Intro
BIOS (pronounced "by-ose") stands for Basic Input/Output System. It is a program that resides on a tiny chip that is integrated into the motherboard. These instructions carry out important tasks and provide the steps in the boot process.
The Process
When you turn on your computer, there's got to be some way to get everything started. Once the processor receives a good power signal, it executes a tiny bit of code in a particular memory address (FFFF:0000) in the BIOS that jumps to the actual memory location where the main BIOS program is stored. The boot process kicks into action. The first task BIOS undertakes is loading information from the CMOS (Complementary Metal Oxide Semiconductor) chip. Many people use the terms BIOS and CMOS interchangeably. They are not the same. The BIOS chip contains a program (Remember a program carries out instructions). The CMOS chip simply contains BIOS settings that are specific to a system, such as the number of hard drives, type of processor, amount of memory, and many other things. These configurations can be altered by the user to reflect changes to his/her computer. For example, if you purchase your own video card, you may have to go into CMOS and disable the built-in video before installing the new card. The BIOS, however, is altered through software. By loading the CMOS, the BIOS knows your system's makeup.
Accessing the CMOS screen varies from system to system and is usually listed in the computer's manual. Another way to find out is to look at the bottom left corner of your screen a few seconds after you power on your computer. A message appears that tells you to press a certain key to enter SETUP. The majority use "DEL", "F1", "ESC", or some other key combination. If you make changes to CMOS, you must save them before exiting. Always double-check your settings before saving them. Wrong settings can cause problems. A small, flat battery, usually made of lithium, maintains these configurations after the computer is turned off. These batteries lasts for years.
After CMOS, interrupt handlers and basic device drivers are loaded. An interrupt handler is software that facilitates communication between the operating system (OS) and the processor. It "gets the processor's attention" when some action occurs. When pressing a key, the keyboard interrupt tells the processor what key was pressed, and that information is sent to the OS. A device driver is another type of software that allows components to be recognized.
Next, if your system has adapter cards, they are checked to see if they contain their own bios.
Then comes the POST (Power On Self Test). During the POST, devices are checked for errors, starting with the video card. The graphics adapter's bios, if present, checks the card's processor and memory. If everything is OK you will see the manufacturer's name and version of the system BIOS, plus the name, model, and amount of memory of the video card at top left of the screen.
From there, the POST checks to see if a cold boot or reboot (also called a warm boot) occurred by looking at the number located at a certain memory address (0000:0472). If the value stored is 123h (memory addresses and values are expressed in hexadecimal, hence the "h" at the end) it's a reboot, otherwise, a cold boot. A cold boot performs a memory test. A reboot skips this test. Other components are then checked for, such as ports, mouse, keyboard, and disk controllers. It then decides which device is the boot device and displays basic information about
your system.
During the POST the BIOS compares its findings to the settings in CMOS. If any errors occur you will hear either some type of beep or see an error message. If all is well, it looks on the boot device for the Master Boot Record (MBR) that tells where the operating system is located. Once found, the BIOS process ends.
Updating BIOS
At some point you may have to update the BIOS to accommodate hardware changes, but generally this is rare. This is often called "flashing the BIOS" because it is stored on flash memory. But if there does come a time when upgrading is necessary, go to the BIOS manufacturer's site, the motherboard's site, or computer's website if it is a name-brand system. Make sure you know the BIOS date and revision. Instructions are usually given that walk you through the process. Follow them carefully.
Two main BIOS makers are Phoenix Technologies and American Megatrends (AMI)