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BIOS

BIOS is the Acronym for Basic Input/Output System

A legacy firmware stored on a small memory chip on the motherboard that initializes a computer's hardware during the booting process. It acts as the intermediary between the hardware and the operating system, providing the fundamental instructions required to start the computer. 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.

Core Functions

BIOS performs several critical functions during the booting process:

  • POST (Power-On Self-Test): A diagnostic testing sequence that runs immediately after powering on to ensure hardware (CPU, RAM, storage) is functioning correctly.
  • Bootstrap Loader: Locates and hands off control to the operating system's bootloader (found in the Master Boot Record).
  • BIOS Drivers: Low-level drivers used to interface with basic hardware like the keyboard, display, and serial ports.
  • CMOS Setup: A configuration utility that allows users to modify hardware settings, such as system clock, boot order, and voltage.

Key Terminology

BIOS includes several key terms and components:

  • CMOS (Complementary Metal-Oxide-Semiconductor): The physical memory chip that stores the BIOS settings. It is kept alive by the RTC battery when the PC is unplugged.
  • MBR (Master Boot Record): The traditional partitioning scheme used by BIOS, limited to 4 partitions and a maximum disk size of 2.2 TB.
  • Interrupts: Signals that allow hardware to communicate with the CPU. BIOS provides "Software Interrupts" (like INT 13h for disk access) that older operating systems used to perform tasks.
  • Flashing: The process of updating the BIOS firmware. Because the BIOS is stored in "Electrically Erasable Programmable Read-Only Memory" (EEPROM), it must be rewritten using a specific utility.
  • Beep Codes: Audible signals used by the BIOS during POST to communicate hardware failures when the screen cannot yet display an image.

BIOS vs. UEFI

BIOS is considered a legacy system, with modern computers typically using UEFI. Key differences include:

  • Interface: BIOS is text-based (keyboard only), while UEFI is graphical (mouse support).
  • Max Disk Size: BIOS is limited to 2.2 TB, while UEFI supports up to ~9.4 Zettabytes.
  • Boot Speed: BIOS is slower (sequential initialization), while UEFI is faster (parallel initialization).
  • Platform: BIOS is 16-bit, while UEFI is 32-bit or 64-bit.
  • Storage: BIOS uses a firmware chip (ROM/EEPROM), while UEFI uses an EFI System Partition (ESP).

Common Use Cases

BIOS is used for various purposes, including:

  • Overclocking: Adjusting CPU multipliers and voltages (common in older enthusiast systems).
  • Boot Order Management: Prioritizing which drive (HDD, SSD, or USB) the system checks first for an OS.
  • Legacy Hardware Support: Running 16-bit operating systems (like MS-DOS) that require BIOS interrupt calls.
  • Hardware Monitoring: Checking CPU temperatures and fan speeds before the OS loads.

Technical Note

While most modern computers use UEFI, they often include a CSM (Compatibility Support Module) that emulates a BIOS environment to support older operating systems.