4.2.2. The boot process
When an x86 computer is booted, the processor looks at the end
of the system memory for the BIOS (Basic Input/Output System) and
runs it. The BIOS program is written into permanent read-only
memory and is always available for use. The BIOS provides the
lowest level interface to peripheral devices and controls the first
step of the boot process.
The BIOS tests the system, looks for and checks peripherals, and
then looks for a drive to use to boot the system. Usually it checks
the floppy drive (or CD-ROM drive on many newer systems) for
bootable media, if present, and then it looks to the hard drive.
The order of the drives used for booting is usually controlled by a
particular BIOS setting on the system. Once Linux is installed on
the hard drive of a system, the BIOS looks for a Master Boot Record
(MBR) starting at the first sector on the first hard drive, loads
its contents into memory, then passes control to it.
This MBR contains instructions on how to load the GRUB (or LILO)
boot-loader, using a pre-selected operating system. The MBR then
loads the boot-loader, which takes over the process (if the
boot-loader is installed in the MBR). In the default Red Hat Linux
configuration, GRUB uses the settings in the MBR to display boot
options in a menu. 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.