The I-V curve of forming, switching between SET and RESET and memory are evaluated theoretically. The key features of the model are: a) drift diffusion of ions and of electrons, b) stoichiometry changes by exchange of oxygen with the ambient, c) temperature changes with current, d) Mott transitions in forming the filament and e) switching by changes in tunneling through a thin insulating layer. Forming, SET and RESET are induced by stoichiometry changes. Both voltage ramp and voltage pulses are considered. For pulses the interplay between pulse duration and pulse voltage is evaluated.