A computer system contains a low-level firmware, generally known as Basic Input/Output System (BIOS), which is a program stored in a read-only memory (ROM) comprising therein many basic subprograms for computer output and input. After the computer is powered on, the basic input/output system runs first for proceeding initial values setup and power-on self test (POST), and reads out the data (information about hard disk drive size, whether there are CD/DVD-Rom drives or not, system time, whether a shadow random access memory (shadow RAM) is used or not, etc.) stored in the complementary metal-oxide semiconductor (CMOS).
Because the progress in computer technology is quite rapid, the development on the legacy BIOS accompanying the development of computer systems has also reached the limit. Therefore, in order to overcome the bottleneck of the development on the legacy BIOS, Intel Corporation has developed an extensible firmware interface (EFI) specification which can avoid the congenital limit of basic input/output system. Also, such an extensible firmware interface allows the use of standard programming language tools to add new components therein, has better extensibility, and is programmed by C language so that the programs are easy to be maintained and read.
It is believed that such an extensible firmware interface will probably replace the basic input/output system completely in the future. However, the extensible firmware interface system can not be used in computer systems currently using the legacy BIOS. Under the extensible firmware interface environment, conventional operating systems worked under the legacy BIOS before, such as Dos, Windows XP or early Windows versions, can not function; this is because the extensible firmware interface is worked as a 32-bit data environment while the legacy BIOS is worked as a 16-bit data environment, and the extensible firmware interface does not provide service routines for the legacy BIOS such as interrupt service routine. Therefore, although the extensible firmware interface system has the above-mentioned advantages, it is unable to change users who use old computer systems to use the extensible firmware interface system widely in a short period of time.