BIOS .
The BIOS (an acronym for Basic Input/Output
System and also known as the System BIOS, ROM BIOS or PC BIOS) is a type of
firmware used during the booting process (power-on startup) on IBM PC
compatible computers. The BIOS firmware is built into
personal computers (PCs), and it is the first software they run when powered
on. The name itself originates from the Basic Input/Output System used in the
CP/M operating system in 1975. Originally proprietary to the IBM
PC, the BIOS has been reverse engineered by companies looking to create
compatible systems and the interface of that original system serves as a de
facto standard.
The fundamental purposes of
the BIOS are to initialize and test the system hardware components, and to load
a boot loader or an operating system from a mass memory device. The BIOS
additionally provides an abstraction layer for the hardware, i.e. a consistent
way for application programs and operating systems to interact with the
keyboard, display, and other input/output (I/O) devices. Variations in the
system hardware are hidden by the BIOS from programs that use BIOS services
instead of directly accessing the hardware. MS-DOS (PC DOS), which was the
dominant PC operating system from the early 1980s until the mid 1990s, relied
on BIOS services for disk, keyboard, and text display functions. MS Windows NT,
Linux, and other protected mode operating systems in general ignore the
abstraction layer provided by the BIOS and do not use it after loading, instead
accessing the hardware components directly.
Every BIOS implementation is
specifically designed to work with a particular computer or motherboard model,
by interfacing with various devices that make up the complementary system
chipset. Originally, BIOS firmware was stored in a ROM chip on the PC
motherboard; in modern computer systems, the BIOS contents are stored on flash
memory so it can be rewritten without removing the chip from the motherboard.
This allows easy updates to the BIOS firmware so new features can be added or
bugs can be fixed, but it also creates a possibility for the computer to become
infected with BIOS rootkits.
