My employer sent me to an ARM assembly language class once. I learned in the course introduction what made ARM special compared to the other high-performance architectures of the day: conditional instructions, multiple-register load/store instructions, current processor status register (CPSR) and the coprocessor hardware model….to name a few.

This general set of guidelines held true through version 7 of the instruction set architecture (ISA). ARM v8 is published, and chips are already in production. It consists of both a new “Aarch64” definition, and an “Aarch32” update that retains instructions which would still be recognizable from an assembly listing. But is 64-bit ARM “really” ARM as we know it? Their own data brief claims that it is a rationalization of the instruction set.

“Rationalization” in this case may just mean the Acorn folks have retired, and this is the new crop of ISA designers who were getting snot wiped off their noses for them in 1980. As you would expect, based on updated statistics of compiled code performance and thirty more years of published papers they are tweaking the design as they see fit.