1. A method for programmed-state detection of memristor stacks, in which each memristor stack comprises two memristors of opposite fabrication polarity such that, in each of two primary stable configurations, the two memristors of each stack are in opposite resistance states, comprising: applying a first secondary switching voltage across a memristor stack to produce a first programmed-state-dependent secondary switching response in a memristor in the memristor stack, the programmed-state-dependent secondary switching response producing a change in the electrical resistance of the memristor stack; measuring a first electrical resistance of the memristor stack; and inferring a programmed state of the memristor stack from the measured electrical resistance by determining the fabrication polarity of a memristor of the stack that is in a first resistance state.