"...for compatibility reasons, we still support the entire ARMv7 machine in the new ARMv8 architecture, but when running 64-bit software, this part of the machi

"...for compatibility reasons, we still support the entire ARMv7 machine in the new ARMv8 architecture, but when running 64-bit software, this part of the machine is not being used, and the area of complex legacy it had built up does not need to be active when running in the 64-bit ISA, unlike other architectures where 64-bit extension was simply added to the historical complexity and legacy of their 32-bit mode. The new ISA drew upon the years of experience of building different micro architecture implementations, so again it was defined so that these new processors can be more easily optimized for low power operation — an opportunity not really offered since the first ARMv4 machine that resulted in the now legendary low power ARM7 processors." http://www.arm.com/files/downloads/ARMv8_white_paper_v5.pdf http://www.arm.com/files/downloads/ARMv8_white_paper_v5.pdf