.\"	$NetBSD: boot.8,v 1.15.2.5 2023/05/13 11:45:53 martin Exp $
.\"
.\" Copyright (c) 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software written and contributed
.\" to Berkeley by William Jolitz.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)boot_i386.8	8.2 (Berkeley) 4/19/94
.\"
.Dd July 15, 2020
.Dt BOOT 8 x86
.Os
.Sh NAME
.Nm boot
.Nd
system bootstrapping procedures
.Sh DESCRIPTION
Intel Architecture, 32-bit (IA-32) computers (the
.Tn IBM PC
and its clones)
that can run
.Nx Ns /i386
or
.Nx Ns /amd64
can use any of the following boot procedures, depending on what the hardware and
.Tn BIOS
support:
.Bl -tag -width "x86/pxeboot(8)"
.It boot
bootstrap
.Nx
from the system
.Tn BIOS
.It efiboot
bootstrap
.Nx
from the system
.Tn UEFI
.It Xr x86/dosboot 8
bootstrap
.Nx
from
.Tn MS-DOS
.It Xr x86/pxeboot 8
network bootstrap
.Nx
from a
.Tn TCP/IP
.Tn LAN
with
.Tn DHCP ,
.Tn TFTP ,
and
.Tn NFS .
.El
.Ss Power fail and crash recovery
Normally, the system will reboot itself at power-up or after crashes.
An automatic consistency check of the file systems will be performed,
and unless this fails, the system will resume multi-user operations.
.Ss Cold starts
The 386
.Tn "PC AT"
clones attempt to boot the floppy disk drive A (otherwise known as drive
0) first, and failing that, attempt to boot the hard disk C (otherwise
known as hard disk controller 1, drive 0).
The
.Nx
bootblocks are loaded and started either by the
.Tn BIOS ,
or by a boot selector program (such as OS-BS, BOOTEASY, the OS/2 Boot Menu or
.Nx Ns 's
.No boot-selecting
master boot record - see
.Xr x86/mbr 8 ) .
.Ss Normal Operation
Once running, a banner similar to the following will appear:
.Bd -literal -offset indent
>> NetBSD BIOS Boot, revision 3.0
>> (user@buildhost, builddate)
>> Memory: 637/15360 k
Press return to boot now, any other key for boot menu
booting hd0a:netbsd - starting in 5
.Ed
.Pp
After a countdown, the system image listed will be loaded.
In the example above, it will be
.Dq Li hd0a:netbsd
which is the file
.Pa /netbsd
on partition
.Dq Li a
of the
.Nx
.Tn MBR
partition of the first hard disk known to the
.Tn BIOS
.Po
which is an
.Tn IDE
or similar device \(em see the
.Sx BUGS
section
.Pc .
.Pp
Pressing a key within the time limit, or before the boot program starts, will
enter interactive mode.
When using a short or 0 timeout, it is often useful to interrupt the boot
by holding down a shift key, as some BIOSes and BIOS extensions will drain the
keystroke buffer at various points during POST.
.Pp
If present, the file
.Pa /boot.cfg
will be used to configure the behaviour of the boot loader including
setting the timeout, choosing a console device, altering the banner
text and displaying a menu allowing boot commands to be easily chosen.
See
.Xr boot.cfg 5 .
.Ss Boot Protocol
The
.Nx Ns /x86
boot loader can boot a kernel using either the native
.Nx
boot protocol, or the
.Dq multiboot
protocol (which is compatible with some other operating systems).
In the native
.Nx
boot protocol, options are passed from the boot loader
to the kernel via flag bits in the
.Va boothowto
variable (see
.Xr boothowto 9 ) .
In the multiboot protocol, options are passed from the boot loader
to the kernel as strings.
.Ss Diagnostic Output
If the first stage boot fails to load the boot, it will print a terse
message indicating the reason for the failure.
The possible error messages and their cause are listed in
.Xr x86/mbr 8 .
.Pp
If the first stage boot succeeds, the banner will be shown and the
error messages should be self-explanatory.
.Ss Interactive mode
In interactive mode, the boot loader will present a prompt, allowing
input of these commands:
.\" NOTE: much of this text is duplicated in the MI boot.8.
.\" Some of it is
.\" also duplicated in the x86-specific x86/dosboot.8 and x86/pxeboot.8;
.\" please try to keep all relevant files synchronized.
