The PC Boot Process - Windows XP

Power supply switched on.

The power supply performs a self-test. When

all voltages and current levels are acceptable,
the supply indicates that the power is stable
and sends the Power Good signal to the
processor. The time from switch-on to Power
Good is usually between .1 and .5 seconds.
 With the arrival of the
Power Good signal the
The microprocessor
timer chip stops sending
timer chip receives the
reset signals to the
Power Good signal.
processor allowing the CPU
to begin operations.
 Acronym for basic input/output system,
the built-in software that determines
what a computer can do without
accessing programs from a disk. On
PCs, the BIOS contains all the code
The CPU starts required to control the keyboard, display
screen, disk drives, serial
executing the ROM communications, and a number of
BIOS code. miscellaneous functions.

The BIOS is typically placed in a ROM chip

The CPU loads the ROM BIOS
that comes with the computer (it is often
starting at ROM memory address
FFFF:0000 which is only 16 bytes called a ROM BIOS). This ensures that the
from the top of ROM memory. As BIOS will always be available and will not
such it contains only a JMP (jump) be damaged by disk failures. It also makes
instruction that points to the actual
address of the ROM BIOS code. it possible for a computer to boot itself.
Because RAM is faster than ROM, though,
many computer manufacturers design
systems so that the BIOS is copied from
ROM to RAM each time the computer is
booted.
Where the BIOS is stored in
rewritable memory, flashing
the BIOS is overwriting the
BIOS contents with a BIOS image.
This is mainly done to update the
BIOS to support newer hardware
than it could otherwise support, or
if it becomes damaged. However,
care must be taken when doing
this because if it isn't done
correctly it may render the system
inoperable.
 Any errors that occur at this
point in the boot process will be
reported by means of 'beep-
codes' because the video
subsystem has not yet been
initialized.
The ROM BIOS
A beep code is an audible
performs a basic
"code" generated during
test of central
the Power On Self Test
hardware to verify
when the BIOS encounters
basic functionality.
an issue but hasn't booted
far enough to display a
POST error message on
the monitor.
Every major BIOS manufacturer has a unique
set of beep codes that can be generated during
their POSTs. Computer systems without an
internal speaker will not be capable of producing
a beep code.

With many motherboards, the BIOS produces a

single, short beep code as an "all systems clear"
notification. This single beep code isn't an issue that
needs troubleshooting.
 Beeps Meaning
Steady, short Power supply may be bad
beeps
Long Memory failure
continuous
beep tone
Steady, long Power supply bad
beeps
No beep Power supply bad, system not plugged in, or
power not turned on
No beep If everything seems to be functioning correctly
there may be a problem with the 'beeper' itself.
One long, two Video card failure
short beeps
 Video adapters provide the most
common source of adapter ROM
BIOS. The start-up BIOS routines
scan memory addresses
C000:0000 through
C780:0000
to find video ROM. An error
loading any adapter ROM
The BIOS searches for
adapters that may need generates an error such as:
to load their own ROM XXXX ROM Error
BIOS routines. where XXXX represents the
segment address of the failed
module.
 To start running a computer program
from the very beginning, without being
able to continue the processing that was
occurring previously when the system
was interrupted.
The ROM BIOS
checks to see if
A resumption of computer operation,
this is a 'cold-
following a problem-generated
start' or a 'warm- shutdown, in which programs running
start' on the system can resume at the
point they were at when the shutdown
occurred and data is not lost.

To determine whether this is a warm-start or a

cold start the ROM BIOS startup routines check
the value of two bytes located at memory location
0000:0472. Any value other than 1234h indicates
that this is a cold-start.
 The POST can be broken
down into three components:

If this is a cold-start the The Video Test initializes the video

ROM BIOS executes a adapter, tests the video card and
full POST (Power On Self video memory, and displays
Test). If this is a warm- configuration information or any
start the memory test errors.
portion of the POST is
switched off. The BIOS Identification displays the
BIOS version, manufacturer, and
date.

The Memory Test tests the memory

chips and displays a running sum of
installed memory.
Errors the occur during the
POST can be classified as
either 'fatal' or 'non-fatal'.

A non-fatal error will typically

display an error message on
screen and allow the system
to continue the boot process.

