Installation guide

Chapter 9. The GRUB Boot Loader

When a computer with Red Hat Enterprise Linux is turned on, the operating system is loaded into

memory by a special program called a boot loader. A boot loader usually exists on the system's

primary hard drive (or other media device) and has the sole responsibility of loading the Linux kernel

with its required files or (in some cases) other operating systems into memory.

9.1. Boot Loaders and Syst em Archit ect ure

Each architecture capable of running Red Hat Enterprise Linux uses a different boot loader. The

following table lists the boot loaders available for each architecture:

T ab le 9 .1. Bo o t Lo ad ers by Arch it ect u re

Arch it ect u re Bo o t Lo ad ers

AMD® AMD64 GRUB

IBM® eServer™ System i™ OS/400®

IBM® eServer™ System p™ YABOOT

IBM® System z® z/IPL

Intel® Itanium™ ELILO

x86 GRUB

This chapter discusses commands and configuration options for the GRUB boot loader included

with Red Hat Enterprise Linux for the x86 architecture.

9.2. GRUB

The GNU GRand Unified Boot loader (GRUB) is a program which enables the selection of the installed

operating system or kernel to be loaded at system boot time. It also allows the user to pass

arguments to the kernel.

9.2.1. GRUB and t he x86 Boot Process

This section discusses the specific role GRUB plays when booting an x86 system. For a look at the

overall boot process, refer to Section 33.2, “ 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 ⁠ . 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.

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⁠Chapt er 9 . T he G RUB Boot Loader