Editing GNU GRUB (section)

==== Startup on systems using [[BIOS]] firmware ====
* See illustration in last image on the right.<ref>{{Cite web|title=GNU GRUB Manual 2.04|url=https://www.gnu.org/software/grub/manual/grub/grub.html#Images|access-date=2020-11-10|website=www.gnu.org|archive-date=April 3, 2018|archive-url=https://web.archive.org/web/20180403023208/http://www.gnu.org/software/grub/manual/grub/grub.html#Images|url-status=live}}</ref>
* <code>boot.img</code> ('''stage 1''') is written to the first 440&nbsp;bytes of the [[Master boot record|Master Boot Record]] (MBR boot code in sector 0), or optionally in a [[partition boot sector]] (PBR). It addresses <code>diskboot.img</code> by a 64-bit [[Logical block addressing | LBA]] address. The actual sector number is written by <code>grub-install</code>. <code>diskboot.img</code> is the first sector of <code>core.img</code> with the sole purpose to load the rest of <code>core.img</code> identified by LBA sector numbers also written by <code>grub-install</code>.
** On MBR partitioned disks, <code>core.img</code> ('''stage 1.5''') is stored in the empty sectors (if available) between the MBR and the first partition. Recent operating systems suggest a 1 MiB gap here for alignment (2047 512-byte, or 255 4KiB, sectors). This gap used to be 62 sectors (31 KiB) as a reminder of the sector number limit of [[Cylinder-Head-Sector]] (C/H/S) addressing used by [[BIOS]] before 1996, therefore <code>core.img</code> is designed to be smaller than 32 KiB.
**On GPT partitioned disks <code>core.img</code> is written to its own partition. It must be flagged "BIOS_grub", must not be [[file system|formatted]] and can be as tiny as 1 MiB.
* '''stage 2:''' <code>core.img</code> loads <code>/boot/grub/i386-pc/normal.mod</code> from the partition configured by <code>grub-install</code>. If the partition index has changed, GRUB will be unable to find the <code>normal.mod</code>, and presents the user with the GRUB Rescue prompt.
* Depending on how GRUB2 was installed, the <code>/boot/grub/</code> is either in the ''root'' partition of the Linux distribution, or in the separate ''/boot'' partition.
* '''after normal.mod loaded:''' <code>normal.mod</code> parses <code>/boot/grub/grub.cfg</code>, optionally loads modules (eg. for graphical UI and file system support) and shows the menu.