BIOS

Overview

The BIOS (Basic Input/Output System) is a firmware program that initialises and manages the hardware components of a computer when it powers on. It is stored on a chip on the motherboard and is the first software to run when the computer starts. The BIOS performs essential checks on hardware, loads the initial system configuration, and prepares the system to boot the operating system. Understanding the role and functions of the BIOS is essential in grasping how a computer starts up and transitions control to the operating system.

Role of the BIOS

• Hardware Initialisation: The BIOS initialises and tests the hardware components (such as CPU, RAM, hard drive, and graphics card) to ensure they are functioning correctly and ready for use.
• Power-On Self-Test (POST): The BIOS performs a self-diagnostic test known as POST to check that the main hardware components are working. If an issue is detected, the BIOS may produce error codes (e.g., beeps) to indicate a problem.
• System Configuration: The BIOS loads system settings and configuration data (e.g., boot order, system clock settings) from a small memory called CMOS (Complementary Metal-Oxide-Semiconductor), which is powered by a battery to retain settings when the computer is off.
• Boot Loader: After completing POST and configuring the system, the BIOS identifies and loads the boot loader, a small program responsible for loading the operating system into memory.

Steps of the BIOS During Computer Startup

1. Power-On: When the computer is turned on, power is supplied to the motherboard, activating the BIOS chip.
2. POST (Power-On Self-Test):

• The BIOS initiates POST, which tests key components like the CPU, RAM, and storage drives to ensure they are functioning.
• If POST detects an error, it may halt the startup and provide error codes (such as a sequence of beeps or displayed messages) to help diagnose the problem.

3. Loading BIOS Configuration from CMOS:

• The BIOS loads system configuration settings stored in the CMOS, such as date, time, and the boot sequence. Users can modify these settings through the BIOS setup utility.

4. Identifying Boot Devices:

• The BIOS checks the boot sequence in the CMOS settings to determine which device to attempt to boot from first (e.g., hard drive, SSD, CD/DVD drive, or USB).
• It locates the bootable drive and finds the boot loader (usually on the primary storage device).

5. Handing Off Control to the Boot Loader:

• Once the boot loader is found, the BIOS hands over control to it. The boot loader then loads the operating system into memory, completing the startup process.
• The BIOS's Role Ends Here: Once the OS is loaded, the BIOS is no longer actively involved. The operating system takes over control of hardware management and system operation.

BIOS vs. UEFI

• BIOS has been largely replaced in modern systems by UEFI (Unified Extensible Firmware Interface), which provides more advanced features like graphical interfaces, support for larger drives, and faster boot times.
• Key Differences:
  • Boot Speed: UEFI is generally faster.
  • Drive Support: UEFI supports larger drives (over 2TB) and more advanced security options.
  • User Interface: UEFI often includes a more user-friendly, graphical interface compared to the text-based BIOS interface.

Examples and Use Cases

BIOS POST Error

• When a computer is powered on, if there is a hardware issue, such as a missing or faulty RAM module, the BIOS will halt startup and emit a series of beeps to indicate the specific error.

BIOS Boot Sequence Configuration

• A user might access the BIOS to change the boot order if they want to boot from a USB drive or CD instead of the primary hard drive.
• This is commonly done when reinstalling the operating system.

Note Summary