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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: 1. 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. 2. 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.
3. The BIOS POST is next. One of the first operations that are performed by the BIOS is the power-on self test (POST). The purpose of the POST is to determine if there are any fatal errors that prevent the proper booting or operation of the computer. Since at this time the video adapter is not started yet, all alarms about fatal errors are communicated in beeps. These beeps vary from manufacturer to manufacturer and their meanings are looked up in the vendors documentation, so if you hear your computer scream, try to distinguish the signals, look up their meaning in the docs and see if you can troubleshoot it on your own. 4. Looking for the video adapter. If the power-on self test is passed without errors up to here, the BIOS starts looking for adapters that also need to load their ROM BIOS program in order to be initialized. After the video adapter has been initialized, you will see on screen all other messages about failed hardware. Actually initializing the video adapter takes place after the video test, which is also part of the POST, has been completed successfully, so maybe it is more precise to say that the video adapter is loaded in the middle of the POST.
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