Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION, Installation Manual

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Chapter 9.
93
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 System Architecture
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:
Architecture Boot Loaders
AMD
®
AMD64 GRUB
IBM
®
eServer
™
System i
™
OS/400
®
IBM
®
eServer
™
System p
™
YABOOT
IBM
®
System z
®
z/IPL
IBM
®
System z
®
z/IPL
Intel
®
Itanium
™
ELILO
x86 GRUB
Table 9.1. Boot Loaders by Architecture
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 the 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
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.