Understanding the BIOS and Its Main Functions

Understanding the BIOS and its main functions starts in the short gap between pressing the power button and seeing your operating system. The BIOS (Basic Input/Output System) checks the connected hardware components during POST, then chooses a boot option and starts the operating system loader.

This is why the BIOS can report a missing part or change where the computer starts from even when the operating system will not load.

What Is BIOS?

BIOS stands for Basic Input/Output System. It is firmware stored on a memory chip on your motherboard.

BIOS is not an app saved with your files on a hard drive or SSD. It can run before a drive has an operating system. Modern UEFI firmware controls the PC boot process and then passes control to Windows or another operating system.

What Is System BIOS?

System BIOS means the firmware that serves the computer as a whole. This separates it from device firmware, which controls one part such as a storage drive or network adapter. Microsoft's UEFI update guide makes the same system and device firmware distinction.

You may see “System BIOS” in an update list or support tool. The label identifies firmware for the full computer. It does not mean a separate type of BIOS or a setting inside Windows.

How Does BIOS Work, and What Are Its Main Functions?

Some BIOS functions run by themselves. Others wait for you to open the setup utility and choose a setting.

POST (Power-On Self-Test)

BIOS starts with POST, or Power-On Self-Test. This series of internal checks looks for a hardware problem that could stop the computer from starting:

CPU: The processor that runs every instruction.
RAM: Fast temporary memory used by active programs.
Storage: Hard drives and SSDs that hold the operating system.
Graphics: The integrated or dedicated GPU that drives your display.

If POST finds a problem, startup may stop and show an error. When the display cannot show the error, some computers use beep codes that help identify the fault.

POST does not need an operating system because it is part of the firmware startup process. This lets it report a hardware failure before Windows or another system begins to load.

A motherboard cycling through its POST diagnostic codes at power-on, before the BIOS hands off to the operating system.

Bootstrap Loader

After POST passes, the firmware follows its boot order and starts a program that can load the operating system. This program is called a bootstrap loader.

On a UEFI computer, the firmware boot manager starts an operating system boot application from the selected device. Windows then takes over and shows its sign-in screen, which you can skip by setting up auto login on Windows 11.

BIOS Setup Utility

The BIOS includes a settings menu. You can usually open it by pressing a maker-specific key during startup. Microsoft's list of common firmware keys includes Esc, Delete, F1, F2, F10, F11, and F12, but you should check the manual for your computer.

An MSI motherboard BIOS guide shows the types of settings you may find:

Boot order: Pick which drive loads first, which is helpful when installing Windows from a USB drive.
Clock speeds: Adjust how fast the CPU or RAM runs on supported boards.
Voltage settings: Give parts more or less power, usually for overclocking or undervolting.
Device toggles: Enable or disable items like onboard audio, integrated graphics, or specific ports.

The names and available controls depend on the motherboard. Use the guide for your exact model, and change only settings you understand.

CMOS Settings Storage

When you change a BIOS setting, it must stay saved after shutdown. Intel defines CMOS as a small amount of motherboard memory that stores BIOS settings.

A small coin-cell battery helps the board keep those settings and its clock when the computer has no power. A weak battery can cause the time or BIOS settings to reset.

Hardware Management

The operating system cannot take control until the hardware needed for startup is ready. Microsoft's UEFI requirements say firmware must prepare the hardware and make the boot device available before it starts the operating system loader.

This early support lets the computer use the devices needed to finish booting. After the handoff, operating system drivers take over normal hardware control.

Security Features

A modern BIOS includes several ways to protect the computer at the firmware level:

Password protection: A system password can stop startup, while an administrator password can protect BIOS settings.
Secure Boot: Checks the operating system loader's digital signature before letting it run.
TPM: Records startup measurements and protects cryptographic keys used by security tools.
Drive encryption support: BitLocker can ask the TPM to unlock a protected Windows drive during startup.

What Does the BIOS Screen Look Like, and What Information Does It Show?

Once the BIOS screen opens, it usually separates system information from settings you can change. The names, colors, and layout depend on the computer or motherboard. MSI's BIOS guide notes that boards with the same chipset can still have different screens and functions.

A page called Overview, Main, or System Information often shows read-only details. Depending on the model, you may see:

Firmware details: The BIOS or UEFI version and its release date.
Detected hardware: The CPU model, installed RAM, storage drives, and graphics hardware listed under system information.
Startup details: The current firmware mode and available boot devices.
Health readings: Temperatures, fan speeds, and voltages on boards that include a hardware monitor.

These details can show whether the firmware detects a part before the operating system loads. If a value is missing or looks wrong, check the BIOS guide for your exact model before changing a setting.

Overclocking Options

Want your CPU or graphics card to run faster than its default settings? Some motherboards include BIOS controls for overclocking. Intel notes that available CPU and RAM controls depend on motherboard support.

Extra speed is not guaranteed. If the computer becomes unstable after overclocking, restore the default values. This is part of AMD's stability guidance.

Legacy BIOS vs UEFI

UEFI, or Unified Extensible Firmware Interface, replaced the older BIOS firmware interface. Both can start a computer, but UEFI supports newer boot and security features.

Feature	Legacy BIOS	UEFI
Boot speed	Slower startup	Faster startup
Drive size	Up to 2 TB only	Supports drives larger than 2 TB with GPT
Security	No UEFI Secure Boot	Can support Secure Boot

Do not switch firmware modes without checking how the operating system was installed. Windows normally keeps using the same firmware mode used during installation.

Common BIOS Vendors

Computer and motherboard makers often build their firmware from a platform supplied by a BIOS vendor. They then adapt it for their own hardware and support tools.

American Megatrends (AMI): Develops Aptio BIOS and UEFI firmware for client devices, servers, and built-in systems.
Insyde Software: Develops InsydeH2O UEFI firmware for PCs, servers, and built-in systems.
OEM-customized firmware: Computer makers can add controls and branding for a specific product line.

For instructions or updates, use the support page for your exact computer or motherboard model. Dell's BIOS update guide, for example, uses the computer's service tag to find a compatible file.

Updating Your BIOS (Flashing)

Install a BIOS update when the maker lists a fix or feature that your computer needs. Check the BIOS release notes for changes such as:

New CPU support: Adds compatibility for processors released after the board.
Bug and stability fixes: Resolves boot, memory, or device-detection issues.
New features: Adds Secure Boot improvements, Resizable BAR, or extra setup options.

Be careful: A failed update can stop the computer from starting. Follow the maker's update steps, keep the computer connected to reliable power, and save your current BIOS settings first.

Common BIOS Problems and Fixes

A startup symptom can point to saved settings, boot order, firmware damage, or a compatibility problem. Match the symptom before choosing a fix.

Dead CMOS Battery

A weak CMOS battery can make the clock or BIOS settings reset after the computer loses power. Intel lists both signs in its board battery guidance.

Corrupted BIOS

A failed firmware update can leave the computer unable to start. Some models provide a maker-specific recovery tool, such as Dell BIOS Recovery.

Wrong Boot Order

A “No bootable device” message can appear when the firmware does not find a working loader on the selected device. Open BIOS setup, confirm that the system drive is detected, and check its place in the firmware boot order.

New Hardware Not Working

If a new CPU or RAM kit stops the computer from starting, first check the motherboard's support list. Some boards need a newer BIOS before they can use a later CPU. MSI includes CPU compatibility in its no-display checks. If the support list and BIOS version are correct, follow the upgrade no-boot checklist to check the power connections, RAM, graphics card, and motherboard debug lights.

Before changing a BIOS setting, record its original value. Change one item at a time so you know which change caused the result.