To extend the answer of perror: 

Perhaps you should take a look into a recently published whitepaper named **Breaking the x86 ISA** of Christopher Domas. It was published on blackhat17. The author describes an approach for digging into x86 chips and extracted some hidden machine instructions.

> **Abstract:**

> A processor is not a trusted black box for running code; on the
> contrary, modern x86 chips are packed full of secret instructions and
> hardware bugs. In this paper, we demonstrate how page fault analysis
> and some creative processor fuzzing can be used to exhaustively search
> the x86 instruction set and uncover the secrets buried in a chipset.
> The approach has revealed critical x86 hardware glitches, previously
> unknown machine instructions, ubiquitous software bugs, and flaws in
> enterprise hypervisors.
>
> **Author:** Christopher Domas
>
> https://www.blackhat.com/docs/us-17/thursday/us-17-Domas-Breaking-The-x86-Instruction-Set-wp.pdf