.Bl -tag -width 04n -offset 04n
.It Ic boot Oo Ar device Ns Ic \&: Oc Ns Oo Ar filename Oc Oo Fl 1234abcdmqsvxz Oc
The default
.Ar device
will be set to the disk from which the boot loader was loaded.
The partition is set to the first match in this list:
.Bl -enum
.It
The first
.Xr gpt 8
partition with the
.Va bootme
attribute set.
.It
The partition from which the boot loader was loaded from, if that
can be detected.
.It
The first partition with a file system that could be bootable.
.It
The first partition.
.El
.Pp
To boot from an alternate disk, the full name of the device should
be given at the prompt.
.Ar device
is of the form
.Li NAME= Ns Ar partition_label
when booting from a
.Xr gpt 8
partitioned disk.
Otherwise, the syntax is
.Xo Ar xd\^ Ns
.Op Ar N\^ Ns Op Ar x
.Xc
where
.Ar xd
is the device from which to boot,
.Ar N
is the unit number, and
.Ar x
is the partition letter.
.Pp
In the latter case, the following list of supported devices may
vary from installation to installation:
.Bl -hang
.It hd
Hard disks as numbered by the BIOS.
This includes ST506, IDE, ESDI, RLL disks on a WD100[2367] or
lookalike controller(s), and SCSI disks
on SCSI controllers recognized by the BIOS.
.It fd
Floppy drives as numbered by the BIOS.
.It cd
CD-ROM drives as numbered by the BIOS.
.It raid
RAIDframe configured from hard disks recognized by the BIOS.
Only RAID level 1 sets are supported by bootstrap code.
If the RAID is partitioned, the first partition is used, or the
first
.Xr gpt 8
partition that has the
.Va bootme
attribute set.
Inner RAIDframe partitions can also be given to the
.Ic dev
command using he
.Li NAME= Ns Ar partition_label
syntax.
.El
.Pp
The default
.Va filename
is
.Pa netbsd ;
if the boot loader fails to successfully
open that image, it then tries
.Pa netbsd.gz
(expected to be a kernel image compressed by gzip), followed by
.Pa onetbsd ,
.Pa onetbsd.gz ,
.Pa netbsd.old ,
and finally
.Pa netbsd.old.gz .
Alternate system images can be loaded by just specifying the name of the image.
.Pp
Options are:
.Bl -tag -width xxx
.It Fl 1
Sets the machine-dependent flag
.Dv RB_MD1
in
.Va boothowto .
In
.Nx Ns /x86 ,
this disables multiprocessor boot;
the kernel will boot in uniprocessor mode.
.It Fl 2
Sets the machine-dependent flag
.Dv RB_MD2
in
.Va boothowto .
In
.Nx Ns /x86 ,
this disables ACPI.
.It Fl 3
Sets the machine-dependent flag
.Dv RB_MD3
in
.Va boothowto .
In
.Nx Ns /amd64 ,
this disables SVS.
.It Fl 4
Sets the machine-dependent flag
.Dv RB_MD4
in
.Va boothowto .
In
.Nx Ns /x86 ,
this has no effect.
.It Fl a
Sets the
.Dv RB_ASKNAME
flag in
.Va boothowto .
This causes the kernel to prompt for the root file system device,
the system crash dump device, and the path to
.Xr init 8 .
.It Fl b
Sets the
.Dv RB_HALT
flag in
.Va boothowto .
This causes subsequent reboot attempts to halt instead of rebooting.
.It Fl c
Sets the
.Dv RB_USERCONF
flag in
.Va boothowto .
This causes the kernel to enter the
.Xr userconf 4
device configuration manager as soon as possible during the boot.
.Xr userconf 4
allows devices to be enabled or disabled, and allows device locators
(such as hardware addresses or bus numbers)
to be modified before the kernel attempts to attach the devices.
.It Fl d
Sets the
.Dv RB_KDB
flag in
.Va boothowto .
Requests the kernel to enter debug mode, in which it
waits for a connection from a kernel debugger; see
.Xr ddb 4 .
.It Fl m
Sets the
.Dv RB_MINIROOT
flag in
.Va boothowto .
Informs the kernel that a mini-root file system is present in memory.
.It Fl q
Sets the
.Dv AB_QUIET
flag in
.Va boothowto .
Boot the system in quiet mode.
