grub.texi

grub4dos-0.4.4-2008- 08-src.zip

install or maintain other boot loaders and GRUB is flexible enough to
load an operating system from an arbitrary disk/partition. However,
the latter is sometimes required, since GRUB doesn't support all the
existing operating systems natively.

@menu
* Loading an operating system directly::
* Chain-loading::
@end menu


@node Loading an operating system directly
@subsection How to boot an OS directly with GRUB

Multiboot (@pxref{Top, Multiboot Specification, Motivation, multiboot,
The Multiboot Specification}) is the native format supported by GRUB.
For the sake of convenience, there is also support for Linux, FreeBSD,
NetBSD and OpenBSD. If you want to boot other operating systems, you
will have to chain-load them (@pxref{Chain-loading}).

Generally, GRUB can boot any Multiboot-compliant OS in the following
steps:

@enumerate
@item
Set GRUB's root device to the drive where the OS images are stored with
the command @command{root} (@pxref{root}).

@item
Load the kernel image with the command @command{kernel} (@pxref{kernel}).

@item
If you need modules, load them with the command @command{module}
(@pxref{module}) or @command{modulenounzip} (@pxref{modulenounzip}).

@item
Run the command @command{boot} (@pxref{boot}).
@end enumerate

Linux, FreeBSD, NetBSD and OpenBSD can be booted in a similar
manner. You load a kernel image with the command @command{kernel} and
then run the command @command{boot}. If the kernel requires some
parameters, just append the parameters to @command{kernel}, after the
file name of the kernel. Also, please refer to @ref{OS-specific notes},
for information on your OS-specific issues.


@node Chain-loading
@subsection Load another boot loader to boot unsupported operating systems

If you want to boot an unsupported operating system (e.g. Windows 95),
chain-load a boot loader for the operating system. Normally, the boot
loader is embedded in the @dfn{boot sector} of the partition on which
the operating system is installed.

@enumerate
@item
Set GRUB's root device to the partition by the command
@command{rootnoverify} (@pxref{rootnoverify}):

@example
grub> @kbd{rootnoverify (hd0,0)}
@end example

@item
Set the @dfn{active} flag in the partition using the command
@command{makeactive}@footnote{This is not necessary for most of the
modern operating systems.} (@pxref{makeactive}):

@example
grub> @kbd{makeactive}
@end example

@item
Load the boot loader with the command @command{chainloader}
(@pxref{chainloader}):

@example
grub> @kbd{chainloader +1}
@end example

@samp{+1} indicates that GRUB should read one sector from the start of
the partition. The complete description about this syntax can be found
in @ref{Block list syntax}.

@item
Run the command @command{boot} (@pxref{boot}).
@end enumerate

However, DOS and Windows have some deficiencies, so you might have to
use more complicated instructions. @xref{DOS/Windows}, for more
information.


@node OS-specific notes
@section Some caveats on OS-specific issues

Here, we describe some caveats on several operating systems.

@menu
* GNU/Hurd::
* GNU/Linux::
* FreeBSD::
* NetBSD::
* OpenBSD::
* DOS/Windows::
* SCO UnixWare::
* QNX::
@end menu


@node GNU/Hurd
@subsection GNU/Hurd

Since GNU/Hurd is Multiboot-compliant, it is easy to boot it; there is
nothing special about it. But do not forget that you have to specify a
root partition to the kernel.

@enumerate
@item
Set GRUB's root device to the same drive as GNU/Hurd's. Probably the
command @code{find /boot/gnumach} or similar can help you
(@pxref{find}).

@item
Load the kernel and the module, like this:

@example
@group
grub> @kbd{kernel /boot/gnumach root=hd0s1}
grub> @kbd{module /boot/serverboot}
@end group
@end example

@item
Run the command @command{boot} (@pxref{boot}).
@end enumerate


@node GNU/Linux
@subsection GNU/Linux

It is relatively easy to boot GNU/Linux from GRUB, because it somewhat
resembles to boot a Multiboot-compliant OS.

@enumerate
@item
Set GRUB's root device to the same drive as GNU/Linux's. Probably the
command @code{find /vmlinuz} or similar can help you (@pxref{find}).

@item
Load the kernel:

@example
grub> @kbd{kernel /vmlinuz root=/dev/hda1}
@end example

If you need to specify some kernel parameters, just append them to the
command. For example, to set @option{vga} to @samp{ext}, do this:

@example
grub> @kbd{kernel /vmlinuz root=/dev/hda1 vga=ext}
@end example

See the documentation in the Linux source tree for complete
information on the available options.

@item
If you use an initrd, execute the command @command{initrd}
(@pxref{initrd}) after @command{kernel}:

@example
grub> @kbd{initrd /initrd}
@end example

@item
Finally, run the command @command{boot} (@pxref{boot}).
@end enumerate

@strong{Caution:} If you use an initrd and specify the @samp{mem=}
option to the kernel to let it use less than actual memory size, you
will also have to specify the same memory size to GRUB. To let GRUB know
the size, run the command @command{uppermem} @emph{before} loading the
kernel. @xref{uppermem}, for more information.


@node FreeBSD
@subsection FreeBSD

GRUB can load the kernel directly, either in ELF or a.out format. But
this is not recommended, since FreeBSD's bootstrap interface sometimes
changes heavily, so GRUB can't guarantee to pass kernel parameters
correctly.

