IBM xSeries 450 Planning And Installation Manual page 20

In comparison to a BIOS-based, legacy system, the EFI is an additional layer
between the operating system and the firmware. In a legacy system, the OS
loader calls BIOS functions directly. Consequently, to provide a stable boot
environment, changes in the OS loader and the platform firmware must go
hand-in-hand.
Note: All operating systems supported on x450 are EFI-aware. The OS loader
communicates with the firmware and hardware through the EFI interface.
The primary goal of this specification is to provide an abstract model both for
operating system and hardware developers. With such a model in a place, OS
loader customizations are not required if there are changes in the platform
hardware or firmware (added new boot or input devices for instance). The EFI
breaks up a tight dependency between the operating system and the firmware,
thus speeding up the process of releasing the new products and introducing the
new features and functionality (both operating system and hardware).
Figure 1-3 The EFI concept
Consider, for example, the situation where a new type of boot device, for example
a USB key, is to be implemented. First the BIOS would have to offer an option to
choose this new device for booting, then new USB key-specific functions would
have to be added to the firmware to support booting from a USB device, and
finally, the OS loader would have to be modified to use these functions.
IBM ^ xSeries 450 Planning and Installation Guide
6
Operating system
Legacy OS
loader
Platform-specific firmware (BIOS)
Platform hardware
EFI OS loader
EFI