The computer industry usually experiences a transition period between the introduction and adoption of a new technology. The Framework also expects a natural transition phase, brief or prolonged. The CSM eases this transition phase by acting as a bridge between EFI and legacy BIOS interfaces. The CSM allows EFI to use legacy BIOS interfaces to boot to a traditional operating system such as DOS, Windows 2000, Windows XP, and so on. The CSM also allows EFI to boot to an EFI-aware operating system from a device controlled by a traditional Option ROM. With the advent of CSM, a system vendor can participate seamlessly in the innovation of new technology EFI without sacrificing any of its traditional functionalities. Therefore, directly and indirectly, CSM plays a vital role in ensuring the existing functionality while the industry pursues the transition from legacy BIOS to EFI.

The CSM uses the Framework for platform and hardware initialization, enumeration, and boot path selected by EFI. Additionally, the CSM provides the EfiCompatibility functionality to EFI in supporting traditional operating systems and traditional Option ROMs. The EfiCompatibility module along with the IBV supplied Compatibility16BIOS and Compatibility16SMM modules constitute the CSM. During the transition from legacy BIOS to EFI, the traditional Option ROM support may be required longer than traditional operating system support. The CSM is modular and hence can be removed if not required. Figure 17.1 represents a block diagram of how a legacy system operates using the CSM under the...