HANDS ON GUIDE

The wake-up routine of a BIOS

Asus' Alex Perng and Gigabyte's Thomas Chang explain exactly what a BIOS does when you wake it up with a nice dose of electricity from the PSU.

Perng: First, the CPU goes to a specific memory position (address 4G - 10h) to fetch its first instruction.

Chang: The BIOS runs from an EPROM on the motherboard and initialises several key components, including the motherboard's clock generator, processors and caches, and chipset. For an Intel system, this entails both the memory controller and I/O controller.

The BIOS initialises the CPU, chipsets and various other devices. After that, the CPU is sent codes by the BIOS, telling it to initialise the RAM. When the memory is alive, the BIOS itself asks the CPU to move a BIOS image from the flash memory on the board into the RAM, where it can then run its codes from memory. Once this is done, the BIOS will then allocate resources such as memory, input/output, DMA and IRQs for the various components that need them.

This all happens before the first beep - the process is called Power On Self Test (POST).

Perng: The BIOS will also perform some calculations and memory shuffling to create an environment that meets industrial specifications. When all the dirty jobs are done, the BIOS will transfer control to the OS loader and let the OS take ownership of all systems.

Chang: After system bootup, the BIOS is responsible for reporting hardware configurations to the operating system, and taking care of power management (such as S3, also known as 'Save to RAM suspend' under Windows XP). The BIOS is also responsible for controlling the runtime interface for hardware such as the keyboard and hard drive under legacy operating systems such as DOS.

The Chinese Wall BIOS

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