Chapter 4.

Boot Loaders

Before Red Hat Linux can run on a system, it must be started by special program called a boot loader.

The boot loader program usually exists on the system's primary hard drive or other media device and

is responsible for loading the Linux kernel its required files, or, in some cases, other operating systems

into memory.

4.1. Boot Loaders and System Architecture

Each system architecture which can run Red Hat Linux uses a different boot loader. For example, the

Alpha architecture uses the

aboot

boot loader, while the Itanium architecture uses the

ELILO

boot

loader.

This chapter discuss commands and configuration options for the two boot loaders supplied with Red

Hat Linux 8.0 for x86 architecture: GRUB and LILO.

4.2. GRUB

GNU GRand Unified Boot loader or GRUB is a program which enable the user to select which in 

stalled operating system or kernel to load at system boot time. It also allows the user to pass arguments

to the kernel.

4.2.1. GRUB and the x86 Boot Process

This section will discuss in more detail the specific role GRUB plays when booting an x86 system.

For a detailed look at the overall boot process, see Section 3.2.

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. The only thing

it does is load 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 only 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 hands 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.

1. For more on the system BIOS and the MBR, see Section 3.2.1.