BIOS (II)

The size of the BIOS, and the capacities of the ROM, EEPROM and other media it may be stored on, has increased over time as new features have been added to the code; BIOS versions now exist with sizes up to 8 megabytes. Some modern motherboards are including even bigger NAND Flash ROM ICs on board which are capable of storing whole compact operating system distribution like some Linux distributions. For example, some recent ASUS motherboards including SplashTop Linux embedded into their NAND Flash ROM ICs.

EEPROM chips are advantageous because they can be easily updated by the user; hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, this advantage had the risk that an improperly executed or aborted BIOS update could render the computer or device unusable. To avoid these situations, more recent BIOSes use a "boot block"; a portion of the BIOS which runs first and must be updated separately. This code verifies if the rest of the BIOS is intact (using hash checksums or other methods) before transferring control to it. If the boot block detects any corruption in the main BIOS, it will typically warn the user that a recovery process must be initiated by booting from removable media (floppy, CD or USB memory) so the user can try flashing the BIOS again. Some motherboards have a backup BIOS (sometimes referred to as DualBIOS boards) to recover from BIOS corruptions. In 2007, Gigabyte began offering motherboards with a QuadBIOS recovery feature

A computer system can contain several BIOS firmware chips. The motherboard BIOS typically contains code to access fundamental hardware components such as the keyboard, floppy drives, ATA (IDE) hard disk controllers, USB human interface devices, and storage devices. In addition, plug-in adapter cards such as SCSI, RAID, Network interface cards, and video boards often include their own BIOS, complementing or replacing the system BIOS code for the given component.

In some devices that can be used by add-in adapters and actually directly integrated on the motherboard, the add-in ROM may also be stored as separate code on the main BIOS flash chip. It may then be possible to upgrade this "add-in" BIOS (sometimes called an option ROM) separately from the main BIOS code.

Add-in cards usually only require such an add-in BIOS if they:

Need to be used prior to the time that the operating system loads (e.g. they may be used as part of the process which loads (bootstraps) the operating system), and:

Are not sufficiently simple, or generic in operation to be handled by the main BIOS directly

PC operating systems such as DOS, including all DOS-based versions of MS Windows, as well as bootloaders, may continue to make use of the BIOS to handle input and output. However, other modern operating systems will interact with hardware devices directly by using their own device drivers to directly access the hardware. Occasionally these add-in BIOSs are still called by these operating systems, in order to carry out specific tasks such as preliminary device initialization.

To find these memory mapped expansion ROMs during the boot process, PC BIOS implementations scan real memory from 0xC0000 to 0xF0000 on 2 kibibyte boundaries looking for the ROM signature bytes of 55h followed by AAh (0xAA55). For a valid expansion ROM, its signature is immediately followed by a single byte indicating the number of 512-byte blocks it occupies in real memory. The BIOS then jumps to the offset located immediately after this size byte; at which point the expansion ROM code takes over, using the BIOS services to register interrupt vectors for use by post-boot applications and provide a user configuration interface, or display diagnostic information.

There are many methods and utilities for dumping the contents of various motherboard BIOS and expansion ROMs. Under a Microsoft OS, DEBUG can be used to examine 64 KiB segments of memory and save the contents to a file. For UNIX systems the dd command can be used by a user with root privileges: "dd if=/dev/mem bs=1k skip=768 count=256 2>/dev/null | strings -n 8".

Data and source mostly taken from Wikipedia.