BIOS And UEFI Overview
Both BIOS and UEFI are low-level software. The software starts when you boot your PC before booting your operating system, but UEFI is a more modern solution, supporting larger hard drives, faster boot times, more security features, and—conveniently—graphics and mouse cursors.
We could see newer PCs that ship with UEFI still refer to it as the “BIOS” to avoid confusing people who are used to a traditional PC BIOS. Although your PC uses the term “BIOS”, modern PCs you use today almost certainly ship with UEFI firmware instead of a BIOS.
BIOS
BIOS (acronym for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is firmware used to perform hardware initialization during the booting process (power-on startup), and to provide runtime services for operating systems and programs. The BIOS firmware comes pre-installed on a personal computer's system board, and it is the first software to run when powered on.
It’s low-level software that resides in a chip on your computer’s motherboard. The BIOS loads when your computer starts up, and the BIOS is responsible for waking up your computer’s hardware components, ensures they’re functioning properly, and then runs the bootloader that boots Windows or whatever other operating system you have installed.
You can configure various settings in the BIOS setup screen. Settings like your computer’s hardware configuration, system time, and boot order are located here. You can access this screen by pressing a specific key—different on different computers, but often Esc, F2, F10, or Delete—while the computer boots. When you save a setting, it’s saved to the memory on your motherboard itself. When you boot your computer, the BIOS will configure your PC with the saved settings.
The BIOS goes through a POST, or Power-On Self Test, before booting your operating system. It checks to ensure your hardware configuration is valid and working properly. If something is wrong, you’ll see an error message or hear a cryptic series of beep codes. You’ll have to look up what different sequences of beeps mean in the computer’s manual.
When your computer boots—and after the POST finishes—the BIOS looks for a Master Boot Record (MBR), stored on the boot device and uses it to launch the bootloader.
You may also see the acronym CMOS, which stands for Complementary Metal-Oxide-Semiconductor. This refers to the battery-backed memory where the BIOS stores various settings on the motherboard. It’s actually not accurate anymore, since this method has been replaced with flash memory (also referred to as EEPROM) in contemporary systems.
Nowadays BIOS is Outdated
The BIOS has been around for a long time, and hasn’t evolved much. Even MS-DOS PCs released in the 1980s had a BIOS!
Of course, the BIOS has evolved and improved over time. Some extensions were developed, including ACPI, the Advanced Configuration and Power Interface. This allows the BIOS to more easily configure devices and perform advanced power management functions, like sleep. But the BIOS hasn’t advanced and improved nearly as much as other PC technology has since the days of MS-DOS.
The traditional BIOS still has serious limitations. It can only boot from drives of 2.1 TB or less. 3 TB drives are now common, and a computer with a BIOS can’t boot from them. That limitation is due to the way the BIOS’s Master Boot Record system works.
The BIOS must run in 16-bit processor mode, and only has 1 MB of space to execute in. It has trouble initializing multiple hardware devices at once, which leads to a slower boot process when initializing all the hardware interfaces and devices on a modern PC.
The BIOS has needed replacement for a long time. Intel started work on the Extensible Firmware Interface (EFI) specification back in 1998. Apple chose EFI when it switched to the Intel architecture on its Macs in 2006, but other PC manufacturers didn’t follow.
In 2007, Intel, AMD, Microsoft, and PC manufacturers agreed on a new Unified Extensible Firmware Interface (UEFI) specification. This is an industry-wide standard managed by the Unified Extended Firmware Interface Forum, and isn’t solely driven by Intel. UEFI support was introduced to Windows with Windows Vista Service Pack 1 and Windows 7. The vast majority of computers you can buy today now use UEFI rather than a traditional BIOS.
UEFI
UEFI (acronym for Unified Extensible Firmware Interface) is the interface firmware default for PC which replaced the environment BIOS. However, as of 2017 the majority of UEFI firmwares are also backwards compatible with the previous (legacy) BIOS technology.
UEFI replaces the legacy Basic Input/Output System (BIOS) firmware interface originally present in all IBM PC-compatible personal computers, with most UEFI firmware implementations providing support for legacy BIOS services. UEFI can support remote diagnostics and repair of computers, even with no operating system installed. There’s no way to switch from BIOS to UEFI on an existing PC. You need to buy new hardware that supports and includes UEFI, as most new computers do. Most UEFI implementations provide BIOS emulation so you can choose to install and boot old operating systems that expect a BIOS instead of UEFI, so they’re backwards compatible.
This new standard avoids the limitations of the BIOS. The UEFI firmware can boot from drives of 2.2 TB or larger—in fact, the theoretical limit is 9.4 zettabytes. That’s roughly three times the estimated size of all the data on the Internet. That’s because UEFI uses the GPT partitioning scheme instead of MBR. It also boots in a more standardized way, launching EFI executables rather than running code from a drive’s master boot record.
UEFI can run in 32-bit or 64-bit mode and has more addressable address space than BIOS, which means your boot process is faster. It also means that UEFI setup screens can be slicker than BIOS settings screens, including graphics and mouse cursor support. However, this isn’t mandatory. Many PCs still ship with text-mode UEFI settings interfaces that look and work like an old BIOS setup screen.
