1. A computer system including no basic input/output system (BIOS) for operating bootstrap used in initial activation of a legacy operation system, comprising: a central processing unit (CPU) and a memory, in which an extended firmware and a bootstrap program are stored, the extended firmware including a BIOS emulator and a plurality of device drivers; the extended firmware using the device driver to make the BIOS emulator perform emulation of BIOS operation in response to a BIOS call issued by the bootstrap program.