HARDWARE
BIOS vs UEFIFirmware, boot modes and Secure Boot
UEFI is the modern replacement for the legacy BIOS firmware that starts a PC. UEFI supports disks larger than 2 TB via GPT partitioning, boots faster, has a graphical setup, and adds Secure Boot, which only loads trusted boot code. Windows 11 requires UEFI with Secure Boot and a TPM. Most systems since about 2012 use UEFI, often with a legacy-BIOS compatibility mode.
What firmware does
When you press the power button, firmware on the motherboard initialises the hardware and hands control to the operating-system boot loader. For decades that firmware was the BIOS (Basic Input/Output System). Since the late 2000s it has been replaced by UEFI (Unified Extensible Firmware Interface), though people still call the setup screen "the BIOS".
Key differences
| Legacy BIOS | UEFI | |
|---|---|---|
| Partition scheme | MBR | GPT (usually) |
| Maximum boot disk | 2 TB | Effectively unlimited |
| Partitions | 4 primary (MBR) | 128 (GPT) |
| Setup interface | Text | Graphical, mouse support |
| Boot speed | Slower | Faster, parallel init |
| Secure Boot | No | Yes |
| Addressing | 16-bit real mode | 32/64-bit |
Secure Boot and Windows 11
UEFI adds Secure Boot, which checks that each piece of boot code is signed by a trusted key before running it, blocking bootkits that would otherwise load before the operating system. Windows 11 requires UEFI, Secure Boot and a TPM 2.0 (a hardware security module for keys). A machine that cannot meet these is running legacy BIOS mode or has the features disabled.
GPT versus MBR
UEFI normally boots from a GPT disk; legacy BIOS uses MBR. Converting an MBR system disk to GPT (with Microsoft's mbr2gpt tool) is often the step needed to switch a Windows 10 machine from legacy to UEFI mode before upgrading to Windows 11. A boot-mode or partition mismatch is a common cause of inaccessible boot device after a disk clone or firmware change.
Checking and changing the mode
- Check: run
msinfo32and read BIOS Mode (UEFI or Legacy). - CSM: the Compatibility Support Module in firmware setup lets UEFI boot legacy systems; disabling it is usually required for Secure Boot and native UEFI Windows 11.
- Switching an installed Windows from legacy to UEFI: convert the disk to GPT first (
mbr2gpt /convert), then enable UEFI and Secure Boot in firmware. Do this carefully, with a backup; changing boot mode without converting the disk leaves the system unbootable.
Frequently asked questions
How do I check whether I am using BIOS or UEFI?
Run msinfo32 (System Information) and read the "BIOS Mode" line: it shows "UEFI" or "Legacy". You can also check whether the system disk uses GPT (UEFI) or MBR (legacy) in Disk Management → disk Properties → Volumes → Partition style.
Do I need Secure Boot?
Windows 11 requires it. Secure Boot verifies that boot code is signed by a trusted key, which blocks a class of boot-level malware (bootkits). It occasionally needs to be adjusted for some Linux distributions or older hardware, but for a standard Windows PC it should be on.
What is CSM?
The Compatibility Support Module lets a UEFI firmware boot legacy BIOS-style (MBR) operating systems. It must usually be disabled to enable Secure Boot and to boot Windows 11 in native UEFI mode.