.It Fl s
Sets the
.Dv RB_SINGLE
flag in
.Va boothowto .
Boot the system in single-user mode.
.It Fl v
Sets the
.Dv AB_VERBOSE
flag in
.Va boothowto .
Boot the system in verbose mode.
.It Fl x
Sets the
.Dv AB_DEBUG
flag in
.Va boothowto .
Boot the system with debug messages enabled.
.It Fl z
Sets the
.Dv AB_SILENT
flag in
.Va boothowto .
Boot the system in silent mode.
.El
.It Ic consdev Ar dev\| Ns Oo Ns Ic \&, Ns Ar speed Oc
[Not available for 
.Xr x86/dosboot 8 ]
Immediately switch the console to the specified device
.Ar dev
and reprint the banner.
.Ar dev
must be one of
.\" .Bl -item -width com[0123]kbd -offset indent -compact
.Li pc , com0 , com1 , com2 ,
.Li com3 , com0kbd , com1kbd , com2kbd ,
.Li com3kbd ,
or
.Li auto .
See
.Sx Console Selection Policy
in
.Xr x86/boot_console 8 .
.Pp
A
.Ar speed
for the serial port is optional and defaults to 9600.
If a value of zero is specified, then the current baud rate (set by the
BIOS) will be used.
Setting the
.Ar speed
with the
.Li pc
device is not possible.
.It Ic dev Op Ar device
Set the default drive and partition for subsequent file system
operations.
Without an argument, print the current setting.
.Ar device
is of the form specified in
.Ic boot .
.It Ic devpath
[Only available for UEFI boot] Dump UEFI device paths.
.It Ic efivar
[Only available for UEFI boot] Dump UEFI environment variables from NVRAM.
.It Ic fs Ar file
[Only available for BIOS and UEFI boot]
Load a file system image from the specified
.Ar file ,
and request the kernel to use it as the root file system.
The
.Xr makefs 8
utility may be used to create suitable file system images.
.It Ic gop Op Va mode_index
[Only available for UEFI boot] Without argument, list the available
video modes. If an argument is given, select a video mode.
.It Ic help
Print an overview about commands and arguments.
.It Ic load Ar module Op Ar arguments
[Not available for 
.Xr x86/dosboot 8 ]
Load the specified kernel
.Ar module ,
and pass it the specified
.Ar arguments .
If the module name is not an absolute path,
.Bd -ragged -offset indent -compact
.Pa /stand/ Ns Xo Ns
.Aq Ar arch Ns
.Pa / Ns
.Aq Ar osversion Ns
.Pa /modules/ Ns
.Aq Ar module Ns
.Pa / Ns
.Aq Ar module Ns
.Pa .kmod
.Xc
.Ed
is used.
Possible uses of the
.Ic load
command include loading a memory disk image before booting a kernel,
or loading a
.Tn Xen
DOM0 kernel before booting the
.Tn Xen
hypervisor.
See
.Xr boot.cfg 5
for examples.
.Pp
In addition to the
.Cm boot
options specified above, the
.Tn Xen
DOM0 kernel accepts
.Po Ar arguments
being separated with spaces
.Pc :
.Bl -tag -width xxx
.It Ic bootdev Ns = Ns Ar dev Po or Ic root Ns = Ns Ar dev Pc
Override the default boot device.
.Ar dev
is of the form
.Li NAME= Ns Ar partition_label
for
.Xr gpt 8
partitioned disks.
It can also be a unit name
.Pq Ql wd0 ,
or an interface name
.Po Ql bge0 ,
.Ql wm0 ,
\&...
.Pc
for cases where the root file system has to be loaded
from network (see the
.Sx BUGS
section in
.Xr x86/pxeboot 8 ) .
.It Ic console Ns = Ns Ar dev
Console used by DOM0 kernel during boot.
.Ar dev
accepts the same values as the ones given for the
.Cm consdev
command.
See
.Sx Console Selection Policy
in
.Xr x86/boot_console 8 .
