BIOS and UEFI both start the PC before Windows loads, initialize hardware, and hand control to the operating system. UEFI is the modern replacement for classic BIOS, although many people still casually call the firmware screen “the BIOS.”
This guide applies to Windows PCs. Whether your computer uses true legacy BIOS or modern UEFI depends mostly on its age and firmware configuration.
Does This Affect You?
UEFI matters when installing Windows, enabling Secure Boot, using large drives, checking Windows 11 requirements, or changing boot settings. A modern PC may still run in Legacy or CSM mode even if the hardware supports UEFI.
What Classic BIOS Did
- Classic BIOS dates back decades and runs in a restrictive 16-bit startup environment.
- It works with the older MBR partition style, which limits boot drives to about 2.2 TB and four primary partitions.
- BIOS setup menus are usually keyboard-driven and text-heavy.
- Classic BIOS has no equivalent to Secure Boot for verifying startup software before the operating system loads.
What UEFI Adds
- UEFI supports GPT partitioning, which handles drives larger than 2.2 TB and far more partitions.
- It can boot faster because it initializes hardware and boot paths more efficiently.
- Many UEFI setup screens support mouse input, higher-resolution graphics, and richer firmware tools.
- UEFI enables Secure Boot, which checks trusted signatures during startup to block untrusted boot code.
Check Which Mode Windows Uses
- Press Windows key + R.
- Type
msinfo32and press Enter. - On System Summary, find BIOS Mode.
- If it says UEFI, Windows is running in UEFI mode. If it says Legacy, Windows is using legacy BIOS/CSM mode.
Why the Difference Matters
Windows 11 Requires UEFI Secure Boot Capability
Windows 11 requires firmware that is UEFI and Secure Boot capable. A PC running in Legacy mode may fail compatibility checks even if the hardware can support UEFI.
Do Not Switch Modes Casually
Changing an installed PC from Legacy to UEFI or the reverse can stop Windows from booting if the drive partitioning and boot files do not match the new mode.
CSM Bridges Old and New
Compatibility Support Module, or CSM, lets UEFI firmware behave like old BIOS for older operating systems or hardware. Disabling CSM often moves a PC closer to pure UEFI booting.
Menu Labels Can Be Misleading
Manufacturers may still label the firmware screen “BIOS Setup” even when it is technically UEFI. The System Information BIOS Mode field is the reliable check from inside Windows.
Sources
- Microsoft Learn: UEFI firmware and Windows
- Microsoft Support: Windows 11 requirements
- Microsoft Support: Ways to check if your device meets Windows 11 requirements