ARMv8-A (aka 64bit ARM) is a completely different instruction set, with basically zero relation to the instruction encoding of 32bit ARM. They redesigned it fro

ARMv8-A (aka 64bit ARM) is a completely different instruction set, with basically zero relation to the instruction encoding of 32bit ARM. They redesigned it from scratch. And it's an instruction set designed for high-performance application cores (hence the -A) like what you would find as the main processor of a mobile phone, or a laptop or even a server. So they don't want anything optimised for lower-end designs. Plenty of other useful things to do with the encoding space. For those low-end designs, ARM have a second ISA called ARMv8-M. Which is basically just the old thumb mode of 32bit arm with a few improvements. The original 32bit arm ISA is basically dead these days (at least for new designs). You can still run some ARMv8-A chips in 32bit ARM mode, for compatibility. But that's actually optional in regards to the spec. Some only support running usermode 32bit ARM code. In the near future, we will see some cores that don't support 32bit arm code at all.