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Chapter 1. Boot Process, Init...
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.
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
for Other Architectures"
configuring and using GRUB, see
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
configuration file —
/boot/grub/grub.conf
Configuration File"
for information on how to edit this file.
Tip
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.
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 System Administration Guide for more information on the ext3 file system.
4
for a brief overview of non-x86 boot loaders. For more information about
Chapter 2, The GRUB Boot
— at boot time. Refer to
Section 2.2.1, "Boot Loaders
Loader.
1
partitions and load its
Section 7, "GRUB Menu
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