Grand Unified Boot Loader (GRUB) 0.98 ("Legacy GRUB")

GRUB consists of three stages:

• Stage 1 may reside on the partition boot record. As such it is limited in size to 512 bytes. Alternately, GRUB Stage 1 may reside on the MBR. In that case it must also contain partition table and signature, the amount of space available is less than 512 bytes. GRUB stage 1 contains the address of the first sector of stage 1.5 which is loads into memory and executes.
• Stage 1.5 consists of three pieces:
  1. Since only the first sector of stage 1.5 is loaded by stage 1, the first portion of stage 1.5 loads the remainder of stage 1.5 into memory. It does so from a list of sector addresses stored within the first sector of stage 1.5.
  2. The majority of stage 1.5 consists of code to read the file system. This contains the necessary code to locate a file within a directory, determine where the file is located within the partition and read the file. There are actually several versions of GRUB stage 1.5, one for each type of supported file system, EXT2, EXT3, FAT, Reiser, etc.
  3. The final portion of GRUB stage 1.5 reads GRUB stage 2 into memory, using the file-system subroutines mentioned above.
• Stage 2 incorporates the bulk of the functionality of GRUB. It consists of a number of components:
  1. A command processor which can execute a number of commands associated with loading OS components.
  2. A menu processor. This processor reads a menu description from the file /boot/grub/menu.lst. Each menu entry concsists of a "title" command whose contents are displayed in the menu, and a series of following commands that are to be executed if the menu item is selected.
  3. A display system that can display menus, and other information on the screen and get input from tne keyboard.

  When GRUB stage 2 is loaded is reads /boot/grub/menu.lst. In addition to individual menu entries this file contains parameters to specify

  • Whether to display the menu by default or only upon request from the user.
  • A timeout value. If the user does make a selection within the specified time, a default menu entry is selected.
  • The default menu item.

  Once the initial screen is shown the user can select to display the menu, can select a menu item, can edit the commands associated with a menu entry, or can enter a command mode where commands can be typed directly by the keyboard. The most common choice is to let the default menu entry be selected.

  Here is a typical menu entry for loading a Linux system:

  title Red Hat Enterprise Linux ES (2.6.9-42.ELsmp)
  root (hd0,0)
  kernel /vmlinuz-2.6.9-42.ELsmp ro root=/dev/VolGroup00/LogVol00 rhgb quiet
  initrd /initrd-2.6.9-42.ELsmp.img
  

  As mentioned above, the title command specifies the text to be displayed in the menu.

  The root command specifies which disk and partition contains the kernel and initrd files.

  The kernel command loads the kernel into memory. The first word following "kernel" specifies the filename of the file containing the kernel. All words following the filename are parameters that are passed to the kernel when it is started.

  The initrd command specifies an "initialization RAM disk" (thus the name initrd). This file contains a simple file-system structure and a series of files to be loaded by the kernel. These files consist of drivers and other dynamic modules that must be loaded by the kernel early in the boot process. Because the file-system module as well as disk drivers might be loaded as dynamic modules, the kernel cannot use either file system or disk driver to load these components. The kernal has built into it a simple-minded file system for the express purpose of reading this RAM disk.

  Thus GRUB loads the initrd into memory, and passes its address to the kernel. The kernel then process that file system during system initialization.

  Once the kernel and initrd are read into memory, GRUB transfers control to the kernel.

Next: The Kernel

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