1. A storage device comprising: plurality of storage strings, each comprising a serial interconnection of a plurality of active storage elements, each storage element having a part for maintaining a storage state and a part of modulating a current through the element according to the storage state; and mapping circuitry for selectively sensing a storage state of a storage element in a storage string by forming current though the storage element that is a non-linear function of the storage state; wherein the mapping circuitry comprises a reference string of active elements, and the mapping circuitry selectively senses a storage state by forming a difference in currents in the sensed storage string and in the reference string that is a non-linear function of the storage state.