stand/boot/boot.8

.\"	$OpenBSD: boot.8,v 1.69 2024/11/05 09:42:48 miod Exp $
.\"
.\" Copyright (c) 1997-2001 Michael Shalayeff
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.Dd $Mdocdate: November 5 2024 $
.Dt BOOT 8 i386
.Os
.Sh NAME
.Nm boot ,
.Nm boot.conf
.Nd i386-specific second-stage bootstrap
.Sh DESCRIPTION
The main purpose of this program is to load the system kernel while dealing
with the pitfalls of the PC BIOS architecture.
.Pp
As described in
.Xr boot_i386 8 ,
this program is loaded by the
.Xr 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.
.Pp
Basic operations include:
.Pp
.Bl -bullet -compact
.It
Detecting and switching between multiple consoles.
.It
Loading kernels from any device supported by your system BIOS.
.It
Loading kernels compressed by
.Xr gzip 1 .
.It
Passing system parameters queried from the BIOS to the kernel.
.It
Providing an interactive command line.
.El
.Pp
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
.Ic set
command).
If the kernel load fails, a second attempt is made with the timeout increased
by one second.
The sequence of
.Nm
operations is as follows:
.Bl -enum
.It
Set up a protected mode environment which catches and reports processor
exceptions and provides a simple protected-mode BIOS interface.
.It
Probe for console devices, which includes the (default) PC VGA+Keyboard
console
.Pq Li pc0
and up to four serial consoles
.Pf ( Li com0
through
.Li com3 )
connected to the serial ports.
Display messages to the default console about the devices found.
.It
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.
.It
Probe for APM support in the BIOS.
Display a message if support is present.
.It
If the file
.Pa /etc/boot.conf
exists on the filesystem
.Nm
was loaded from, open and parse it.
Lines beginning with the
.Sq #
character,
as well as whitespace at the beginning of lines,
are ignored.
The file may contain any commands
.Nm
accepts at the interactive prompt.
Though default settings usually suffice, they can be changed here.
.Pp
.Pa boot.conf
processing can be skipped, and the automatic boot cancelled, by holding
down either Control key as
.Nm
starts.
.It
The header line
.Pp
.Dl >> OpenBSD/i386 BOOT [x.xx]
.Pp
is displayed to the active console, where
.Ar x.xx
is the version number of the
.Nm
program, followed by the
.Pp
.Dl boot>
.Pp
prompt, which means you are in interactive mode and may enter commands.
If you do not,
.Nm
will proceed to load the kernel with the current parameters after the
timeout period has expired.
.El
.Pp
By default,
.Nm
attempts to load the kernel executable
.Pa /bsd .
If it fails to find the kernel and no alternative kernel image has
been specified, the system will be unable to boot.
.Pp
.Nm
supports booting from
.Xr softraid 4
RAID 1 and CRYPTO volumes.
.Sh COMMANDS
The following commands are accepted at the
.Nm
prompt:
.Bl -tag -width shorten
.It Ic boot Oo Oo Ar device : Oc Ns Ar image Oc Op Fl acds
Boots the specified kernel image
with any options given.
If
.Ar device
or
.Ar image
are omitted, values from
.Nm
variables will be used.
.Pp
When selecting the
.Ar device
to boot from,
.Nm
makes no distinction between SCSI and IDE type drives;
they are detected as
.Sq hd
devices.
Therefore, to boot kernel
.Pa /bsd
from slice
.Sq a
on the first hard drive
.Pq irrespective of device type ,
specify
.Dq boot hd0a:/bsd .
.Bl -tag -width _a_
.It Fl a
Causes the kernel to ask for the
.Nm root
device to use.
.It Fl c
Causes the kernel to go into
.Xr boot_config 8
before performing
.Xr autoconf 4
procedures.
.It Fl d
Causes the kernel to drop into
.Xr ddb 4
at the earliest convenient point.
.It Fl s
Causes the kernel to boot single-user.
.El
.It Ic echo Op Ar args
Displays
.Ar args
on the console device.
.It Ic help
Prints a list of available commands and machine dependent
commands, if any.
.It Ic hexdump Ar addr size
Show
.Ar size
bytes of memory at address
.Ar addr .
.It Ic machine Op Ar command
Issues machine-dependent commands.
These are defined for i386 architecture:
.Bl -tag -width diskinfo
.It Ic boot
Boots the specified partition boot block in place of the original (MBR) boot
block:
.Bd -unfilled -offset indent
machine boot hd0a
.Ed
.Pp
Where
.Ar a
is the first MBR partition table entry, and
.Ar d
the last.
