Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. The most widely used form of primary storage today is a volatile form of random access memory, meaning that when the computer is shut down, anything contained in RAM is lost. However, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory costs more, provides lower performance, or has a limited lifetime compared to volatile random access memory. Non-volatile data storage can be categorized into electrically addressed systems and mechanically addressed systems. Generally speaking, electrically addressed systems are expensive, have limited capacity, but are fast, whereas mechanically addressed systems are more cost effective per bit, but are slower.
Electrically addressed
Electrically addressed semiconductor non-volatile memories can be categorized according to their write mechanism. Mask ROMs are factory programmable only, and typically used for large-volume products which are not required to be updated after manufacture. Programmable read-only memory can be altered after manufacture, but require a special programmer and usually cannot be programmed while in the target system. The programming is permanent and further changes require replacement of the device. Data is stored by physically altering storage sites in the device.
Read-mostly devices
An EPROM is an erasable ROM that can be changed more than once. However, writing new data to an EPROM requires a special programmer circuit. EPROMs have a quartz window that allows them to be erased with ultraviolet light, but the whole device is cleared at one time. A one-time programmable device may be implemented using an EPROM chip without the quartz window; this is less costly to manufacture. An electrically erasable programmable read-only memory EEPROM uses voltage to erase memory. These erasable memory devices require a significant amount of time to erase data and to write new data; they are not usually configured to be programmed by the processor of the target system. Data is stored by use of floating-gate transistors which require special operating voltages to trap or release electric charge on an insulated control gate to store information.
Flash memory is a solid-state chip that maintains stored data without any external power source. It is a close relative to the EEPROM; it differs in that erase operations must be done on a block basis and capacity is substantially larger than that of an EEPROM. Flash memory devices use two different technologies—NOR and NAND—to map data. NOR flash provides high-speed random access, reading and writing data in specific memory locations; it can retrieve as little as a single byte. NAND flash reads and writes sequentially at high speed, handling data in blocks, however it is slower on read when compared to NOR. NAND flash reads faster than it writes, quickly transferring whole pages of data. Less expensive than NOR flash at high densities, NAND technology offers higher capacity for the same-size silicon.
Ferroelectric RAM (F-RAM)
Ferroelectric RAM is a random-access memory similar in construction to DRAM both use a capacitor and transistor but instead of using a simple dielectric layer the capacitor, a F-RAM cell contains a thin ferroelectric film of lead zirconate titanate , commonly referred to as PZT. The Zr/Ti atoms in the PZT change polarity in an electric field, thereby producing a binary switch. Due to the PZT crystal maintaining polarity, F-RAM retains its data memory when power is shut off or interrupted. Due to this crystal structure and how it is influenced, F-RAM offers distinct properties from other nonvolatile memory options, including extremely high, although not infinite, endurance, ultra low power consumption, single-cycle write speeds, and gamma radiation tolerance.
Magnetoresistive RAM (MRAM)
Magnetoresistive RAM stores data in magnetic storage elements called magnetic tunnel junctions. The first generation of MRAM, such as Everspin Technologies' 4 Mbit, utilized field-induced writing. The second generation is developed mainly through two approaches: Thermal-assisted switching which is being developed by Crocus Technology, and Spin-transfer torque which Crocus, Hynix, IBM, and several other companies are developing.
FeFET memory
uses a transistor with ferroelectric material to permanently retain state.
Mechanically addressed systems
Mechanically addressed systems use a recording head to read and write on a designated storage medium. Since the access time depends on the physical location of the data on the device, mechanically addressed systems may be sequential access. For example, magnetic tape stores data as a sequence of bits on a long tape; transporting the tape past the recording head is required to access any part of the storage. Tape media can be removed from the drive and stored, giving indefinite capacity at the cost of the time required to retrieve a dismounted tape. Hard disk drives use a rotating magnetic disk to store data; access time is longer than for semiconductor memory, but cost per stored data bit is very low, and they provide random access to any location on the disk. Formerly, removable disk packs were 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 were used to retrieve and mount disks under direct program control.
Organic
produces rewriteable non-volatile organic ferroelectric memory based on ferroelectric polymers. Thinfilm successfully demonstrated roll-to-rollprinted 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. This gives a non-volatile memory comparable to ferroelectric RAM technologies and offer the same functionality as flash memory.
Non-volatile main memory
Non-volatile main memory is primary storage with non-volatile attributes. This application of non-volatile memory presents security challenges.