.It Xo Ic ip Ns Li \&= Ns
.Ar my_ip Ns Li \&: Ns Ar serv_ip Ns Li \&: Ns Ar gw_ip Ns Li \&: Ns
.Ar mask Ns Li \&: Ns Ar host Ns Li \&: Ns Ar iface
.Xc
Specify various parameters for a network boot (IPs are in
dot notation),
each one separated by a colon:
.Bl -tag -width xxxxxxx
.It Ar my_ip
address of the host
.It Ar serv_ip
address of the NFS server
.It Ar gw_ip
address of the gateway
.It Ar mask
network mask
.It Ar host
address of the host
.It Ar iface
interface
.Po e.g., Dq Li xennet0
or
.Dq Li eth0
.Pc
.El
.It Ic nfsroot Ns = Ns Ar address Ns Li \&: Ns Ar rootpath
Boot the system with root on NFS.
.Ar address
is the address of the NFS server, and
.Ar rootpath
is the remote mount point for the root file system.
.It Ic pciback.hide Ns = Ns Ar pcidevs
Pass a list of PCI IDs for use with the PCI backend driver,
.Xr pciback 4 .
.Ar pcidevs
is formed of multiple IDs (in
.Ar bus Ns Li \&: Ns Ar device Ns Li \&. Ns Ar function
notation),
each ID being surrounded with brackets.
PCI domain IDs are currently ignored.
See
.Xr pciback 4 .
.El
.It Ic ls Op Ar path
[Not available for
.Xr x86/pxeboot 8 ]
Print a directory listing of
.Ar path ,
containing inode number, filename, and file type.
.Ar path
can contain a device specification.
.It Ic memmap
[Only available for UEFI boot] Dump UEFI memory map.
.It Ic menu
[Only available for BIOS and UEFI boot]
Display the boot menu and initiate a countdown,
similarly to what would have happened if interactive mode
had not been entered.
.It Ic modules Bro Li \^on \
           No \(or Li off \
           No \(or Li enabled \
           No \(or Li disabled\^ \
           Brc
[Not available for
.Xr x86/dosboot 8 ]
The values
.Ql enabled ,
.Ql on
will enable module loading for
.Ic boot
and
.Ic multiboot ,
whereas
.Ql disabled ,
.Ql off
will turn off the feature.
.It Ic mode Va fstype
[Only available for
.Xr x86/dosboot 8 ]
Switch file system type;
.Va fstype
should be one of
.Ar dos
or
.Ar ufs .
.It Ic multiboot Ar kernel Op Ar arguments
[Not available for
.Xr x86/dosboot 8 ]
Boot the specified
.Ar kernel ,
using the
.Dq multiboot
protocol instead of the native
.Nx
boot protocol.
The
.Ar kernel
is specified in the same way as with the
.Ic boot
command.
.Pp
The multiboot protocol may be used in the following cases:
.Bl -tag -width indent
.It Nx Ns / Ns  Xen No kernels
The
.Tn Xen
DOM0 kernel must be loaded as a module using the
.Ic load
command, and the
.Tn Xen
hypervisor must be booted using the
.Ic multiboot
command.
Options for the DOM0 kernel (such as
.Dq -s
for single user mode) must be passed as options to the
.Ic load
command.
Options for the hypervisor (such as
.Dq Li dom0_mem=256M
to reserve
.Pf 256 Tn MB
of memory for DOM0)
must be passed as options to the
.Ic multiboot
command.
See
.Xr boot.cfg 5
for examples on how to boot
.Nx Ns / Ns  Xen.
.It Nx No multiboot kernels
A
.Nx
kernel that was built with
.Cd options MULTIBOOT
(see
.Xr x86/multiboot 8 )
may be booted with either the
.Ic boot
or
.Ic multiboot
command, passing the same
.Ar arguments
in either case.
.It Non- Ns Nx No kernels
A kernel for a
.No non- Ns Nx
operating system that expects to be booted using the
multiboot protocol (such as by the GNU
.Dq GRUB
boot loader)
may be booted using the
.Ic multiboot
command.
See the foreign operating system's documentation for the available
.Ar arguments .
.El
.It Ic pkboot
[Only available for BIOS and UEFI boot] Boot a kernel that has
the
.Cd KASLR
option set, for Kernel Address Space Layout Randomizaton.
.It Ic quit
Reboot the system.
.It Ic reloc Op Va default No \(or Va none No \(or Va address
[Only UEFI boot] Sets where the kernel is copied by bootstrap
before it is started. Values other than default require a kernel
built with the
.Cd SELFRELOC
option, so that can relocate itself at the right address,
otherwise a crash occurs at boot time.