UEFI is packed with other features. It supports Secure Boot, which means the operating system can be checked for validity to ensure no malware has tampered with the boot process. It can support networking features right in the UEFI firmware itself, which can aid in remote troubleshooting and configuration. With a traditional BIOS, you have to be sitting in front of a physical computer to configure it.
It’s not just a BIOS replacement, either. UEFI is essentially a tiny operating system that runs on top of the PC’s firmware, and it can do a lot more than a BIOS. It may be stored in flash memory on the motherboard, or it may be loaded from a hard drive or network share at boot.
Different PCs with UEFI will have different interfaces and features. It’s all up to your PC manufacturer, but the basics will be the same on each PC.
How to Access UEFI Settings on Modern PCs
If you’re a normal PC user, switching to a computer with UEFI won’t be a noticeable change. Your new computer will boot up and shut down faster than it would have with a BIOS, and you can use drives of 2.2 TB or more in size.
If you need to access low-level settings, there may be a slight difference. You may need to access the UEFI settings screen through the Windows boot options menu rather than pressing a key while your computer starts. With PCs now booting so quickly, PC manufacturers don’t want to slow down the boot process by waiting to see if you press a key. However, we’ve also seen PCs with UEFI that allow you to access the BIOS in the same way, by pressing a key during the boot-up process.
Boot to UEFI Mode or legacy BIOS mode
1. Open the firmware menus. You can use any of these methods:
- Boot the PC, and press the manufacturer’s key to open the menus. Common keys used: Esc, Delete, F1, F2, F10, F11, or F12. On tablets, common buttons are Volume up or Volume down. During startup, there’s often a screen that mentions the key. If there’s not one, or if the screen goes by too fast to see it, check your manufacturer’s site.
- Or, if Windows is already installed, from either the Sign on screen or the Start menu, select Power > hold Shift while selecting Restart. Select Troubleshoot > Advanced options > UEFI Firmware settings.
2. From the firmware menus, boot to drive or network while in UEFI or BIOS mode:
On the boot device menu, select the command that identifies both the firmware mode and the device. For example, select UEFI: USB Drive or BIOS: Network/LAN.
2. From the firmware menus, boot to drive or network while in UEFI or BIOS mode:
On the boot device menu, select the command that identifies both the firmware mode and the device. For example, select UEFI: USB Drive or BIOS: Network/LAN.
You might see separate commands for the same device. For example, you might see UEFI USB Drive and BIOS USB Drive. Each command uses the same device and media, but boots the PC in a different firmware mode.
Some devices only support one mode (either UEFI or BIOS). Other devices will only allow you to boot to BIOS mode by manually disabling the UEFI security features. To disable the security features, go to Security > Secure Boot and disable the feature.
Note :
Some older PCs (Windows 7-era or earlier) support UEFI, but require you to browse to the boot file. From the firmware menus, look for the option: "Boot from file", then browse to \EFI\BOOT\BOOTX64.EFI on Windows PE or Windows Setup media.
UEFI and BIOS modes in WinPE
Detect if WinPE is booted into BIOS or UEFI Mode
Query the registry to determine which mode the device is in. You can do this from the command line:
Copy
reg query HKLM\System\CurrentControlSet\Control /v PEFirmwareType
Use it in a script:
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wpeutil UpdateBootInfo
for /f "tokens=2* delims= " %%A in ('reg query HKLM\System\CurrentControlSet\Control /v PEFirmwareType') DO SET Firmware=%%B
:: Note: delims is a TAB followed by a space.
if %Firmware%==0x1 echo The PC is booted in BIOS mode.
if %Firmware%==0x2 echo The PC is booted in UEFI mode.
Note that between delims= and " %%A is a tab, followed by a space.
Make sure you boot into the right mode every time
Here are a couple of ways you can make sure you're booted into the right firmware mode every time you start your PC.
Use preformatted hard drives, and use a method that doesn't automatically format the drive.
If you want to ensure that your drive boots into a certain mode, use drives that you've preformatted with the GPT file format for UEFI mode, or the MBR file format for BIOS mode. When the installation starts, if the PC is booted to the wrong mode, Windows installation will fail. To fix this, restart the PC in the correct firmware mode.
Remove the UEFI or BIOS boot files
If you want a PC to only boot into a certain mode, you can remove the files that Windows PE or Windows Setup use to boot in UEFI or BIOS mode. Remove the following files, depending on the mode you want to boot to.
Boot only when in UEFI mode
Remove the bootmgr file from the root of the Windows PE or Windows Setup media. This prevents the device from starting in BIOS mode.
Boot only when in BIOS mode
Remove the efi folder from the root of the Windows PE or Windows Setup media. This prevents the device from starting in UEFI mode.
While UEFI is a big upgrade, it’s largely in the background. Most PC users will never notice—or need to care—that their new PCs use UEFI instead of a traditional BIOS. They’ll just work better and support more modern hardware and features.
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