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How BIOS Works

written by: D Lamberti•edited by: M.S. Smith•updated: 11/6/2011

If you ever wondered what happens behind the scenes when you start the computer, you will get your answer when you read this article. You will learn of the steps involved in getting your computer to your desktop.

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    What The BIOS Does

    Every time the computer starts it runs the BIOS. BIOS stands for Basic Input/Output System, and can also be referred to as ROM BIOS or the System BIOS, though all three terms refer to the same thing. The BIOS is a bit of code that is programmed in a ROM chip at the factory when the computer was first made. In older computers this was non-programmable. But in newer computers, the ROM chip is now referred to as Flash memory, meaning the programming can be changed with a device that connects to the chip, even while the chip is installed on the motherboard. As of 2011, BIOS systems are being replaced by more complex pieces of hardware called Extensible Firmware Interfaces (EFIs), but the terminology hasn't caught on, and for most computer users BIOS will refer to both the BIOS or EFI.

    The BIOS contains two parts. The actual code that runs the pre-start programs is a part of the BIOS. The second part is a setup program you can go in using Setup on your computer. This is the only part of the BIOS where you can configure settings for the A BIOS Menu computer. This part of memory is stored on what is referred to as CMOS (complementary metal oxide semiconductor). You can access this part of the BIOS by hitting a key or combination of keys when you first start your computer. Some computers have a BIOS that requires the F2 key to be hit while booting the computer. Some computers require a combination of CTL, ALT, ENTER, or some other combination of keys upon startup.

    Either way you do it, once you enter the setup program, you are now able to enter configuration information for such things as specs on your hard drive, CD-ROM, how the boot process goes, setting up your network, virus protection, and many other tasks. Newer computers have a BIOS that was designed to allow for password set up and power management features. This helps control the computer during and after boot up.

    There are several types of BIOS out there including Phoenix, IBM, and American Megatrend, to name a few. Each one is created differently, but with the same purpose of allowing certain functions on your motherboard and in your computer to be changed when necessary.

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    How the BIOS Works

    When the computer first starts, the BIOS copies itself to the upper part of memory where it continues its processing. The BIOS checks the information stored on the 64 bytes part of RAM that is located in the CMOS. Once the CMOS has been checked, the BIOS performs various routines in a specific sequence including:

    • Loading the interrupt handlers and device drivers
    • Initializing registers and power management
    • Performing the power-on self-test (POST)
    • Displaying system settings
    • Determining which devices are bootable
    • Initiating the bootstrap sequence

    The BIOS is also responsible for initializing several motherboard components and peripherals. These include the following:

    • The clock generator
    • The CPU
    • External and internal cache
    • The chipset (this makes up the memory and I/O controller)
    • System memory
    • All PCI devices
    • Graphics controller
    • All mass storage controllers
    • Other I/O controllers such as those that control the keyboard and mouse and any other USB devices

    Once the configuration information for every component is loaded into memory, the boot loader executes, which in turn starts the operating system. When the operating system begins, the BIOS has done its job and no longer functions. At this time the operating system kicks in and begins loading files and programs the system needs to operate.

    The BIOS is not only confined to the motherboard. Every time you buy a device that includes a board, that plugs into a slot, there will be a BIOS programmed on that board. When the computer first starts, the BIOS scans the computer for any boards installed and loads the BIOS from those boards. This way the device configuration will be in memory from the start, allowing the device to work immediately after the operating system completes the loading sequence.

    You can overclock certain models of BIOS chips, which results in your CPU running at a higher clock rate, causing your computer to run faster. Keep in mind that this can cause serious damage to your computer's hardware, and shouldn't be done by most computer users; have a computer expert do it for you instead.

    Although the BIOS stops functioning when the operating system takes over, there is a set of low-level routines that the operating system uses to interface with different hardware devices. These devices include the keyboard, the screen, the serial port, and parallel port. The BIOS does load these settings at boot, but they also provide the settings during normal computer operation.