Table of Contents
Introduction
Have you ever asked yourself how many steps your computer performs from the moment you press the power switch to turn it on till the moment your favorite desktop wallpaper shines in its full glory in front of you? Well, there are many steps that the computer performs in order for you to be able to enjoy your powerful hardware and your slick software. 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 to turn your ultra high-tech computer into a useless piece of metal and plastic.
Besides being interested 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 depending 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. Truth be told, 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 can learn something new from this article.
Although there are slight differences between the way different systems boot, generally the process can be divided into two areas — 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 things go wrong and ways to fix the problem.
Turning the Machine On
The following is a detailed step-by-step break down of the hardware side of the boot sequence.
- 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 is dead — a more common 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. Remember, the BIOS is the Basic Input/Output System that prepares the machine to run other software. It is commonly stored on ROM (Read Only Memory), or recently, flash memory that can be updated. 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 is quite insufficient for the program itself to reside, and there is absolutely enough for a JMP (jump) instruction, which tells the processor the actual address of the ROM BIOS code.
- The BIOS POST is next. One of the first operations 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 can be found in the vendor’s documentation. 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.
- Looking for the video adapter. If the power-on self test is passed without errors up to here, it continues by 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.
- POST continued. Besides checking the central hardware and the video adapter, the POST reads the BIOS identification and displays the data on screen. Another portion of the POST is the memory test, which is skipped if it is a warm-startup (warm startup is when you restart the computer, while cold startup is when you have to switch it on). The output of the memory test is displayed on screen — i.e. how much installed memory you have. If you see that the installed memory is less than what you physically have inside the computer box, this could mean that some of your memory might have stopped functioning (i.e. you have 2 blocks of 512MB each, which is 1GB in total but the memory test displays that you have only 512MB, which means that one of the two blocks is not working). Depending on how many blocks of RAM you have, you might be able or you might be not able to continue booting. If you have 2 or more blocks of memory and at least one of them is working, you will be able to go further, though your computer will be slower because of the reduced memory. This memory scenario is an example of a non-critical error. The other less dire reason for missing memory is with a system uses an onboard video card; it often “borrows” RAM from primary memory to use as video RAM.
Like what you read?
If so, please join over 28,000 people who receive our exclusive weekly newsletter and computer tips, and get FREE COPIES of 5 eBooks we created, as our gift to you for subscribing. Just enter your name and email below:
