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Chapter 3. Boot Process, Init, and Shutdown

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 first checks any floppy drives and CD ROM drives

present for bootable media, then it looks to the system's hard drives. The order of the drives searched

for booting can often be controlled with a setting in BIOS. Often, the first hard drive set to boot is

the C drive or the master IDE device on the primary IDE bus. The BIOS loads whatever program is

residing in the first sector of this device, called the Master Boot Record or MBR, into memory. The

MBR is only 512 bytes in size and contains machine code instructions for booting the machine along

with the partition table. Once found and loaded the BIOS passes control whatever program is on the

MBR.

3.2.2. The Boot Loader

This section looks at the boot process for the x86 platform. Depending on your system's architecture,

the boot process may differ slightly. Please see Section 3.4 for a brief overview of non x86 boot

processes.

Linux boot loaders for the x86 platform are 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. Under Red Hat Linux you can install one of two boot loaders: GRUB

or LILO. GRUB is the default boot loader, but LILO is available for those who require it for their

hardware setup or who prefer it. For more information about configuring and using GRUB or LILO,

see Chapter 4.

If you are using LILO under Red Hat Linux, the second stage boot loader uses information on the

MBR to determine what boot options are available to the user. This means that any time a configu 

ration change is made or you upgrade your kernel manually, you must run the

/sbin/lilo  v  v

command to write the appropriate information to the MBR. For details on doing this, see Section 4.8.

GRUB, on the other hand, can read ext2 partitions and therefore simply loads its configuration file  

/boot/grub/grub.conf

  when the second stage loader is called. See Section 4.7 for information

on how to edit this file.

Tip

If

you

upgrade

the

kernel

using

the

Red Hat

Update Agent, the

MBR or

the

/boot/grub/grub.conf will be updated automatically. For more information on RHN, refer to the

following URL, https://rhn.redhat.com

Once the second stage boot loader is in memory, it presents the user with the Red Hat Linux initial,

graphical screen showing the different operating systems or kernels it has been configured to boot. If

you have only Red Hat Linux installed and have not changed anything in the

/etc/lilo.conf

or

/boot/grub/grub.conf

, you will only see one option for booting.

If you have configured the boot loader to boot other operating systems, this screen gives you the

opportunity to select it. Use the arrow keys to highlight the operating system and press [Enter]. If you

do nothing, the boot loader will load the default selection.

Note

If you installed Symmetric Multi Processor (SMP) kernel support, you will see more than one option

the first time you boot your system. Under LILO you will see linux and linux up. Under GRUB

you will see Red Hat Linux (kernel version) and Red Hat Linux (kernel version smp).

Thelinux or Red Hat Linux (kernel version smp) option is the SMP kernel. If you experience

any problems with the SMP kernel, select the linux up or non SMP kernel upon rebooting.