We demonstrate on the basis of ab initio calculations that metal-oxide-nitride-oxide-semiconductor (MONOS) memory is one of the most promising future high-density archive memories. We find that O related defects in a MONOS memory cause irreversible structural changes to the SiO2/Si3N4 interface at the atomistic level during program/erase (P/E) cycles. Carrier injection during the programming operation makes the structure energetically very stable, because all the O atoms in this structure take on three-fold-coordination. The estimated lifespan of the programmed state is of the order of a thousand years.