* docs/grub.texi (BIOS installation): New section, partly based on
previous text in other sections. (Installing GRUB using grub-install): Replace BIOS discussion with a cross-reference. (Images): Likewise.
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Colin Watson
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2 changed files with 79 additions and 29 deletions
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@ -1,3 +1,11 @@
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2011-03-29 Colin Watson <cjwatson@ubuntu.com>
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* docs/grub.texi (BIOS installation): New section, partly based on
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previous text in other sections.
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(Installing GRUB using grub-install): Replace BIOS discussion with a
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cross-reference.
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(Images): Likewise.
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2011-03-29 Vladimir Serbinenko <phcoder@gmail.com>
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* grub-core/kern/emu/hostdisk.c (find_partition_start)
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100
docs/grub.texi
100
docs/grub.texi
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@ -550,6 +550,7 @@ the @dfn{boot directory}.
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* Installing GRUB using grub-install::
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* Making a GRUB bootable CD-ROM::
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* Device map::
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* BIOS installation::
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@end menu
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@ -565,13 +566,8 @@ always. Therefore, GRUB provides you with a map file called the
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@dfn{device map}, which you must fix if it is wrong. @xref{Device
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map}, for more details.
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On BIOS platforms GRUB has to use a so-called embedding zone. On msdos
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partition tables, this is the space between the MBR and the first partition
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(called the MBR gap or the boot track), while on GPT partition tables it
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uses a BIOS Boot Partition (a partition with GUID
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21686148-6449-6e6f-744e656564454649). If you use GRUB on a BIOS system, make
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sure that the embedding zone is at least 31 KiB (512KiB or more
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recommended).
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For information on where GRUB should be installed on PC BIOS platforms,
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@pxref{BIOS installation}.
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If you still do want to install GRUB under a UNIX-like OS (such
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as @sc{gnu}), invoke the program @command{grub-install} (@pxref{Invoking
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@ -744,6 +740,72 @@ comments in the file if needed, as the GRUB utilities assume that a line is
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just a comment if the first character is @samp{#}.
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@node BIOS installation
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@section BIOS installation
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@heading MBR
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The partition table format traditionally used on PC BIOS platforms is called
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the Master Boot Record (MBR) format; this is the format that allows up to
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four primary partitions and additional logical partitions. With this
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partition table format, there are two ways to install GRUB: it can be
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embedded in the area between the MBR and the first partition (called by
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various names, such as the "boot track", "MBR gap", or "embedding area", and
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which is usually at least 31 KiB), or the core image can be installed in a
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file system and a list of the blocks that make it up can be stored in the
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first sector of that partition.
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Each of these has different problems. There is no way to reserve space in
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the embedding area with complete safety, and some proprietary software is
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known to use it to make it difficult for users to work around licensing
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restrictions; and systems are sometimes partitioned without leaving enough
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space before the first partition. On the other hand, installing to a
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filesystem means that GRUB is vulnerable to its blocks being moved around by
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filesystem features such as tail packing, or even by aggressive fsck
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implementations, so this approach is quite fragile; and this approach can
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only be used if the @file{/boot} filesystem is on the same disk that the
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BIOS boots from, so that GRUB does not have to rely on guessing BIOS drive
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numbers.
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The GRUB development team generally recommends embedding GRUB before the
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first partition, unless you have special requirements. You must ensure that
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the first partition starts at least 31 KiB (63 sectors) from the start of
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the disk; on modern disks, it is often a performance advantage to align
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partitions on larger boundaries anyway, so the first partition might start 1
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MiB from the start of the disk.
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@heading GPT
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Some newer systems use the GUID Partition Table (GPT) format. This was
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specified as part of the Extensible Firmware Interface (EFI), but it can
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also be used on BIOS platforms if system software supports it; for example,
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GRUB and GNU/Linux can be used in this configuration. With this format, it
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is possible to reserve a whole partition for GRUB, called the BIOS Boot
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Partition. GRUB can then be embedded into that partition without the risk
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of being overwritten by other software and without being contained in a
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filesystem which might move its blocks around.
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When creating a BIOS Boot Partition on a GPT system, you should make sure
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that it is at least 31 KiB in size. (GPT-formatted disks are not usually
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particularly small, so we recommend that you make it larger than the bare
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minimum, such as 1 MiB, to allow plenty of room for growth.) You must also
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make sure that it has the proper partition type. Using GNU Parted, you can
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set this using a command such as the following:
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@example
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# @kbd{parted /dev/@var{disk} set @var{partition-number} bios_grub on}
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@end example
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If you are using gdisk, set the partition type to @samp{0xEF02}. With
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partitioning programs that require setting the GUID directly, it should be
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@samp{21686148-6449-6e6f-744e656564454649}.
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@strong{Caution:} Be very careful which partition you select! When GRUB
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finds a BIOS Boot Partition during installation, it will automatically
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overwrite part of it. Make sure that the partition does not contain any
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other data.
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@node Booting
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@chapter Booting
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@ -2021,28 +2083,8 @@ target operating systems, and so on) from the file system at run-time. The
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modular design allows the core image to be kept small, since the areas of
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disk where it must be installed are often as small as 32KB.
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On PC systems using the traditional MBR partition table format, the core
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image is usually installed in the "MBR gap" between the master boot record
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and the first partition, or sometimes it is installed in a file system and
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read directly from that. The latter is not recommended because GRUB needs
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to encode the location of all the core image sectors in @file{diskboot.img},
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and if the file system ever moves the core image around (as it is entitled
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to do) then GRUB must be reinstalled; it also means that GRUB will not be
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able to reliably find the core image if it resides on a different disk than
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the one to which @file{boot.img} was installed.
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On PC systems using the more recent GUID Partition Table (GPT) format, the
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core image should be installed to a BIOS Boot Partition. This may be
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created by GNU Parted using a command such as the following:
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@example
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# @kbd{parted /dev/@var{disk} set @var{partition-number} bios_grub on}
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@end example
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@strong{Caution:} Be very careful which partition you select! When GRUB
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finds a BIOS Boot Partition during installation, it will automatically
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overwrite part of it. Make sure that the partition does not contain any
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other data.
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@xref{BIOS installation}, for details on where the core image can be
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installed on PC systems.
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@item *.mod
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Everything else in GRUB resides in dynamically loadable modules. These are
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