It sounds like an oxymoron. Given the meaning of hacking today it seems a computer by definition is hackable - if you can't hack it what's the point? There's surely some mathematical way to prove that the usefulness of a computing system declines in proportion to it's hackability, such that the least hackable system resembles more or less a rock.
Wouldn't abandoning the von Neumann architecture do this immediately? Store the code and data in separate memories. I'd think that would take out most all exploits in one strike, no?
