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Most likely yes. In Google v. Oracle, the Supreme Court ignored whether software APIs are copyrightable and instead performed a fair use test. Oracle's Java APIs remain under copyright, but Google's use of the APIs were fair use.

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I rephrased the question. The instruction set may be copyrightable, but as in Google v. Oracle, we don't have to answer that question. Even if an instruction set is copyrightable, then under Google, it is probably fair use to copy it for interoperability purposes (e.g., making a compatible processor or emulator). For the actual binary encoding of instructions, there is even less of an argument for copyright protection because the encoding is purely functional.

It’s certainly (?) legal to implement an emulator for a given instruction set. I’m not sure how implementing a CPU in hardware would be different.

What clearly is subject to copyright is the documentation describing the instruction set and its semantics.

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