“x86-64/AMD64 was solely developed by AMD. AMD holds patents on techniques used in AMD64; those patents must be licensed from AMD in order to implement AMD64. Intel entered into a cross-licensing agreement with AMD, licensing to AMD their patents on existing x86 techniques, and licensing from AMD their patents on techniques used in x86-64. In 2009, AMD and Intel settled several lawsuits and cross-licensing disagreements, extending their cross-licensing agreements.”
⇒ I think only Intel and AMD currently can freely implement x64.
There’s also https://newsroom.intel.com/editorials/x86-approaching-40-sti...:
“However, there have been reports that some companies may try to emulate Intel’s proprietary x86 ISA without Intel’s authorization. Emulation is not a new technology, and Transmeta was notably the last company to claim to have produced a compatible x86 processor using emulation (“code morphing”) techniques. Intel enforced patents relating to SIMD instruction set enhancements against Transmeta’s x86 implementation even though it used emulation.”
That was seen as a message to Microsoft (https://arstechnica.com/information-technology/2017/06/intel...)
Some industries are awfully complex and getting into it requires insane amounts of work. x86 processor making is one of these. It's the same as making new browser engines/js JIT/etc. There is just so much work that catching up with the incumbents is almost impossible.
¹Not sure what happened to that x86 license that Cyrix had.