The Legacy BIOS
The BIOS dates back to the IBM PC in 1981, when the machine operated in 16-bit mode, and BIOS code was jammed into unused blocks within the first 1MB of memory. BIOS enhancements remain limited to the original 16-bit architecture (see PC memory map).
UEFI provides compatibility with legacy PCs by providing BIOS compatibility. UEFI-enabled PCs use UEFI routines to perform the test and configuration phases and then load a compatibility system module (CSM) that sets up legacy BIOS interfaces (interrupts) for use by the system software. In 2006, more than 12 million PCs shipped with EFI/UEFI booting and the CSM, but without making the EFI/UEFI interface available. See BIOS.
Windows support for UEFI started with the 64-bit versions of Windows Server 2008 and Windows Vista Service Pack 1 (SP1). In addition, Apple's Intel-based Macs are also EFI/UEFI compliant.