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Added GRUB docs, Added netboot.xyz

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Eric-Teunis de Boone 2020-11-13 00:50:49 +01:00
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11 GRUB image files
*******************
GRUB consists of several images: a variety of bootstrap images for
starting GRUB in various ways, a kernel image, and a set of modules
which are combined with the kernel image to form a core image. Here is
a short overview of them.
'boot.img'
On PC BIOS systems, this image is the first part of GRUB to start.
It is written to a master boot record (MBR) or to the boot sector
of a partition. Because a PC boot sector is 512 bytes, the size of
this image is exactly 512 bytes.
The sole function of 'boot.img' is to read the first sector of the
core image from a local disk and jump to it. Because of the size
restriction, 'boot.img' cannot understand any file system
structure, so 'grub-install' hardcodes the location of the first
sector of the core image into 'boot.img' when installing GRUB.
'diskboot.img'
This image is used as the first sector of the core image when
booting from a hard disk. It reads the rest of the core image into
memory and starts the kernel. Since file system handling is not
yet available, it encodes the location of the core image using a
block list format.
'cdboot.img'
This image is used as the first sector of the core image when
booting from a CD-ROM drive. It performs a similar function to
'diskboot.img'.
'pxeboot.img'
This image is used as the start of the core image when booting from
the network using PXE. *Note Network::.
'lnxboot.img'
This image may be placed at the start of the core image in order to
make GRUB look enough like a Linux kernel that it can be booted by
LILO using an 'image=' section.
'kernel.img'
This image contains GRUB's basic run-time facilities: frameworks
for device and file handling, environment variables, the rescue
mode command-line parser, and so on. It is rarely used directly,
but is built into all core images.
'core.img'
This is the core image of GRUB. It is built dynamically from the
kernel image and an arbitrary list of modules by the 'grub-mkimage'
program. Usually, it contains enough modules to access
'/boot/grub', and loads everything else (including menu handling,
the ability to load target operating systems, and so on) from the
file system at run-time. The modular design allows the core image
to be kept small, since the areas of disk where it must be
installed are often as small as 32KB.
*Note BIOS installation::, for details on where the core image can
be installed on PC systems.
'*.mod'
Everything else in GRUB resides in dynamically loadable modules.
These are often loaded automatically, or built into the core image
if they are essential, but may also be loaded manually using the
'insmod' command (*note insmod::).
For GRUB Legacy users
=====================
GRUB 2 has a different design from GRUB Legacy, and so correspondences
with the images it used cannot be exact. Nevertheless, GRUB Legacy
users often ask questions in the terms they are familiar with, and so
here is a brief guide to how GRUB 2's images relate to that.
'stage1'
Stage 1 from GRUB Legacy was very similar to 'boot.img' in GRUB 2,
and they serve the same function.
'*_stage1_5'
In GRUB Legacy, Stage 1.5's function was to include enough
filesystem code to allow the much larger Stage 2 to be read from an
ordinary filesystem. In this respect, its function was similar to
'core.img' in GRUB 2. However, 'core.img' is much more capable
than Stage 1.5 was; since it offers a rescue shell, it is sometimes
possible to recover manually in the event that it is unable to load
any other modules, for example if partition numbers have changed.
'core.img' is built in a more flexible way, allowing GRUB 2 to
support reading modules from advanced disk types such as LVM and
RAID.
GRUB Legacy could run with only Stage 1 and Stage 2 in some limited
configurations, while GRUB 2 requires 'core.img' and cannot work
without it.
'stage2'
GRUB 2 has no single Stage 2 image. Instead, it loads modules from
'/boot/grub' at run-time.
'stage2_eltorito'
In GRUB 2, images for booting from CD-ROM drives are now
constructed using 'cdboot.img' and 'core.img', making sure that the
core image contains the 'iso9660' module. It is usually best to
use the 'grub-mkrescue' program for this.
'nbgrub'
There is as yet no equivalent for 'nbgrub' in GRUB 2; it was used
by Etherboot and some other network boot loaders.
'pxegrub'
In GRUB 2, images for PXE network booting are now constructed using
'pxeboot.img' and 'core.img', making sure that the core image
contains the 'pxe' and 'pxecmd' modules. *Note Network::.