common, allowing storage capacity to be expanded. Optical discs store data by altering a pigment layer on a plastic disk and are similarly random access. Read-only and read-write versions are available; removable media again allows indefinite expansion, and some automated systems (e.g. optical jukebox) were used to retrieve and mount disks under direct program control. Domain-wall memory (DWM) stores data in a magnetic tunnel junctions (MTJs), which works by controlling domain wall (DW) motion in ferromagnetic nanowires. Organic Thinfilm produces rewriteable non-volatile organic ferroelectric memory based on ferroelectric polymers. Thinfilm successfully demonstrated roll-to-roll printed memories in 2009. In Thinfilm's organic memory the ferroelectric polymer is sandwiched between two sets of electrodes in a passive matrix. Each crossing of metal lines is a ferroelectric capacitor and defines a memory cell. Non-volatile main memory Non-volatile main memory (NVMM) is primary storage with non-volatile attributes. This application of non-volatile memory presents security challenges. References External links Supporting filesystems in persistent memory, LWN.net, 2 September 2014, by Jonathan Corbet Research paper about perspective usage of magnetic photoconductors in magneto-optical data storage. Computer memory