Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION, Руководство по установке

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Chapter 33. Boot Process, Init, and Shutdown
354
process, but also provides the lowest level interface to peripheral devices. For this reason it is written
into read-only, permanent memory and is always available for use.
Other platforms use different programs to perform low-level tasks roughly equivalent to those of
the BIOS on an x86 system. For instance, Itanium-based computers use the
Extensible Firmware
Interface
(
EFI
)
Shell
.
Once loaded, the BIOS tests the system, looks for and checks peripherals, and then locates a valid
device with which to boot the system. Usually, it checks any diskette drives and CD-ROM drives
present for bootable media, then, failing that, looks to the system's hard drives. In most cases, the
order of the drives searched while booting is controlled with a setting in the BIOS, and it looks on the
master IDE device on the primary IDE bus. The BIOS then loads into memory whatever program is
residing in the first sector of this device, called the
Master Boot Record
or
MBR
. The MBR is only 512
bytes in size and contains machine code instructions for booting the machine, called a boot loader,
along with the partition table. Once the BIOS finds and loads the boot loader program into memory, it
yields control of the boot process to it.
33.2.2. The Boot Loader
This section looks at the default boot loader for the x86 platform, GRUB. Depending on the system's
architecture, the boot process may differ slightly. Refer to
Section 33.2.2.1, “Boot Loaders for Other
Architectures”
for a brief overview of non-x86 boot loaders. For more information about configuring
and using GRUB, see
Chapter 9, The GRUB Boot Loader
.
A boot loader for the x86 platform is broken into at least two stages. The first stage is a small machine
code binary on the MBR. Its sole job is to locate the second stage boot loader and load the first part of
it into memory.
GRUB has the advantage of being able to read ext2 and ext3
1
partitions and load its configuration file
—
/boot/grub/grub.conf
— at boot time. Refer to
Section 9.7, “GRUB Menu Configuration File”
for information on how to edit this file.
Note
If upgrading the kernel using the
Red Hat Update Agent
, the boot loader configuration file
is updated automatically. More information on Red Hat Network can be found online at the
following URL:
https://rhn.redhat.com/
.
Once the second stage boot loader is in memory, it presents the user with a graphical screen showing
the different operating systems or kernels it has been configured to boot. On this screen a user can
use the arrow keys to choose which operating system or kernel they wish to boot and press
Enter
. If
no key is pressed, the boot loader loads the default selection after a configurable period of time has
passed.
Once the second stage boot loader has determined which kernel to boot, it locates the corresponding
kernel binary in the
/boot/
directory. The kernel binary is named using the following format —
/
boot/vmlinuz- <kernel-version>
file (where
<kernel-version>
corresponds to the kernel
version specified in the boot loader's settings).
1
GRUB reads ext3 file systems as ext2, disregarding the journal file. Refer to the chapter titled
The ext3 File System
in the
Red
Hat Enterprise Linux Deployment Guide
for more information on the ext3 file system.