BIOS

The Basic Input–Output System, widely known as the BIOS, is a foundational form of firmware embedded within a computer’s motherboard. Its principal functions include the initialisation of hardware during power-on startup and the delivery of essential runtime services to operating systems and application software. Historically, the BIOS has defined the interaction between the system hardware and the software environment, forming a crucial bridge in the booting process.

Origins and Early Development

The term BIOS was introduced in the mid-1970s within early microcomputer operating systems, where it denoted the machine-specific code responsible for interfacing with hardware. As personal computing emerged, the IBM PC adopted a proprietary BIOS, which soon became a de facto standard. Third-party manufacturers engineered compatible versions to support the growing market of PC-compatible systems, contributing to the widespread adoption of the BIOS interface model.
During the era of MS-DOS and related systems, the BIOS played a central role. It offered low-level interrupt services for devices such as keyboards, displays and storage drives, enabling a standard interface accessible to application software. This uniformity helped establish the PC ecosystem as an open and extensible platform.

Firmware Architecture and Storage

Originally, BIOS firmware resided in read-only memory chips fixed to the motherboard. This arrangement provided stable and non-modifying code but limited flexibility. Later developments introduced the use of flash memory, allowing updates to be applied without physical replacement. This enabled manufacturers and users to correct firmware faults, improve compatibility and extend hardware support. However, rewritable firmware introduced new security risks, including the potential installation of stealth rootkits and the danger of irrecoverable system failure if updates were interrupted.
Most BIOS implementations are tailored to specific motherboard models. They must interact closely with system chipsets and peripheral hardware, ensuring correct initialisation and configuration before the operating system takes control.

BIOS and UEFI

As computing evolved, the limitations of legacy BIOS became more apparent, particularly concerning boot speed, security and capacity for modern hardware. The Unified Extensible Firmware Interface (UEFI) was developed to replace the traditional BIOS model. UEFI supports larger disk partitions, improved boot performance and enhanced security features. Many UEFI systems offer legacy BIOS compatibility to support older operating systems and hardware, although this legacy mode has been phased out in recent Intel-based systems since 2020. Modern operating systems increasingly require UEFI-compliant hardware, reflecting the shift towards more advanced and secure firmware standards.

User Interaction and Configuration

Early personal computers provided no interactive setup interface. Configuration options were fixed through physical switches and jumpers, and any hardware errors were communicated through simple screen messages or diagnostic beep codes. Throughout the 1990s, manufacturers incorporated BIOS setup utilities accessible at startup via specific key sequences. These utilities enabled users to alter configuration parameters such as boot order, memory settings and device behaviour.
Some earlier systems stored configuration data in battery-backed RAM and required a separate bootable disk to modify settings. Later systems integrated configuration menus directly into ROM-based utilities, a practice retained in modern BIOS and UEFI environments. Current systems typically store configuration settings in flash memory and provide extensive menus for system tuning, diagnostics and firmware management.

Extension ROMs and Hardware Initialisation

Peripheral devices such as video cards, SCSI controllers and RAID adapters often contain option ROMs: embedded firmware modules that extend or replace BIOS services. These ROMs execute during the boot sequence to initialise their respective hardware and register additional capabilities. For example, an adapter card may add support for booting from attached storage or implement network-based boot protocols.
Option ROMs can modify the system’s boot process significantly, and some early systems employed ROM cartridges to provide additional modules. Although modern systems no longer rely on such cartridges, the principle of firmware extensibility remains central to system expansion and hardware compatibility.
During startup, the system’s processor begins execution at a predefined memory address, where the BIOS code resides. From this point, the firmware performs a sequence of tasks: initial hardware checks, memory configuration, enumeration of devices and selection of a boot device. This process ensures that the operating system is loaded into a stable and fully initialised environment.

Role in Modern Computing

While many functions of the traditional BIOS have been superseded by UEFI and modern operating-system capabilities, the concept of firmware remains vital. Initial hardware testing, secure boot mechanisms, compatibility layers and low-level configuration are still the responsibility of the firmware environment. Legacy BIOS interfaces continue to influence contemporary system architecture, even as newer standards extend functionality, efficiency and security.