.Bl -tag -width default
.It Va default
Copy the kernel at ELF header load address, this is the historical
behavior.
.It Va none
Leave the kernel where it was loaded and start it as is. 
.It Va address
Copy the kernel at given
.Va address .
.El
.It Ic rndseed Ar file
[Only available for BIOS and UEFI boot]
Load the specified
.Ar file
and request the kernel to use it as a seed for the
.Xr rnd 4
random number generator.
The
.Ar file
should be in the private format used by
.Xr rndctl 8 ,
and should have been saved by
.Ql "rndctl -S"
shortly before the previous shutdown.
See the
.Va random_seed
and
.Va random_file
variables in
.Xr rc.conf 5 ,
and the
.Pa /etc/rc.d/random_seed
script, for a way to manage the seed file.
Using the same seed file on more then one host,
or for more than one boot on the same host,
will reduce the quality of random numbers
and may impact system security.
.It Ic splash Ar file
[Only available for BIOS and UEFI boot]
Load a graphical image from the specified
.Ar file
and request the kernel to use it as a splash screen.
The
.Ar file
should contain an image in one of these formats:
JPEG (baseline only, not progressive),
PNG (8-bit only),
TGA,
BMP (non-1bpp, non-RLE),
GIF,
PSD (composited view only),
or
PIC.
.It Ic text Op Va mode_index
[Only available UEFI boot] Without argument, list the available
text modes (displayed as column x line in hexadecimal, therefore
.Li 50x19 
means 
.Li 80
columns and
.Li 25
lines). With an argument, select a text mode.
.It Ic userconf Ar command
[Not available for
.Xr x86/dosboot 8 ]
Pass command
.Ar command
to
.Xr userconf 4
at boot time.
These commands are processed before the interactive
.Xr userconf 4
shell is executed, if requested.
.It Ic version Op Ar full
[Only available UEFI boot] Display UEFI bootstrap version. With the
.Op full
argumznt, also display information about UEFI itself.
.It Ic vesa Bro Ar \^modenum \
        No \(or Li on \
        No \(or Li off \
        No \(or Li enabled \
        No \(or Li disabled \
        No \(or Li list Brc
[Only available for BIOS and
.Xr x86/pxeboot 8 ]
Initialise the video card to the specified resolution and bit depth.
The
.Ar modenum
should be in the form of
.Ql 0x100 ,
.Ql 800x600 ,
.Ql 800x600x32 .
The values
.Ql enabled ,
.Ql on
put the display into the default mode, and
.Ql disabled ,
.Ql off
returns the display into standard vga mode.
The value
.Ql list
lists all supported modes.
.El
.Pp
In an emergency, the bootstrap methods described in the
.Nx
installation notes for the x86 architectures
can be used to boot from floppy or other media,
or over the network.
.Ss Locating the root file system
The kernel uses information from the bootloader to locate the
file system to mount as root.
There are three methods:
.Bl -tag -width 04n -offset 04n
.\" XXX: what this .Va is supposed to mean?
.It Dv BTINFO_ROOTDEVICE Va from
.Xr boot.cfg 5
or multiboot.
The bootloader passes the root device name as driver, unit, and
partition (like
.Ql sd0a Ns ).
This will be automatically substituted by a
.Xr dk 4
wedge if one is discovered.
.Pp
If the bootloader passes a wedge name as
.Dq Li wedge:
or
.Dq Li NAME=
followed by the name.
The kernel will search for a
.Xr dk 4
device with that name.
.It Dv BTINFO_BOOTWEDGE Va determined by bootblock
The bootloader passes start offset and length of a hard disk partition
and a offset, size and hash of a
.Dq boot area .
Then kernel searches
all disks and wedges for a block sequence at that offset with a
matching hash.
If one is found, the kernel will look for a wedge
on that device at the same offset.
.Pp
An additional partition number is provided if the bootloader also
passed a
.Dv BTINFO_BOOTDISK
record.
This (or partition
.Ql a )
will be used
by the kernel as a fallback if there is no matching wedge.
.It Dv BTINFO_BOOTDISK Va determined by bootblock
This uses the device number passed by the
.Tn BIOS
that distinguishes between floppy, hard drive and
.Tn CD-ROM
boot.
.Bl -tag -width xxx
.It Floppy
The kernel searches for the
.Xr fd 4
device with the correct unit, the partition number is used
to select a specific disk format.
