[bitwize]

You can patent an ISA. And then everyone who implements that ISA, in hardware or emulation, is infringing. Intel actually threatened lawsuits against Microsoft for implementing emulation of recent x86 instructions on ARM Windows.

[eschaton]

You cannot patent an instruction set architecture, but you can patent mechanisms used in the implementation of the instruction set architecture. This is an extremely important distinction.

Usually these patents are written such that it’s difficult or impossible to implement the covered portions of the instruction set without violating the patent, but that’s by no means guaranteed.

[neonsunset]

Do you have a link covering that? AFAIK anything up to SSE4.2 was effectively public domain and AVX2 has recently joined that.

[kragen]

arm thumb-2 is over 20 years old now, so any patents that cover necessary techniques to implement it are expired by now (unless they are lemelson-style submarine patents from much longer ago that issued very recently)