Both. Copyright applies to some aspects, patents to others, and then you also get into trademark rights. With the US having embraced effectively permanent copyright, ISAs will not be freely able to be implemented unless their owner expressly allows it (my understanding is that the Oracle vs. Google case solidified this thinking).

Fundamentally, that means you get to pick from RISC-V, POWER, or SPARC when selecting a well known ISA for your new device, or start signing licensing agreements (which always come with restrictions and oftentimes significant royalties).