[bluGill]

Depends on how you count. Is the instruction that adds register one to register two and stores it in register three different from the instruction that adds register one to register two and stores it in register four? The only difference is the register the data is stored in after the add. I can argue this either way, and you should be able to as well (though you may find one side is a lot more compelling).

Really an undocumented instruction is just anything that isn't documented, it is good practice to leave some blank space in your instruction set so that you can implement the next feature that needs a new instruction. As such we expect to find millions of potential instructions: there is nothing there but the next CPU might have something.

[wolfgke]

It is a difference to reserve space in the instruction encodings vs. having an undocumented instruction.

For the former when trying to encode it an "undefined instruction" interrupt should occur.