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GRUB image files

GRUB consists of several images: two essential stages, optional stages called Stage 1.5, and two network boot images. Here is a short overview of them. See Internals, for more details.

stage1

This is an essential image used for booting up GRUB. Usually, this is embedded in a 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.

stage2

This is the core image of GRUB. This does all things but booting up itself. Usually, this is put in a filesystem, but that is not required.

e2fs_stage1_5

fat_stage1_5

ffs_stage1_5

jfs_stage1_5

minix_stage1_5

reiserfs_stage1_5

vstafs_stage1_5

xfs_stage1_5

These are called Stage 1.5, because the purpose is 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 but the latter does an filesystem (e.g. e2fs_stage1_5 understands ext2fs). So you can move the location of Stage 2 to another safely, even after GRUB has been installed.

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 a MBR, or the boot loader area of a ReiserFS or a FFS.

nbgrub

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 this also sets up a network and loads a configuration file from the network.

pxegrub

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.