A fatal error, on the other

hand, stops the process of
booting the computer and is
generally signaled by a series
of beep-codes.
 CMOS
(Complementary Metal-Oxide
Semiconductor)
is a small area of memory (64
bytes) which is maintained by the
current of a small battery attached
The BIOS locates to the motherboard.
and reads the Most importantly for the ROM
configuration BIOS startup routines CMOS
indicates the order in which drives
information stored
should be examined for an
in CMOS. operating systems - floppy first,
CD-Rom first, or fixed disk first.
Complementary metal oxide semiconductor
(CMOS) is a type of semi-conductor
chip that holds data without requiring an
external power source. In a personal
computer (PC), CMOS holds the basic
instructions the computer needs to initialize
its hardware components and boot up.
These settings are known as the basic input
output settings (BIOS), also referred to as
CMOS settings
CMOS controls a variety of functions,
including the Power On Self Test (POST).
When the computer’s power supply fires
up, CMOS runs a series of checks to make
sure the system is functioning properly.
One of these checks includes counting up
random access memory (RAM). This
delays boot time, so some people disable
this feature in the CMOS settings, opting for
a quick boot. If installing new RAM it is
better to enable the feature until the RAM
has been checked.
Once POST has completed, CMOS runs
through its other settings. Hard disks and
formats are detected, along with Redundant
Array of Independent Disk (RAID)
configurations, boot preferences, the presence
of peripherals, and over clocking tweaks. Many
settings can be manually changed within the
CMOS configuration screen to improve
performance. However, changes should be
made by experienced users. Changing settings
improperly can make the system unstable,
cause crashes, or even prevent the computer
from booting.
The CMOS configuration screen is
accessible during the POST phase of boot
up, by pressing a key before the operating
system initializes. Normally this is the Del
key but it might be another. A line of text will
indicate which key will take the user into the
CMOS or BIOS setup screen. Changes
cannot be made from within an operating
system such as Microsoft Windows, but
must be made within a true DOS session.
There is also an option to protect CMOS
settings by requiring a password to change
settings. Changes are saved upon exit by
pressing the F10 key, then the computer
reboots to utilize the new settings.
When was the last time you thought about the battery in
your PC? Didn't know there was a Battery?
Thats the battery that protects your valuable CMOS
data when your desktop isn't powered on. Most recent
motherboards come with coin shaped
lithium/manganese-dioxide batteries that look like large
watch batteries. These batteries are advertised to have
a life span of five years. That's longer than many people
keep their PCs!
While the lifespan of the battery might be five years under the best of
circumstances, why would you wait until failure to replace? Remember that the
battery is the only thing standing between you and the loss of your CMOS
settings when you power down your PC.
If you keep your PC long term without swapping out the motherboard, you
should consider changing the battery every two to three years. Why change
the battery sooner? Simply this. If you wait until the battery begins to die, your
PC can forget its CMOS settings. And remember its only about $3.00 to
replace if you do it yourself.
Often, a dying battery will first manifest itself intermittently. Sometimes, you
will boot with no problem. At other times, you will see an error indicating that
the system doesn't match the configuration. When the battery fails entirely, the
message will either come up every time you reboot, or your BIOS may report
an error number with a message stating that the battery is dead.
 The most common battery used in
Coin Cell Batteries todays computers .3 volt Lithium
coin cell battery. Replacement for
CR-2032 computer battery

4.5V Alkaline Batteries 4 pin connector. Velcro strip

provided to attach it out of the
way on the power supply or case.

3.6V CMOS Batteries 3.6 volt ni-cad battery - 60mah -

2 pin connector - solder on.

6V Lithium battery replacement for

PS2 Computer Clock Battery PS2 50 and up computer battery.
 Replacing a CMOS Battery on a PC
When you replace your battery, the CMOS settings
will immediately be lost. In addition, battery corrosion
(more common in older PCs) or other battery
problems may cause the battery to die more suddenly
than its expected lifetime. For this reason, it is
important to keep a record of your CMOS settings.
Many utility suites, such as Norton Utilities, will allow
you to create a
floppy disk backup of your settings. This can be
extremely handy for battery problems as well as
recovering from other CMOS errors .

Warning!!!
Before proceeding remember to observe proper anti-static precautions
when operating inside the case of your PC. If you do not have a anti-static
strap, rub your hands on the metal portion of the case to get rid of static.