BIOS

...Understanding BIOS...
BIOS of your computer is always present with your system.You can say it's omnipresent.Basic Input Output System is something like without which your computer can't live.The definitions of the computer are given by the BIOS only.It is the thing which knows how to keep your system in line.It holds the most of the secrets of your computer.A lot of things are to be tested when the system turns on.This job is done by the BIOS itself(..behind the screens..).Windows always tries its best to lock it up,but BIOS is smart enough to work in backgrounds as well..
The BIOS is, in fact, the one essential constituent that distinguishes one computer from another, even when they both share the same microprocessor, motherboard, and support hardware.
...Now comes the point of FIRMWARE...
Strictly speaking, however, the BIOS isn't hardware at all, even though it is an essential part of your computer's hardware. The BIOS is special program code that's permanently encapsulated in ROM chips or, as is most often the case with newer computers, Flash memory. Because of the two-sided aspects of the BIOS, existing between hardware and software, it and other pieces of program code encapsulated in ROM or Flash memory are often termed "Firmware".
The importance of the BIOS arises from its function. The BIOS tests your computer every time you turn it on. It may even allocate your system's resources for you automatically, making all the adjustments necessary to accommodate new hardware. It also determines the compatibility of your computer with both hardware and software and can even determine how flexible your computer is in setup and use.
If some computer-maker wanted to, it could put the entire operating system and application software into the Firmware. Some portable computers actually came close... Early Hewlett-Packard Omnibook,sub-notebook,computers packed Windows and part of Microsoft Office into their ROMs. Of course, these machines are not very useful any more.The storing capacity of the required software would fill more chips than u could carry,of course,comfortably.As we know,ROM chips have few kilobytes of memory to store.It can't store a huge software of many Gigabytes...
...Functions...
BIOS has various operations to perform.It has routines that allows the computer software to take the responsibility of its hardware.Also it checks,which new expansion boards have been installed.It also ensures that the newly installed hardware may not interfere with the previously installed peripherals and hardware components.
First of all,what it does is,instructs the microprocessor to check the previously installed hardware components whether they are functioning properly or not.If they are,it then instructs the processor to check the newly installed hardware.
When the microprocessor runs out of add-in peripherals, it begins the actual bootup process, which is the Initial Program Load (IPL). The BIOS code tells the microprocessor to jump to a section of code that tells the chip how to read the first sector of your floppy or hard disk. Program code then takes over from the BIOS and tells the microprocessor how to load the operating system from the disk to start the computer running.
With Windows, however, the BIOS gets pushed out of the way. After the BIOS has assured the operating integrity of your system, Windows takes over. First, the operating system starts loading its own boot code. Then, it installs drivers that take over the various interface functions of the BIOS, one by one.
The operating system replaces the BIOS interface routines for several reasons. Because software drivers load into RAM, they are not limited in the amount of space available for their code. Software drivers also extend the capabilities, whereas the BIOS limits them. Using only the BIOS, your computer cannot do anything that the BIOS does not know about. It enables you to use the wide variety of peripherals you're apt to connect to your computer. Moreover, because the BIOS is designed to run at bootup when your computer is in real mode, it uses only real-mode code, with all the limitations that implies (in particular, a total address space of 1MB). Software drivers can (and nowadays invariably do) run in protected mode. Not only can they access more memory, but protected mode drivers can be written in 32-bit code that executes faster on modern microprocessors.
The BIOS starts to work as soon as you switch your system on. When all modern Intel microprocessors start to work, they immediately set themselves up in real mode and look at a special memory location that is exactly 16 bytes short of the top of the 1MB real-mode addressing range—absolute address 0FFFF0(hex). This location holds a special program instruction, a jump that points to another address where the BIOS code actually begins.
...Power-On Self Test...
POST(Power-On Self Test) comes into play as soon as the system starts.It checks whether everything is functioning well or not.
...Error Codes...The BIOS tests are relatively simple. The BIOS sends data to a port or register and then looks to see the results. If it receives expected results, the BIOS assumes all is well. If it finds a problem, however, it reports the failure. If the display system is working, it posts an error-code number on your monitor screen.If your computer is so ill that the display system will not even work, the BIOS sends out a coded series of beeps through your system's loudspeaker.This doesn't mean that your system is not working well.Displaying the code on the screen proves that at least one of the components is working well...These errors are palyed via sounds.
...Initial Program Load...
After all the testings,it sends the control to the operating system which in turn gives life to the system.This transfer is called Initial Program Load.
...Storage Functions...
Because the BIOS comes in the physical form of ROM chips, its storage functions should not be surprising. For example, the BIOS stores its own name and the date it was written inside its own code. But the BIOS also incorporates storage functions that go beyond the bytes encapsulated in its ROM silicon. Although physically separate from the BIOS chips themselves, the setup memory of your computer is controlled by BIOS functions and is often considered with—and even as part of—the BIOS. This memory records vital details about your computer hardware so that you don't have to set up each expansion board and disk drive every time you switch on your computer. In addition, the BIOS tells your computer to reserve several small blocks of memory for dynamic data that your computer and its operating system use for tracking several system functions.

