Windows support for UEFI was first introduced in 64-bit editions of Microsoft Windows Server® 2008 and Windows Vista® Service Pack 1. Windows 7 and Windows Server 2008 R2 continued to support UEFI. The main reason for early adoption was for GPT and large boot disk support.
Many computers built prior to Windows 8 took advantage of UEFI architecture to reduce costs and standardize firmware stacks, but were still used in BIOS mode to remain compatible with existing factory processes, tools, legacy operating systems, drivers, option ROMs, and some applications. These UEFI computers are called “Class Two” UEFI as defined by Intel. This means that they have the capability to boot into native UEFI mode, but in practice most computers boot into legacy BIOS mode using a Compatibility Support Module (CSM). The figure below demonstrates boot flows for Class 2 systems.
Figure 1: UEFI Startup Paths
The gold elements in the diagram indicate legacy BIOS-style startup. The green arrows show native UEFI mode boot. The blue arrow indicates a system that attempts to boot into UEFI mode to an OS that does not support it. UEFI configured with CSM enabled reverts the boot process to BIOS mode via the CSM. This is called “progressive boot.”
UEFI and Windows 8
It is expected that there will be more Class 2 systems designed for use with Windows 8 that boot into native UEFI mode with the CSM disabled – this is represented by the crossed-out CSM path in the diagram below. This enables the use of Secure Boot, and compatibility with the Windows 8 Logo requirements.
Figure 2: Native UEFI Mode with CSM support
So on a Windows 8 UEFI computer, there are three likely configurations:
· Computer with native UEFI startup. CSM is not used. Green arrows above.
o Improved Boot Performance: Yes
o Secure Boot: Yes
· UEFI computer boots into native UEFI mode, but still loads the CSM for legacy device or OS support: Green arrows. Some interaction with gold component remains.
o Improved Boot Performance: Some improvement vs. BIOS configuration
o Secure Boot: Yes
· UEFI computer boots via the CSM into BIOS mode: Boot via legacy path shown in Figure 1.
o Improved Boot Performance: No improvement
o Secure Boot: No
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