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How does the boot process[init levels] work on Linux? How
is it different from Solaris?
Answer Posted / dayashankar prajapat
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.
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