26 Facts About BIOS

The BIOS firmware comes pre-installed on an IBM PC or IBM PC compatible's system board and exists in UEFI-based systems too.

Originally, BIOS firmware was stored in a ROM chip on the PC motherboard.

In later computer systems, the BIOS contents are stored on flash memory so it can be rewritten without removing the chip from the motherboard.

BIOS uses the boot devices set in Nonvolatile BIOS memory, or, in the earliest PCs, DIP switches.

At boot time, all BIOS services are available, and the memory below address 0x00400 contains the interrupt vector table.

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Motherboard BIOS typically contains code for initializing and bootstrapping integrated display and integrated storage.

Once an option ROM returns, the BIOS continues searching for more option ROMs, calling each as it is found, until the entire option ROM area in the memory space has been scanned.

BIOS ROM is customized to the particular manufacturer's hardware, allowing low-level services to be provided in a standardized way to programs, including operating systems.

Since the AT-compatible BIOS ran in Intel real mode, operating systems that ran in protected mode on 286 and later processors required hardware device drivers compatible with protected mode operation to replace BIOS services.

In modern PCs running modern operating systems the BIOS interrupt calls is used only during booting and initial loading of operating systems.

The BIOS contain patches to the processor microcode that fix errors in the initial processor microcode; microcode is loaded into processor's SRAM so reprogramming is not persistent, thus loading of microcode updates is performed each time the system is powered up.

Some BIOS implementations allow overclocking, an action in which the CPU is adjusted to a higher clock rate than its manufacturer rating for guaranteed capability.

BIOS interrupt calls are not used by modern multitasking operating systems after they initially load.

Since 2010, BIOS technology is in a transitional process toward UEFI.

Early BIOS versions did not have passwords or boot-device selection options.

The BIOS was hard-coded to boot from the first floppy drive, or, if that failed, the first hard disk.

In modern PCs the BIOS is stored in rewritable EEPROM or NOR flash memory, allowing the contents to be replaced and modified.

Original IBM PC BIOS was stored on mask-programmed read-only memory chips in sockets on the motherboard.

The effect of the publication of the BIOS listings is that anyone can see exactly what a definitive BIOS does and how it does it.

In May 1984 Phoenix Software Associates released its first ROM-BIOS, which enabled OEMs to build essentially fully compatible clones without having to reverse-engineer the IBM PC BIOS themselves, as Compaq had done for the Portable, helping fuel the growth in the PC-compatibles industry and sales of non-IBM versions of DOS.

New standards grafted onto the BIOS are usually without complete public documentation or any BIOS listings.

Former major BIOS vendors included American Megatrends, Insyde Software, Phoenix Technologies, Byosoft, Award Software, and Microid Research.

BIOS Meningitis has relatively harmless, compared to a virus like CIH.

Second BIOS virus was CIH, known as the "Chernobyl Virus", which was able to erase flash ROM BIOS content on compatible chipsets.

Third BIOS virus was a technique presented by John Heasman, principal security consultant for UK-based Next-Generation Security Software.

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Researchers Anibal Sacco and Alfredo Ortega, from Core Security Technologies, demonstrated how to insert malicious code into the decompression routines in the BIOS, allowing for nearly full control of the PC at start-up, even before the operating system is booted.