UEFI versus BIOS

UEFI (Unified Extensible Firmware Interface) is a standard firmware interface for personal computers. It was designed to replace BIOS (basic input/output system). BIOS has been the PC firmware standard for decades, but with the recent advancements in computer hardware, the stage is being set to remediate its shortcomings with the UEFI standard. More than 140 technology companies participate in the Unified EFI Forum, including Microsoft. New devices being shipped with Windows 10 must have UEFI firmware by default and Secure Boot technology turned on. Regardless of this requirement, legacy BIOS systems will still function with Windows 10 installed.

Some of the limitations of BIOS are as follows:

• 16-bit
• 1 MB address space
• Slow performance on ROM startup
• Master Boot Record (MBR) maximum bootable disk size of 2.2 TB

The advantages of UEFI over BIOS are the following:

• Security features such as Secure Boot and encrypted drives that prevent malware from running before the OS is loaded
• Faster startup and resume times
• Support for drives larger than 2.2 TB as well as drives with more than four partitions
• Support for modern, 64-bit firmware device drivers that the system can use to address more than 17.2 billion GB of memory during startup
• Backward compatibility to use BIOS with UEFI hardware, although Secure Boot must be turned off
• Support for multicast image deployments

Many current computers have the capability to use a BIOS or UEFI firmware mode—switching between one and the other is a fairly simple task. However, there are items to take into consideration with regard to OS deployment and changing existing systems that are on BIOS mode to UEFI mode:

• Changing from BIOS to UEFI requires changing the MBR/NTFS to GPT/FAT32 and NTFS. This translates to reinstalling the OS. This would be the equivalent to a wipe-and-load deployment.
• Ensure that the startup option you select matches the setting you want to have. It is common for old machines to have several startup options for BIOS but only a few for UEFI, or vice versa.
• When deploying from media, the media must be FAT32 for UEFI, and FAT32 has a file-size limitation of 4 GB.
• UEFI does not support cross-platform startup, you will need to have the correct startup media (32 or 64-bit).
• For UEFI-based PCs that support both UEFI and legacy BIOS modes, WinPE needs to be started in the correct mode in order to correctly install Windows. For more information, see WinPE: Boot in UEFI or legacy BIOS mode.

Source: Deploying Windows 10 Press Book