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2.2. GRUB

GNU GRand Unified Boot loader or GRUB is a program which enables the user to select which installed operating system or kernel to load at system boot time. It also allows the user to pass arguments to the kernel.

2.2.1. GRUB and the x86 Boot Process

This section discusses in more detail the specific role GRUB plays when booting an x86 system. For an look at the overall boot process, see Section 1.2 A Detailed Look at the Boot Process.

GRUB loads itself into memory in the following stages:

  1. The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR[1]. The primary boot loader exists on less than 512 bytes of disk space within the MBR and is capable of loading either the Stage 1.5 or Stage 2 boot loader.

  2. The Stage 1.5 boot loader is read into memory by the Stage 1 boot loader, if necessary. Some hardware requires an intermediate step to get to the Stage 2 boot loader. This is sometimes true when the /boot partition is above the 1024 cylinder head of the hard drive or when using LBA mode. The Stage 1.5 boot loader is found either on the /boot partition or on a small part of the MBR and the /boot partition.

  3. The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays the GRUB menu and command environment. This interface allows you to select which operating system or Linux kernel to boot, pass arguments to the kernel, or look at system parameters, such as available RAM.

  4. The secondary boot loader reads the operating system or kernel and initrd into memory. Once GRUB determines which operating system to start, it loads it into memory and transfers control of the machine to that operating system.

The boot method used to boot Red Hat Linux is called the direct loading method because the boot loader loads the operating system directly. There is no intermediary between the boot loader and the kernel.

The boot process used by other operating systems may differ. For example, Microsoft's DOS and Windows operating systems, as well as various other proprietary operating systems, are loaded using a chain loading boot method. Under this method, the MBR points to the first sector of the partition holding the operating system. There it finds the files necessary to actually boot that operating system.

GRUB supports both direct and chain-loading boot methods, allowing it to boot almost any operating system.


During installation, the Microsoft's DOS and Windows installation program completely overwrites the MBR, destroying any existing boot loader. If creating a dual-boot system, it is best to install the Microsoft operating system first. For instructions on how to do this, see the appendix titled Installing Red Hat Linux in a Dual-Boot Environment in the Red Hat Linux Installation Guide.

2.2.2. Features of GRUB

GRUB contains a number of features that make it preferable to other boot loaders available for the x86 architecture. Below is a partial list of some of the more important features:

  • GRUB provides a true command-based, pre-OS environment on x86 machines. This affords the user maximum flexibility in loading operating systems with certain options or gathering information about the system. For years, many non-x86 architectures have employed pre-OS environments that allow system booting from a command line. While some command features are available with LILO and other x86 boot loaders, GRUB is more feature rich.

  • GRUB supports Logical Block Addressing (LBA) mode. LBA places the addressing conversion used to find files in the hard drive's firmware, and is used on many IDE and all SCSI hard devices. Before LBA, boot loaders could encounter the 1024-cylinder BIOS limitation, where the BIOS could not find a file after that cylinder head of the disk. LBA support allows GRUB to boot operating systems from partitions beyond the 1024-cylinder limit, so long as the system BIOS supports LBA mode. Most modern BIOS revisions support LBA mode.

  • GRUB can read ext2 partitions. This functionality allows GRUB to access its configuration file, /boot/grub/grub.conf, every time the system boots, eliminating the need for the user to write a new version of the first stage boot loader to MBR when configuration changes are made. The only time a user would need to reinstall GRUB on the MBR is if the physical location of the /boot partition is moved on the disk. For details on installing GRUB to the MBR, see Section 2.3 Installing GRUB.



For more on the system BIOS and the MBR, see Section 1.2.1 The BIOS.

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