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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.

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.

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.
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.

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.
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.