See
.Xr fd 4
for details.
.It Hard drive
The bootloader passed a partition number and disklabel
data (offset, type, checksum, packname).
The kernel searches
all disks for a matching disklabel.
If one is found, the
kernel will use that device and partition number.
.It Tn CDROM
The
.Tn BIOS
does not distinguish between multiple
.Tn CD
devices.
The kernel searches for the first
.Xr cd 4
device.
So you can only boot from unit 0.
.El
.El
.Sh FILES
.Bl -tag -width /usr/mdec/bootxx_fstype -compact
.It Pa /boot
boot program code loaded by the primary bootstrap
.It Pa /boot.cfg
optional configuration file
.It Pa /netbsd
system code
.It Pa /netbsd.gz
gzip-compressed system code
.It Pa /usr/mdec/boot
master copy of the boot program (copy to /boot)
.It Pa /usr/mdec/bootxx_fstype
primary bootstrap for file system type fstype, copied to the start of
the
.Nx
partition by
.Xr installboot 8 .
.It Pa /usr/mdec/bootia32.efi
.It Pa /usr/mdec/bootx64.efi
.Tn UEFI
bootstraps for
.Nx Ns /i386
and
.Nx Ns /amd64 ,
which should be copied to the
.Pa /EFI/boot
directory in a
.Tn FAT
formatted partition of type
.Tn EFI
(Either
.Xr x86/mbr 8
and
.Xr gpt 8 ,
see the
.Sx BUGS
section).
.Nx
.Tn UEFI
bootstrap reads its configuration from the
.Pa /EFI/NetBSD/boot.cfg
file in the
.Tn EFI
partition.
.El
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr fd 4 ,
.Xr pciback 4 ,
.Xr userconf 4 ,
.Xr boot.cfg 5 ,
.Xr halt 8 ,
.Xr installboot 8 ,
.Xr reboot 8 ,
.Xr rescue 8 ,
.Xr shutdown 8 ,
.Xr x86/boot_console 8 ,
.Xr x86/dosboot 8 ,
.Xr x86/mbr 8 ,
.Xr x86/multiboot 8 ,
.Xr x86/pxeboot 8 ,
.Xr boothowto 9
.Sh BUGS
The kernel file name must be specified before, not after, the boot options.
Any
.Ar filename
specified after the boot options, e.g.:
.Pp
.Dl boot -d netbsd.test
.Pp
is ignored, and the default kernel is booted.
.Pp
Hard disks are always accessed by
.Tn BIOS
functions.
Unit numbers are
.Tn BIOS
device numbers which might differ from numbering in the
.Nx
kernel or physical parameters
.Po
e.g.,
.Tn SCSI
slave numbers
.Pc .
There isn't any distinction between
.Dq sd
and
.Dq wd
devices at the bootloader level.
This is less a bug of the bootloader code than
a shortcoming of the PC architecture.
The default disk device's name printed in the starting message
is derived from the
.Dq type
field of the
.Nx
disklabel (if it is a hard disk).
.Pp
.Tn UEFI
implementations are supposed to support either
.Xr x86/mbr 8
or
.Xr gpt 8
partitioning, but some do not handle the latter.
.Tn UEFI
booting
from a
.Xr gpt 8
partitioned disk is still possible in this case, by adding
an overlapping
.Tn EFI
partition in the protective
.Xr x86/mbr 8
block.
This can be achieved using the following commands
(you must adapt the hard disk and
.Tn EFI
partition start end size to fit your setup):
.Bd -literal -offset indent
dd if=/dev/rwd0d bs=512 count=1 of=mbr
fdisk -FIfaui1s 4/34/32768 -c /usr/mdec/mbr mbr
dd if=mbr bs=512 count=1 of=/dev/rwd0d conv=notrunc
.Ed
.Pp
The resulting
.Xr x86/mbr 8
partition table will look like this:
.Bd -literal -offset indent
0: GPT Protective MBR (sysid 238)
    start 1, size 2097151 (1024 MB, Cyls 0-130/138/8)
        PBR is not bootable: Bad magic number (0x0000)
1: Primary DOS with 16 bit FAT <32M (sysid 4)
    start 34, size 32768 (16 MB, Cyls 0/0/35-2/10/42), Active
2: <UNUSED>
3: <UNUSED>
.Ed