UK What separates UEFI from BIOS
UEFI vs BIOS is the comparison between the two kinds of firmware that start a PC: the legacy BIOS (Basic Input/Output System) that shipped with the IBM PC in 1981, and the UEFI (Unified Extensible Firmware Interface) specification that replaced it. Both test the hardware at power-on and hand control to an operating system, but they do it in very different ways. Our guide to what UEFI is and how it works covers the newer standard on its own.
The BIOS was designed for a 16-bit processor with 1 MB of address space, and kept that design for 30 years as PCs grew around it. UEFI started as Intel’s Extensible Firmware Interface for Itanium servers, and the UEFI Forum has published it as an open industry specification since 2005. PCs moved to it in large numbers around 2011, and Windows 8 was the first Windows release to require it for certified hardware.
Most people still say “BIOS” when they mean the firmware setup screen. On a computer made in the last decade, that screen is almost always UEFI.
How each one boots
The clearest way to see the difference is to follow a start-up step by step.
| Boot step | Legacy BIOS | UEFI |
|---|---|---|
| Processor mode | 16-bit real mode, 1 MB addressable | 32-bit or 64-bit mode from early in start-up |
| Hardware set-up | POST, then option ROMs on expansion cards | Firmware drivers, including UEFI drivers on cards |
| Finding the boot code | Reads sector 0 of each disk in the boot order | Reads named boot entries stored in NVRAM |
| Disk format | MBR, up to about 2 TiB with 512-byte sectors | GPT, with MBR still readable for data |
| Boot code | 446 bytes in the MBR, chaining to a loader | A .efi file on the FAT32 EFI System Partition |
| Checking the code | None | Secure Boot signature checks |
| Hand-over | Loader calls BIOS interrupts until the OS takes over | Boot services end; runtime services stay for the OS |
The firmware setup screen differs too. BIOS setup is a text menu driven by the keyboard, while UEFI setup can be graphical, with mouse support, and can reach the network before any operating system loads.
UEFI vs legacy boot mode
When a firmware menu offers “UEFI or legacy”, it is not choosing between two chips. The firmware is UEFI in both cases. Legacy mode runs a Compatibility Support Module (CSM) that imitates a BIOS, so older operating systems and boot media can start as if the computer had one.
Intel described the move in classes: Class 1 firmware is UEFI that only ever runs CSM, Class 2 offers both, and Class 3 is UEFI with no CSM at all. Intel stopped supporting CSM on its client platforms around 2020, and many current embedded boards are Class 3, so on those boards legacy mode is simply not there. Several of the boards we stock are listed as booting in UEFI mode only.
Why UEFI replaced BIOS
UEFI was adopted because the BIOS had run out of room in several directions at once.
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Larger disks. A BIOS boots from MBR disks, which top out at about 2 TiB with 512-byte sectors. UEFI boots from GPT, which removes that limit; our page on MBR vs GPT partition tables explains the change.
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A trusted boot chain. Secure Boot checks the signature of each bootloader and driver, which a BIOS has no way to do.
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Modern drivers. UEFI drivers run in 32-bit or 64-bit mode and are not squeezed into the small option ROM space a BIOS allows.
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Network and recovery. UEFI firmware can boot over HTTP as well as PXE, and receive capsule firmware updates from the operating system.
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Faster start-up. UEFI can skip initialising devices it does not need, so boot times are often shorter, although this depends on the board.
How to check and switch modes
On Windows, open System Information (msinfo32) and look at BIOS Mode, which reads either UEFI or Legacy. On Linux, the folder /sys/firmware/efi exists only when the system started in UEFI mode.
Moving a Windows 10 or 11 installation from legacy to UEFI takes two steps, in this order. First convert the system disk from MBR to GPT with Microsoft’s MBR2GPT tool, which keeps the data in place. Then restart into the firmware settings, disable CSM or legacy boot, and enable Secure Boot. Switching the firmware without converting the disk leaves a computer that cannot find anything to boot. Take a full image backup before either step.
When legacy BIOS mode is still needed
On industrial and embedded computers, UEFI is the default for anything new. Legacy mode survives only where something older depends on it: a 32-bit Windows 7 or Windows Embedded image, DOS-based configuration or test tools, an older PXE imaging server, or an expansion card whose firmware has no UEFI driver. In those cases the replacement system has to keep CSM, so the board choice depends on it.
Where the software can move, it is usually better to move it. Windows 11 IoT Enterprise needs UEFI and Secure Boot capability, and our guide to upgrading to Windows 11 IoT Enterprise covers the other requirements. When we replace an older system, we check which mode its image needs before choosing the industrial PC to replace it.
Quick answers on UEFI and BIOS
UEFI and BIOS: common questions
Why is UEFI considered better than BIOS?
It removes the BIOS limits. UEFI boots from disks larger than 2 TiB, checks boot code with Secure Boot, runs 64-bit drivers and supports network boot over HTTP.
Do I have UEFI or BIOS?
Check BIOS Mode in System Information. On Windows, msinfo32 shows UEFI or Legacy. Most computers made since around 2012 have UEFI firmware, even if they are set to legacy mode.
Is UEFI faster than BIOS?
Usually, at start-up. UEFI can skip unneeded device checks and load the bootloader directly. Once the operating system is running, the firmware type makes no difference to speed.
Can Windows 11 run in legacy BIOS mode?
Not on supported hardware. Microsoft requires UEFI with Secure Boot capability and TPM 2.0 for Windows 11, so the system disk must be GPT.
Why do people still call UEFI the BIOS?
Habit. The setup screen does the same job as the old BIOS menu, so board makers and users kept the name.
We set the firmware mode, boot order and Secure Boot on every industrial PC we supply as standard, and image and validate the operating system in that mode before it ships. Tell us which operating system and images you run, and we will match the board and firmware to them.