So, you are the decision maker for whether or not Intel CPU's get designed into your product? If so, I'm sure several Intel salespeople have you on speed dial.
The 286 had purposely undocumented instructions, of this sort: "Ooops this is b0rk3d. It will always be b0rk3d. Let's pretend it didn't happen." So for generations there were holes in the op code map that people tip-toed around. Especially since Intel (meaning the internal grey-beard collective) also forgot exactly what those opcodes were and what they were supposed to do. You care, why, exactly?
It's not like the NSA slips extra opcodes into executables that you compile with your own compiler in order to spy on you. They have much easier ways to spy on you.
Also, it's not like it is that hard to throw unused opcodes at the decoder and see which ones give you the illegal instruction exception, and which ones do something else. You now have a homework assignment. Have fun, let us know what you find.
The 286 had purposely undocumented instructions, of this sort: "Ooops this is b0rk3d. It will always be b0rk3d. Let's pretend it didn't happen." So for generations there were holes in the op code map that people tip-toed around. Especially since Intel (meaning the internal grey-beard collective) also forgot exactly what those opcodes were and what they were supposed to do. You care, why, exactly?
It's not like the NSA slips extra opcodes into executables that you compile with your own compiler in order to spy on you. They have much easier ways to spy on you.
Also, it's not like it is that hard to throw unused opcodes at the decoder and see which ones give you the illegal instruction exception, and which ones do something else. You now have a homework assignment. Have fun, let us know what you find.