10. GRUB image files

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 section Hacking GRUB, for more details.

‘stage1’

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.

‘stage2’

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.

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

‘stage2_eltorito’

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)’.

‘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 it 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.

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