GRUB consists of several images: two essential stages, optional stages
called Stage 1.5, one image for bootable CD-ROM, and two network
boot images. Here is a short overview of them. See Internals, for
This is an essential image used for booting up GRUB. Usually, this is
embedded in an MBR or the boot sector of a partition. Because a PC boot
sector is 512 bytes, the size of this image is exactly 512 bytes.
All stage1 must do is to load Stage 2 or Stage 1.5 from a local
disk. Because of the size restriction, stage1 encodes the
location of Stage 2 (or Stage 1.5) in a block list format, so it never
understand any filesystem structure.
This is the core image of GRUB. It does everything but booting up
itself. Usually, this is put in a filesystem, but that is not required.
These are called Stage 1.5, because they serve as a bridge
between stage1 and stage2, that is to say, Stage 1.5 is
loaded by Stage 1 and Stage 1.5 loads Stage 2. The difference between
stage1 and *_stage1_5 is that the former doesn't
understand any filesystem while the latter understands one filesystem
(e.g. e2fs_stage1_5 understands ext2fs). So you can move the
Stage 2 image to another location safely, even after GRUB has been
While Stage 2 cannot generally be embedded in a fixed area as the size
is so large, Stage 1.5 can be installed into the area right after an MBR,
or the boot loader area of a ReiserFS or a FFS.
This is a boot image for CD-ROMs using the no emulation mode in
El Torito specification. This is identical to Stage 2, except that
this boots up without Stage 1 and sets up a special drive `(cd)'.
This is a network boot image for the Network Image Proposal used by some
network boot loaders, such as Etherboot. This is mostly the same as
Stage 2, but it also sets up a network and loads a configuration file
from the network.
This is another network boot image for the Preboot Execution Environment
used by several Netboot ROMs. This is identical to nbgrub, except
for the format.
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