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Page 1 of 4 Introduction
Have you ever asked yourself how many steps does your
computer perform from the moment you press the power switch to turn it on till
the moment your favorite desktop wallpaper shines it its full glory in front of
you? Well, there are many steps that the computer performs in order you to be
able to enjoy your powerful hardware and your slick software but when a step or
two fails to execute and you can't access the contents of your computer, you
become more and more interested to know what exactly happens when you boot your
computer and what could have potentially gone wrong that turns your ultra high-tech
computer into a useless piece of metal (and some plastic, to be more precise).
Besides that it is interesting to know what exactly
happens when you boot your computer, being familiar with the basic steps that
are executed during system boot can help you troubleshoot problems on your own,
rather than be dependent on Technical Support for minor issues, like an
unplugged power supply cable, for example.
If you are in Technical Support, then certainly it is a
must to know the boot sequence of the type of machines you are supporting.
Well, if you are in Technical Support, you definitely need to know more about
booting than what is presented in this article but I believe that even
experienced Technical Support maniacs might have something to learn from the next
chapters.
Although there are slight differences between the way
different systems boot, generally the process can be divided into two – booting
the hardware and starting the operating system. Basically, all PCs follow the
same routine and if there are differences (mainly in the BIOS-related steps),
you may want to check your vendors documentation before you take any steps to
troubleshoot it further. Differences in the way the operating system boots are
also possible and I have mentioned them occasionally. Also, though it is hardly
possible to make an exhaustive list of all possible problems and their
solutions for each of the stages, I have tried to suggest some of the reasons
for things to go wrong and the ways to fix the problem.
Turning the
Machinery On
Briefly, the hardware part of the boot sequence can be
described like that: The boot process starts with pressing the power button to
turn the computer on. Then, after a short self-test of the power supply, a
signal is sent to the processor and it starts executing the ROM BIOS code. Then
the ROM BIOS performs a short test of the available hardware and if everything
is OK, the BIOS starts reading the configuration information in the CMOS
(Complimentary Metal-Oxide Semiconductor) – i.e. where to start the operating
system from (from a floppy, CD or the harddisk). In case the operating system
is to be loaded from a harddisk, the BIOS locates its Master Boot Record (MBR)
and loads it into memory. Then the partition loader (also called Boot Loader)
takes charge and reads the partition table to find the active partition and a
boot record there. After that the operating system starts booting. After that
brief explanation of the hardware part of the boot process, let's look in more
detail at each of the steps:
- Switching
on the power. When the power supply is switched on, its first task is
to perform a self-test to ensure that power is stable - all voltages and
current levels are normal. The self-test takes less than a half second and
if you didn't know about it, you would certainly never notice it, unless
the computer freezes at this point. If the power supply does not pass the
power-self test, this means that either the power supply unit is faulty,
or that the voltage and current levels are not normal. So, when you turn
on your computer and it just does not make any noise at all, one possible
reason is a problem with the power supply. However, this does not
necessarily mean that the power supply has gone off – a more prosaic
reason can be an unplugged power supply cable.
- Here
comes the CPU. Before the CPU receives the signal and the power is
stable, the processor receives continuous reset signals and just waits.
After the power supply has made the power self-test, it sends a signal to
the processor that the power is OK. The CPU starts operations and the
first thing it does is to look in the BIOS ROM for the start of the BIOS
boot program. Generally, the start of the BIOS boot program resides right
at the end of system memory and usually it is only 16 bytes from the top
of ROM memory. Of course, 16 bytes are quite insufficient for the program
itself to reside there but they are absolutely enough for a JMP (jump)
instruction, which tells the processor the actual address of the ROM BIOS
code.
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