Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 77)

Chapter 4.

GRUB

Before Red Hat Linux can load on a system, it must be told to boot by special instructions placed on

a boot loader, a program that exists on the system’s primary hard drive or other media device that

knows how to start the Linux kernel.

If using an x86 system that only boots Red Hat Linux and only one version of the Linux kernel, then

the exact process used by the boot loader to start Red Hat Linux is not very important. The Red Hat

Linux installation program allows users to quickly and easily conﬁgure the boot loader in the primary

hard drive’s Master Boot Record (MBR) to load the operating system.

However, in order to have the choice to boot from multiple Linux kernels or other operating systems,

it is essential to have the knowledge of the method used by Red Hat Linux to provide the necessary

boot options, as well as an understanding of the boot process and what to change.

This chapter discusses GRUB, the default method Red Hat Linux uses to launch operating systems

on x86 systems. The chapter will also detail various commands and conﬁguration options that enable

control over the boot process.

4.1. Deﬁning GRUB

GNU GRUB (GRand Uniﬁed Bootloader) is a program that installs a boot loader to the MBR, which

exists at the beginning sectors of a disk. It allows the placement of speciﬁc instructions in the MBR

that loads a GRUB menu or command environment, which enables users to start the operating system

of choice, pass special instructions to kernels when they boot, or discover system parameters (such as

available RAM) before booting.

4.1.1. x86 Boot Process

When an x86 machine ﬁrst starts up, the BIOS of the system checks for particular system parameters,

such as the amount of RAM in the machine, the date and time, disk devices present, and the order

in which those disk devices should be checked for bootable media. Usually, the BIOS is conﬁgured

to check the machine’s ﬂoppy disk drive or CD-ROM drive (or both) before attempting to boot from

the hard drive. If no bootable media can be found in those removable devices, the BIOS usually turns

to the ﬁrst few sectors of the ﬁrst hard drive for instructions on where to ﬁnd and load an operating

system. These ﬁrst few sectors — the MBR — start the process of loading a pre-selected operating

system, a GRUB menu of operating system options, or a GRUB command line interface to execute

special options.

The process of loading GRUB, and then the operating system, involves several stages of operation:

1. Load the primary boot loader, commonly called Stage 1. The primary boot loader must exist in

the very small space allocated for the MBR, which is less than 512 bytes. Therefore, the only

thing the primary boot loader accomplishes is loading the secondary boot loader, due to the fact

that there is not enough space in the MBR for anything else.

2. Load the secondary boot loader, commonly called Stage 2. The secondary boot loader actually

brings up the advanced functionality that allows you to load a speciﬁc operating system. With

GRUB, this is the code that allows you to display a menu or type commands.

3. Load the operating system, such as the Linux kernel, on a speciﬁed partition. Once GRUB has

received the correct instructions for the operating system to start, either from its command line

or conﬁguration ﬁle, it ﬁnds the necessary boot ﬁle and hands off control of the machine to that

operating system.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 77

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