Troubleshooting Boot Problems

Diagnosing PC bootup problems can
be the most difficult of tasks for
several reasons. Diagnosis requires
logical reasoning, which in itself
requires that the person doing the
diagnosis understand the system
events that are supposed to occur,
and in which order. Further, many
of the clues that might be available
require specialized diagnostic
equipment that the average user
does not possess. Fortunately, even
without the use of diagnostic tools,
the most common boot-time errors
can be resolved by with a little
knowledge, a few common
components and a bit of patience.
The BIOS (Basic I/O System) is the
very first program run by the PC,
and resides in a ROM chip (Read
Only Memory), which is also called
Non-Volatile memory. The BIOS
program initializes the standard
devices, such as the memory
controller, video controller, IDE
controller and floppy controller.
Using stored parameters, it
initializes the motherboard chipset,
and sets timing parameters. It also
creates an interrupt vector table
and provides a set of services,
accessible through interrupts, that
allow access to the standard I/O
devices. Following is a brief
description of the entire PC boot
process, which can be extremely
helpful in identifying and resolving
problems.
When power is applied to the
motherboard, the reset signal is
turned on to the CPU. This
prevents it from fetching any
instructions until all power supply
voltages are valid and the system is
ready for initialization.
Once the output voltages from the
power supply have stabilized, it will
send a power good signal to the
motherboard. The motherbord
timer receives this signal, and turns
off the reset signal, allowing the
processor to fetch the first
instruction
The first instruction fetched by the
processor is from address
FFFF:0000h, which is generally a
branch instruction to the ROM
BIOS starting address.
The first BIOS routine initializes
and tests the various system
controllers, which are now mostly
embedded into the chipset, and
initializes the video controller. If
there are any errors initializing the
memory or video controllers, an
error will be returned from this
routine and the system will stop.
Generally, there will be some type
of audible error signal emitted
from the PC speaker
The next BIOS routine will then will
look for any additional ROM chips
that may be installed on adapter
cards (such as SCSI controllers
cards), and run their routines.
At this point, a specific location of
memory is tested for a value of
1234h, which indicates that this is
a ‘cold’ start. A cold start means
that this is the first time power has
been applied, while a warm start
means that the reset button has
been hit (or the vulcan death grip
has been applied). If it is a cold
start, the full POST (power-on self
test) routine is run, which performs
additional tests on the
motherboard, memory, video card,
keyboard, floppy, etc. If it is a
warm start, this routine is skipped
Once the full POST routine is
completed successfully, the boot
devices stored in the CMOS area
are checked one at a time for a
valid OS loader program, which
then initializes the operating
system and will most likely return
control to the user.
Each of the tests run during the
POST routine have their own code,
which can be displayed with a POST
card that is inserted into an ISA or
PCI slot. In this manner,
technicians can determine exactly
which routine has failed, and can
determine what, if any, corrective
measures can be taken. Recently,
some motherboard manufacturers
have included an LED on the
motherboard which displays these
codes, making it much easier for
the user to determine the problem
and communicate that to tech
support personnel.
The codes and audible signals vary
by BIOS manufacturer, so it is
necessary to know which BIOS your
motherboard has in order to
diagnose any errors. At the end of
the next page, there are links to
the most common BIOS
manufacturer websites, which
contain downloadable documents
with all of the information
necessary to determine what each
error code means.
The next page contains the actual
troubleshooting process, which is
presented as a sort of ‘cookbook’
approach that outlines the various
steps that can be taken to identify
the cause of the most common
problems seen during the boot
process.
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