Basic input-output systems

In computing, BIOS or the Basic Input/Output System is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the booting process (power-on startup). On a computer using BIOS firmware, the firmware comes pre-installed on the computer's motherboard. The name originates from the Basic Input/Output System used in the CP/M operating system in 1975. The BIOS firmware was originally proprietary to the IBM PC; it was reverse engineered by some companies, such as Compaq, Phoenix Technologies, AMI and others, looking to create compatible systems. The interface of that original system serves as a de facto standard. The BIOS in older PCs initializes and tests the system hardware components (power-on self-test or POST for short), and loads a boot loader from a mass storage device which then initializes a kernel. In the era of DOS, the BIOS provided BIOS interrupt calls for the keyboard, display, storage, and other input/output (I/O) devices that standardized an interface to application programs and the operating system. More recent operating systems do not use the BIOS interrupt calls after startup. Most BIOS implementations are specifically designed to work with a particular computer or motherboard model, by interfacing with various devices especially system chipset. 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. This allows easy, end-user 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. Furthermore, a BIOS upgrade that fails could brick the motherboard. Unified Extensible Firmware Interface (UEFI) is a successor to the legacy PC BIOS, aiming to address its technical limitations. UEFI firmware may include legacy BIOS compatibility to maintain compatibility with operating systems and option cards that do not support UEFI native operation. Intel phased out providing support for booting from the BIOS interface from 2020 to 2024 depending on the platform. The last version of Microsoft Windows to officially support running on PCs which use legacy BIOS firmware is Windows 10 as Windows 11 requires a UEFI-compliant system (except for IoT Enterprise editions of Windows 11 since version 24H2).