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.