OpenBSD/amd64 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.

     boot supports booting from softraid(4) RAID 1 and CRYPTO volumes.


     The following commands are accepted at the boot prompt:

     boot [[device:]image] [-acds]
              Boots the specified kernel image with any options given.  If
              device or image are omitted, values from boot variables will be

              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 amd64

              boot      Boots the specified partition boot block in place of
                        the original (MBR) boot block:

                              machine boot hd0a

                        Where a is the first MBR partition table entry, and d
                        the last.

              comaddr   Set the I/O base address for the serial port to be
                        used as serial console.

              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.  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:

                              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 32MB of memory right after the first

                              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 15-16MB 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), softraid(4), biosboot(8), boot_amd64(8),
     boot_config(8), fdisk(8), installboot(8), pxeboot(8), reboot(8)


     This program was written by Michael Shalayeff for OpenBSD 2.1 on the i386
     platform, and was later ported to the amd64 platform.

OpenBSD 5.9                    November 23, 2015                   OpenBSD 5.9

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