.It Ic comaddr
Set the I/O base address for the serial port to be used as serial console.
.It Ic diskinfo
Prints a list of hard disks installed on your system including:
BIOS device number, and the BIOS geometry.
.It Ic 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 of:
.Pp
.D1 =size[KMG]
.D1 [+-]size@address
.Pp
Meaning to add(+), exempt(-) or limit(=) the amount of memory specified by
.Ar size
at the location specified by
.Ar address .
Both size and base address can be specified as octal,
decimal, or hexadecimal numbers, as accepted by the
.Xr 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.
.Pp
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:
.Bd -unfilled -offset indent
machine mem =64M
.Ed
.Pp
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 16MB:
.Bd -unfilled -offset indent
machine mem +0x2000000@0x1000000
.Ed
.Pp
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\(en16MB range from the map of useful memory:
.Bd -unfilled -offset indent
machine mem -0x100000@0xf00000
.Ed
.It Ic regs
Prints contents of processor registers if compiled with
.Em DEBUG .
.El
.It Ic ls Op Ar directory
Prints contents of the specified
.Ar directory
in long format including: attributes and file type, owner, group,
size, filename.
.It Ic reboot
Reboots the machine by initiating a warm boot procedure.
.It Ic set Op Ar varname Op Ar value
If invoked without arguments, prints a list of variables and their values.
If only
.Ar varname
is specified, displays contents of that variable.
If
.Ar varname
and
.Ar value
are both specified, sets that variable to the given value.
Variables include:
.Pp
.Bl -tag -compact -width db_console
.It Ic db_console
Boolean (0 or 1) to permit entry into the kernel debugger before the
.Em ddb.console
sysctl gets effective.
.It Ic debug
Debug flag if
.Nm
.It Ic debug
Debug flag if
.Nm
was compiled with DEBUG defined.
.It Ic device
Boot device name (e.g.,
.Li fd0a ,
.Li hd0a ) .
.It Ic howto
Options to pass to the loaded kernel.
.It Ic image
File name containing the kernel image.
.It Ic timeout
Number of seconds boot will wait for human intervention before
booting the default kernel image.
.It Ic tty
Active console device name (e.g.,
.Li com0 ,
.Li com1 ,
.Li pc0 ) .
.El
.It Ic stty Op Ar device Op Ar speed
Displays or sets the
.Ar speed
for a console
.Ar device .
If changing the baudrate for the currently active console,
.Nm
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
.Em not
for the active console, the baudrate is set for the
.Em next
time you switch to a serial console.
The baudrate value is not used for the
.Li pc0
console.
.Pp
The default baudrate is 9600bps.
.It Ic time
Displays system time and date.
.El
.Sh FILES
.Bl -tag -width /usr/mdec/biosbootxx -compact
.It Pa /usr/mdec/biosboot
first stage bootstrap
.It Pa /usr/mdec/pxeboot
PXE bootstrap
.It Pa /boot
system bootstrap
.It Pa /etc/boot.conf
system bootstrap's startup file
.It Pa /bsd
kernel image
.It Pa /bsd.sp
kernel image for single processor machines
.It Pa /bsd.mp
kernel image for multiprocessor machines
.It Pa /bsd.rd
kernel image for installation/recovery
.El
.Sh EXAMPLES
Boot the default kernel:
.Pp
.Dl boot> boot
.Pp
Remove the 5 second pause at boot-time permanently, causing
.Nm
to load the kernel immediately without prompting:
.Pp
.Dl # echo \&"boot\&" > /etc/boot.conf
.Pp
Use serial console.
A null modem cable should connect the specified serial port to a terminal.
Useful for debugging.
.Pp
.Dl boot> set tty com0
.Pp
Invoke the serial console at every boot:
.Pp
.Dl # echo \&"set tty com0\&" > /etc/boot.conf
.Pp
Boot the kernel named
.Pa /bsd
from the second hard disk in
.Dq User Kernel Configuration
mode (see
.Xr 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
.Xr config 8 Ns 's
.Fl e
option.
.Pp
.Dl boot> boot hd1a:/bsd -c
.Sh SEE ALSO
.Xr gzip 1 ,
.Xr autoconf 4 ,
.Xr ddb 4 ,
.Xr softraid 4 ,
.Xr biosboot 8 ,
.Xr boot_config 8 ,
.Xr boot_i386 8 ,
.Xr fdisk 8 ,
.Xr installboot 8 ,
.Xr pxeboot 8 ,
.Xr reboot 8
.Sh HISTORY
This program was written by Michael Shalayeff for
.Ox 2.1 .