BIOS, which stands for Basic Input/Output System, is a built-in software program stored on a small memory chip on your computer’s motherboard.
It is the very first software that runs when you press the power button on your computer. Its primary job is to bridge the gap between the computer hardware components (like the CPU, hard drive, and RAM) and the operating system (like Windows, macOS, or Linux). Without the BIOS, your computer wouldn’t even know how to look for your operating system to start it up.
The 4 Core Functions of a BIOS
The BIOS performs four critical operations during the startup process:
1. POST (Power-On Self-Test)
Before anything else, the BIOS performs a hardware checkup called the POST. It tests your computer’s hardware to ensure everything is connected and working properly—including the processor, memory modules, graphics card, and storage devices.
Note: If the POST detects a critical error (like missing RAM or a failing video card), the computer will halt and emit a series of coded audio beeps or show an error code on the screen to help you troubleshoot.
2. Bootstrap Loader
Once the POST finishes successfully, the BIOS looks for an operating system. It checks the computer’s storage drives based on a pre-configured priority list (e.g., checking the SSD, then a USB drive, then an optical drive). When it finds the OS, it passes control of the computer over to it.
3. BIOS Drivers
The BIOS contains low-level drivers that give the computer basic control over peripheral hardware—such as the keyboard, mouse, and display—before the operating system’s advanced drivers take over.
4. CMOS Setup (BIOS Configuration)
The BIOS provides a built-in configuration interface (often called the BIOS Setup Utility) that allows users to change hardware settings, system time, and security options.
Where is the BIOS stored?
The BIOS is stored on a ROM (Read-Only Memory) chip on the motherboard, specifically a type called EEPROM (Electrically Erasable Programmable ROM). This ensures that the BIOS software is permanent and won’t be wiped out when you turn off the computer.
However, the custom settings you choose in the BIOS (like changing your boot order or setting a hardware password) cannot be saved on a Read-Only chip. Instead, those settings are saved on a separate, tiny chip called the CMOS (Complementary Metal-Oxide-Semiconductor). The CMOS chip requires a constant trickle of power to retain its memory, which is provided by a small, coin-sized CMOS battery (usually a CR2032) located on the motherboard. If this battery dies, your computer will reset its clock and BIOS settings back to factory defaults every time it loses power.
The Evolution: BIOS vs. UEFI
If you have bought a computer in recent years, it likely uses UEFI (Unified Extensible Firmware Interface) instead of the traditional legacy BIOS. UEFI is a modern successor designed to overcome the limitations of the older BIOS framework.
| Feature | Legacy BIOS | Modern UEFI |
| Interface | Text-only (Blue/grey screen, keyboard navigation only) | Graphical User Interface (Supports mouse, animations, and color) |
| Boot Speed | Slower | Faster (optimized initialization) |
| Max Drive Size | $2.2 \text{ TB}$ (uses MBR partition style) | Up to $9.4 \text{ ZB}$ (uses GPT partition style) |
| Security | Minimal security at boot | Secure Boot prevents malware from executing during startup |
| Bit Architecture | 16-bit | 32-bit or 64-bit |
When Do You Need to Access the BIOS?
Most casual users never need to open the BIOS, but it is necessary for advanced tasks such as:
- Changing the Boot Order (e.g., forcing the computer to boot from a USB drive to install a new OS).
- Overclocking the CPU or adjusting RAM speeds (XMP/EXPO profiles).
- Enabling virtualization extensions (Intel VT-x or AMD-V) required to run virtual machines.
- Checking hardware temperatures and fan speeds.
- Updating (Flashing) the BIOS to support newer CPUs or fix system stability bugs.
To access it, you must press a specific key (usually F2, F10, Del, or F12) immediately after turning on the computer, right before the Windows or manufacturer logo appears.