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The typical computer
system boots
over and over again with no problems, starting the
computer's operating
system (OS)
and identifying its hardware
and software
components that all work together to provide the
user with the complete computing experience. But
what happens between the time that the user powers
up the computer
and when the GUI
icons
appear on the desktop?
In order for a computer
to successfully boot,
its BIOS,
operating
system and hardware
components must all be working properly; failure of
any one of these three elements will likely result
in a failed boot
sequence.
When the computer's
power is first turned on, the CPU
initializes itself, which is triggered by a series
of clock
ticks generated by the system clock. Part
of the CPU's
initialization is to look to the system's ROM
BIOS
for its first instruction in the startup program.
The ROM
BIOS
stores the first instruction, which is the
instruction to run the power-on
self test (POST), in a predetermined
memory address. POST
begins by checking the BIOS
chip and then tests CMOS
RAM.
If the POST
does not detect a battery failure, it then continues
to initialize the CPU,
checking the inventoried hardware
device
(such as the video
card), secondary storage
devices, such as hard
drives and floppy
drives, ports
and other hardware
devices,
such as the keyboard
and mouse,
to ensure they are functioning properly.
Once the POST
has determined that all components are functioning
properly and the CPU
has successfully initialized, the BIOS
looks for an OS
to load.
The BIOS
typically looks to the CMOS
chip to tell it where to find the OS,
and in most PCs,
the OS
loads from the C drive on the hard
drive even though the BIOS
has the capability to load the OS
from a floppy
disk, CD
or Zip
drive. The order of drives
that the CMOS
looks to in order to locate the OS
is called the boot
sequence, which can be changed by
altering the CMOS
setup. Looking to the appropriate boot
drive,
the BIOS
will first encounter the boot
record,
which tells it where to find the beginning of the OS
and the subsequent program
file
that will initialize the OS.
Once the OS
initializes, the BIOS
copies its files
into memory and the OS
basically takes over control of the boot
process. Now in control, the OS
performs another inventory of the system's memory
and memory availability (which the BIOS
already checked) and loads the device
drivers
that it needs to control the peripheral
device, such as a printer,
scanner,
optical
drive, mouse
and keyboard.
This is the final stage in the boot
process, after which the user can access the system’s
applications
to perform tasks. |