Prior to ARMv8, all licensees could add additional specialized instructions or even entire instruction sets using the ARM coprocessor model. By changing a few

Prior to ARMv8, all licensees could add additional specialized instructions or even entire instruction sets using the ARM coprocessor model. By changing a few registers, you could alter how instructions were interpreted. This was used by some vendors for hardware-software interfacing, and by others to provide specific acceleration primitives for their specialized workloads. ARM used this mechanism themselves to offer an optional floating point unit, with specialized vectorized floating point instructions. Unfortunately, support for coprocessors has been removed from ARMv8.