NON VOLATILE MEMORY

Non-volatile memory (NVM) is a type of computer storage that retains stored data even when power is removed.

There are several types of non-volatile memory and they are; read only memory (ROM), flash memory (NAND flash), NOR flash, solid state drives (SSDs), hard disk drive (HDD), optical storage, magnetic tape, and emerging NON-volatile memory technologies.

Read only memory (ROM) has data permanently encoded during manufacturing by physical connecting or disconnecting circuit paths. Once written it cannot be altered under normal operating conditions.

Flash memory (NAND flash) stores data as electrical charge in floating gate transistors (or charge trap layers in 3D NAND). Data is written by forcing electrons through a thin oxide layer via quantum tunneling and erased by removing them. Operations occur in pages (read/writes) and blocks (erases).

NOR flash has similar floating gate principle as NAND but cells are arranged in parallel (like NOR gates). Allows random byte level access and direct code execution (eXecute-in place/XIP).

Solid state drives (SSDs) combine NAND flash arrays with a sophiscated controller, DRAM cache, and firmware that manages wear leveling, garbage collection, error correction and encryption hence presenting a standard storage interface to the host.

Hard disk drive (HDDs) has data stored as magnetic patterns on spinning platters coated with ferromagnetic material. A read/write head mounted on an actuator arm floats nanometers above the platter surface to read and write data.

Optical storage has a laser beam read and write data encoded as microscopic pits and lands on a reflective disc. Variations in reflectivity are interpreted as binary os and 1s.

Magnetic tape has data recorded as magnetic patterns on a long strip of magnetic tape wound on a reel. A tape drive uses read/write heads to access data sequentially.

Emerging non-volatile memory technologies such as 3D Xpoint/optane uses bulk resistance switching in a crystalline material between two selectors. It is neither a transistor nor a floating gate but a fundamentally different storage mechanism offering DRAM like speed with non-volatile retention. Other emerging non-volatile memory technologies are; magneto resistive RAM (MRAM), resistive RAM (ReRAM)/RRAM), phase change memory (PCM), ferroelectric RAM (FeRAM) etc.

The advantages of non-volatile memory are; non volatile memory retains all stored information even when there is no power. They retain data for as long as possible. Non-volatile memory has high storage density and scalability. NVM are cost efficient at scale, portable and very flexible. NVM are highly resistant to physical shock, vibration, altitude changes and orientation especially for solid state memory. Above all they provide storage resources for Bios,UEFI firmware, boot loaders and operating systems so without NVM storage no modern computer can boot or be secure since hardware encryption rely on it.

The disadvantages of non-volatile memory are; they have slower access speed compared to volatile memory. They have limited write endurance since their physical cells have finite program/erase cycles. Moreover they have complex controller requirements among a host of others.

Non-volatile memory finds widespread application in the following; consumer electronics such as smartphones, tablets, laptops etc. where they are used to store data files, photos, music etc. NVM are used in data centers to store Exabyte of data across tiered NVM architectures. Modern vehicles contain dozens of NVM-dependent systems such as engine control unit (ECU) which stores firmware in NOR flash, infotainment systems use NAND SSDs, event data recorders (black boxes) capture crash data to flash and ADS (advanced driver assistance systems) rely on high reliability NVM to store maps and calibration data. They are also used on satellites, aerospace and defense industry as well as health care and medical devices, IoT systems, financial services and scientific research.

The future of non-volatile memory depends on the advances and development of the following technologies; 300+ layer 3D NAND and beyond, MRAM (STT-MRAM), DNA data storage, holographic storage and so on. AI optimized storage will be the norm in the nearest future. As data volumes grow exponentially and computing moves deeper into AI, edge and embedded domains, the evolution of NVM will remain one of the most remarkable stories of the future of NVM storage.

 

SOURCES:

• Memory systems: Cache, DRAM, DISK by Bruce Jacob, Spencer Ng and David Wang.
• Computer organization and architecture: Designing for performance by William Stallings.
• The memory book: A guide to memory organization and architecture by John L. Hennessy and David Patterson.
• Solid state drives: the NAND flash memory handbook by Michael Comwell et all.
• NAND flash memory technologies by Seiichi Aritome.
• Emerging non-volatile memories edited by Seiichi Aritome.