PC Technicians - Slaithwaite Adult Education

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Often I will offer some leading questions to assist your revision - EG

What does BIOS stand for?
What does POST stand for?
What is one difference between a Quick POST and Normal POST?
What is the MBR?
Name three operating system configuration files?
What are POST codes? Why are they needed?
Are POST codes always beeps?
Are POST codes the same for every motherboard?
What is a Shell?
What is a Kernel?
What is GUI? Is it a Shell?
Where is the BIOS stored?
What is CMOS?
What are BIOS defaults?
Can you think how faulty memory would effect the boot process?
What is a silent boot in the BIOS set-up?
What are four common causes of startup problems. How might you determine the fault in each?

When the PC is switched on the CPU is held at reset until the power supply unit (PSU) provides a power good signal. This is to prevent the CPU performing millions of instructions while the PSU reaches full voltage in the first fractions of a second.

The CPU decompresses the code stored in the BIOS CMOS chip and the CPU begins processing. One of the first tasks the BIOS program performs is to signal various busses on the motherboard testing for correctly connected components including memory, keyboard, PCI cards and video card. This process is known as POST or Power On Self Test. As the video card is not yet initialised the motherboard reports errors using a series of beeps or coded lights (LEDs).

Most BIOSs have an option to Quick POST. In this mode only simple tests are performed and the POST can miss things such as faulty memory addresses.

After the video card's BIOS is started and the display shows the video BIOS revision on the screen, the system BIOS performs a memory scan (depending on either Quick POST or Standard POST), initialises the IDE busses and PCI busses and then begins searching the first boot device for a Master Boot Record (MBR) to hand over the boot sequence to an installed operating system. This stage is known as the bootstrap. This is could be in the sequence A: C: CDROM but can be changed within the BIOS setup and usually looks to the primary hard disk partition.

The MBR is a small area of code at the beginning of a drive with instructions about the drives file system, partitioning information and operating system (OS) kernel location. The MBR is a prime target for virus attacks (boot sector virus) as it is critical for the startup of a PC.

Using the following Fdisk command in DOS:

A:\>fdisk   /mbr

will restore a default MBR, erasing the virus. If you have a multi-boot PC it will also remove any boot loader information - so be warned ;o)

This link on the MBR is an excellent overview of the common errors associated with corruption in this area and steps to resolve them.

The OS kernel is processed and established in system memory, followed by the OS shell. The shell is the user environment and commands - Typical shells are:

DOS command line
Windows Graphical User Interface (GUI)
Linux command line (BASH -Bourne Again SHell)
Linux GUI

The next stage is the user/system configuration files that can be modified to alter the start-up environments. Examples of these include MSDOS.SYS, AUTOEXEC.BAT, CONFIG.SYS, WIN.INI in older operating systems, largely replaced by the registry in XP. Then come all the device drivers needed to make the hardware work with the operating system , shared program libraries (DLLs) and finally the individual user profiles and any programs instructed to commence at start-up by the user.

Tools for solving start-up problems

Pressing the F8 key in DOS before the MBR loads the kernel in a Windows operating systems provides selective start-up options, including safe modes, debugging modes and in XP - Last known working condition.

A brute force approach to fixing start up problems in XP is done with the XP CD and repairing an installation.

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Tools for solving start-up problems

Further reading