Specifications

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Figure 1 Compatibility Overview
Differences between Traditional BIOS and EFI
An EFI system differs from the traditional BIOS POST in that only minimal system
configuration takes place until the Boot Device Selection (BDS) phase (equivalent to
traditional POST INT19). Video is not required until the BDS phase, nor are other
OpROMs dispatched until the BDS phase. Likewise, BIOS Setup is entered from the BDS
phase. These differences place the policy to invocate (or not invocate) the CSM in BDS
and make this policy an integral part of BDS.
The policy is predominately set by the following three classes of information: OS being
booted, boot drive selection, and Device OpROM selection.
2.3.2.1 OS Being Booted
The selection of booting a traditional (non-EFI-aware) OS dictates that any OpROM being
dispatched must be a traditional OpROM rather than an EFI OpROM. This requirement
means that the CSM code must be activated and invoked.
2.3.2.2 Boot Device Selection
A boot device that has only a traditional OpROM associated with it requires the CSM code
to be activated and invoked.