How is the BIOS used by a modern OS?

Question

What's the function of the BIOS in a modern OS? Is it still used after booting? And is there some kind of BIOS API?

Answer 1

The BIOS is still the first thing that runs on the just-started CPU and responsible for getting the motherboard hardware turned on, setting basic chipset modes and registers, initializing some hardware, and running the code that loads the kernel.

The BIOS is usually not used once the kernel is loaded, and depends on a 16-bit execution environment as opposed to the 32- or 64-bit protected mode environment that a modern kernel operates in.

The boot loader normally does require the BIOS IO calls to get the kernel into memory. The BIOS is being replaced even in this role by newer boot-time software such as Coreboot to provide faster boot times. EFI will one day replace the traditional BIOS, and hopefully the boot loader, passing control directly to the kernel after loading it from storage.

The discovered hardware configuration, memory range settings, and ACPI metadata tables are probably the only BIOS-based data used by the OS after the kernel is loaded. Any runnable ACPI code is encoded as ACPI Machine Language and is interpreted by the OS.

Any good traditional book on MS-DOS assembly programming will include information on the BIOS programming interface. Check out The Art of ASSEMBLY LANGUAGE PROGRAMMING

Answer 2

I wrote BIOS for notebook computers for several years. The BIOS does a lot of things while the OS is running.

A major task is to inform the OS when many events happen so the OS can look smart (as if it somehow figured these things out on its own). For example, the BIOS tells the OS when: the power button is pressed, batteries are inserted or removed, AC power comes or goes, the system connects to or disconnects from a docking station, hard drives and or certain types of optical drives are inserted or removed.

Most portable computers have features that you can access/control through Fn keys and through OS-level applications provided by the manufacturers. The BIOS responds to these hotkeys and has code to interface with the OS-level apps. Features like controlling screen brightness (which certain OSes want to appear to control) or controlling bling LEDs fall into this category.

Perhaps the most important task of the BIOS is to shut down the system when the power button is held down for more than 4 seconds (to recover from OS hangs!).