...CMOS...
When the world and computers were young, all the differences between computers could be coded by one or two banks of DIP switches. But as the options began to pile up, the switch proved to be more a problem than a panacea. A reasonable number of switches couldn't allow for the number of options possible in a modern computer. Another problem with switches is that they are prone to mechanical problems—both of their own making and otherwise. Switch contacts naturally go bad, and they can be helped along the path of their own destruction by people who attempt to adjust them with pencils (the graphite that scrapes off the point is conductive and can short out the switches). People often set switches wrong and wonder what is awry.
...Resetting CMOS...
The chief reason you're advised not to tinker with the advanced settings of your computer's BIOS is that the control they afford allows you to set your computer up so that it does not work. For example, if you err by setting too few wait states for the memory you have installed in your computer, you may encounter a memory error as soon as your computer switches on and checks its BIOS settings. You won't have an opportunity to jump back into setup to fix the problem. You'll be stuck with a dead computer and have no way of bringing it back to life.
Well, not quite. Most computer BIOSs have factory defaults that are set conservatively enough that the machine will operate with whatever you've installed. The only trick is to restore the factory defaults.
...Switch or Jumper Reset...
Many computers have jumpers or a DIP switch that forces the reset. Check the documentation of your computer or motherboard to see if this option is available to you. Typically you'll find this information using the index of your instruction manual and looking under "factory defaults" or "BIOS defaults."
The exact procedure usually takes one of two forms, depending on the design of your computer. The easiest requires only that you move a jumper or slide a DIP switch and then turn your computer back on. The position of the switch or jumper doesn't matter. All that counts is that you move it. The most complicated procedure isn't much more difficult. You move the jumper or slide the switch, wait a few minutes, and then move it back. The delay allows the tiny amount of power that's locked in the CMOS circuitry to drain away.

...Power Deprivation...
The alternate procedure for resetting your CMOS works for nearly every computer and is based on the same power-depravation principle. You only need to deprive your computer's CMOS of its battery power so that the contents of its memory evaporate. Exactly how to deprive your CMOS of its lifeblood electricity depends on the design of your system.
If your computer's motherboard uses an external battery, simply unplug it from the motherboard. If your computer uses a replaceable disc-style battery in a matching battery holder, pop the battery out. In either case, allow ten minutes for the residual power in the CMOS to drain away before reconnecting or reinstalling the battery.
Some motherboards use permanently installed rechargeable nickel-cadmium batteries instead of replaceable cells. Usually these computers have some provision for electrically disconnecting the battery power from the CMOS. A few computer motherboards make no provision for disconnecting their nickel-cadmium batteries (also known as ni-cads). If you have such a computer and have put your BIOS in a nonfunctional configuration, as a last resort you can sometimes force a reset to the factory defaults by discharging the battery. The battery will recharge the next time you operate your computer.
Never short out a nickel-cadmium battery to discharge it. The low resistance of ni-cad cells produces high currents (even with small batteries), sufficient to melt circuit board traces and even set your computer on fire. Instead of shorting out the battery, discharge it through a resistor. A half-watt, 39-ohm resistor will safely discharge a ni-cad cell in about half an hour without danger to you or your computer. Alternately, you can use a six-volt lamp, such as one designed for battery-powered lanterns, as a battery load. The lamp will show you the progress of the discharge, glowing brightly at first and dimming as the battery's charge gets drained away. Connect either the resistor or the lamp directly between the terminals of the battery using clip leads.
...Bus Clock...
Many ISA systems allow you to adjust the clock speed of their expansion buses so that you can eke the most performance from your old expansion boards. Some systems give you a choice of clock speeds in megahertz; others express the speed in terms of the microprocessor clock (for example, CLOCKIN/4 implies one-quarter the microprocessor clock speed—8.25MHz with a 33MHz system). Higher speeds (lower divisors) can deliver more performance, but rates above about 12MHz may sacrifice reliability. In VL Bus and PCI systems, the bus clock setting has less effect on overall performance because the devices that need the most bus bandwidth (that is, a faster bus speed) will be connected to the higher-speed local bus. The clock speed of the local bus is not affected by the bus clock setting.
That's all for this section.Being somewhat brief,most of the essential components have been covered.