Most parts of present computer systems are made of volatile
devices, and the power to supply them to avoid information loss causes huge
energy losses. We can eliminate this meaningless energy loss by utilizing the
non-volatile function of advanced spin-transfer torque magnetoresistive
random-access memory (STT-MRAM) technology and create a new type of computer,
i.e., normally off computers . Critical tasks to achieve normally off computers
are implementations of STT-MRAM technologies in the main memory and low-level
cache memories. STT-MRAM technology for applications to the main memory has
been successfully developed by using perpendicular STT-MRAMs, and faster
STT-MRAM technologies for applications to the cache memory are now being
developed. The present status of STT-MRAMs and challenges that remain for
normally off computers are discussed.