Re: "I did not know that ARM actually prohibited adding instructions"
So what about the AMX instructions in Apple's A13? Are those not custom, but rather Apple just implemented them before ARM even announced them?
I thought the deal was that if you licensed the cores from ARM you could only tweak them not add to them. If you had an architectural license like Apple and design your own cores, you had more freedom - you still had to implement the ARM ISA but I didn't think it stopped you from 'enhancing' the ISA with your own instructions.