Thus, we'd recommend loading the very flexible loader
@file{/boot/loader} instead. See this example:

@example
@group
grub> @kbd{root (hd0,a)}
grub> @kbd{kernel /boot/loader}
grub> @kbd{boot}
@end group
@end example


@node NetBSD
@subsection NetBSD

GRUB can load NetBSD a.out and ELF directly, follow these steps:

@enumerate
@item
Set GRUB's root device with @command{root} (@pxref{root}).

@item
Load the kernel with @command{kernel} (@pxref{kernel}). You should
append the ugly option @option{--type=netbsd}, if you want to load an
ELF kernel, like this:

@example
grub> @kbd{kernel --type=netbsd /netbsd-elf}
@end example

@item
Run @command{boot} (@pxref{boot}).
@end enumerate

For now, however, GRUB doesn't allow you to pass kernel parameters, so
it may be better to chain-load it instead. For more information, please
see @ref{Chain-loading}.


@node OpenBSD
@subsection OpenBSD

The booting instruction is exactly the same as for NetBSD
(@pxref{NetBSD}).


@node DOS/Windows
@subsection DOS/Windows

GRUB cannot boot DOS or Windows directly, so you must chain-load them
(@pxref{Chain-loading}). However, their boot loaders have some critical
deficiencies, so it may not work to just chain-load them. To overcome
the problems, GRUB provides you with two helper functions.

If you have installed DOS (or Windows) on a non-first hard disk, you
have to use the disk swapping technique, because that OS cannot boot
from any disks but the first one. The workaround used in GRUB is the
command @command{map} (@pxref{map}), like this:

@example
@group
grub> @kbd{map (hd0) (hd1)}
grub> @kbd{map (hd1) (hd0)}
@end group
@end example

This performs a @dfn{virtual} swap between your first and second hard
drive.

@strong{Caution:} This is effective only if DOS (or Windows) uses BIOS
to access the swapped disks. If that OS uses a special driver for the
disks, this probably won't work.

Another problem arises if you installed more than one set of DOS/Windows
onto one disk, because they could be confused if there are more than one
primary partitions for DOS/Windows. Certainly you should avoid doing
this, but there is a solution if you do want to do so. Use the partition
hiding/unhiding technique.

If GRUB @dfn{hide}s a DOS (or Windows) partition (@pxref{hide}), DOS (or
Windows) will ignore the partition. If GRUB @dfn{unhide}s a DOS (or
Windows) partition (@pxref{unhide}), DOS (or Windows) will detect the
partition. Thus, if you have installed DOS (or Windows) on the first
and the second partition of the first hard disk, and you want to boot
the copy on the first partition, do the following:

@example
@group
grub> @kbd{unhide (hd0,0)}
grub> @kbd{hide (hd0,1)}
grub> @kbd{rootnoverify (hd0,0)}
grub> @kbd{chainloader +1}
grub> @kbd{makeactive}
grub> @kbd{boot}
@end group
@end example


@node SCO UnixWare
@subsection SCO UnixWare

It is known that the signature in the boot loader for SCO UnixWare is
wrong, so you will have to specify the option @option{--force} to
@command{chainloader} (@pxref{chainloader}), like this:

@example
@group
grub> @kbd{rootnoverify (hd1,0)}
grub> @kbd{chainloader --force +1}
grub> @kbd{makeactive}
grub> @kbd{boot}
@end group
@end example


@node QNX
@subsection QNX

QNX seems to use a bigger boot loader, so you need to boot it up, like
this:

@example
@group
grub> @kbd{rootnoverify (hd1,1)}
grub> @kbd{chainloader +4}
grub> @kbd{boot}
@end group
@end example


@node Making your system robust
@section How to make your system robust

When you test a new kernel or a new OS, it is important to make sure
that your computer can boot even if the new system is unbootable. This
is crucial especially if you maintain servers or remote systems. To
accomplish this goal, you need to set up two things:

@enumerate
@item
You must maintain a system which is always bootable. For instance, if
you test a new kernel, you need to keep a working kernel in a
different place. And, it would sometimes be very nice to even have a
complete copy of a working system in a different partition or disk.

@item
You must direct GRUB to boot a working system when the new system
fails. This is possible with the @dfn{fallback} system in GRUB.
@end enumerate

The former requirement is very specific to each OS, so this
documentation does not cover that topic. It is better to consult some
backup tools.

So let's see the GRUB part. There are two possibilities: one of them
is quite simple but not very robust, and the other is a bit complex to
set up but probably the best solution to make sure that your system
can start as long as GRUB itself is bootable.

@menu
* Booting once-only::
* Booting fallback systems::
@end menu


@node Booting once-only
@subsection Booting once-only

You can teach GRUB to boot an entry only at next boot time. Suppose
that your have an old kernel @file{old_kernel} and a new kernel
@file{new_kernel}. You know that @file{old_kernel} can boot
your system correctly, and you want to test @file{new_kernel}.

To ensure that your system will go back to the old kernel even if the
new kernel fails (e.g. it panics), you can specify that GRUB should
try the new kernel only once and boot the old kernel after that.

First, modify your configuration file. Here is an example:

@example
@group
default saved        # This is important!!!
timeout 10