Non-volatile memory (NVM) is a form of persistent data storage that retains data while unconnected to a power source.
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What makes NVM different?
Unlike RAM or any other type of main memory, which is closely connected to the central processing unit, non-volatile memory (NVM) does not have a direct path to the CPU. It’s much slower than main memory, but it stores more data. If data is running in main memory on a computer, it’s being immediately processed, but the moment the computer shuts down, all that immediate memory is gone. Non-volatile memory is much slower, and it stays on the corresponding hardware indefinitely without requiring a connection to computing power.
There are two types of non-volatile memory: electrically addressed and mechanically addressed. One of the most popular examples of electrically addressed NVM systems is flash memory. Flash memory saves large amounts of data without needing a constant connection to power. Flash drives use flash memory; they connect to another device using a USB port and store and modify data. Flash drives are very convenient because USB ports connect to multiple devices and can store large numbers of files. Examples of mechanically addressed NVM storage include disk drives and magnetic tapes.
The non-volatile memory can be categorized into two main systems: electrically addressed systems and mechanically addressed systems.
Electrically Addressed Systems
Electrically addressed NVMs are expensive and faster rather than mechanically addressed systems. Electrically addressed systems are again categorized based on their writing mechanism. Here are some examples for them:
Read-only memory (ROM) devices such as PROM, EPROM, and EEPROM are examples of electrically addressed non-volatile memory. A PROM or programmable read-only memory is used to store programs permanently. It can be usually seen in mobile phones, implantable medical devices, and more.
EPROM or erasable programmable read-only memory is another type of non-volatile memory chip. It can be easy to erase a programmed EPROM by exposing it to UV lights and retaining data for ten to twenty years.
EEPROM or electrically erasable programmable read-only memory is a storage device used to store small amounts of data. The data can be saved even in the absence of a power supply.
Magnetoresistive random-access memory (MRAM) is a type of non-volatile memory in which data is stored in magnetic domains as magnetic tunnel junctions (MTJs) instead of an electric charge in the conventional RAM. MRAM has two-generation, and the first generation utilizes 4 Mbit memory, developed by Everspin Technologies, to read and write. The second generation is common, and it uses two methods for writing: Thermal-assisted switching (TAS) developed by Corcus Technology and Spin-transfer torque (STT), which is developed by several companies, including Corcus and IBM.
Fe FET Memory
The ferroelectric field-effect transistor (Fe FET) is a non-volatile memory data storage system that includes ferroelectric material fixed between the FET and gate electrode. FETs control the emission of current and the ferroelectric material helps Fe FET devices to retain the state of transistors even in the absence of electricity.
Ferroelectric RAM is a type of non-volatile random-access memory that possesses the same functionalities as Flash memory, an NVM storage that is erasable and reprogrammable. It uses low power consumption, performs faster, and has greater read/write endurance. Fe RAM contains ferroelectric material, also referred to as lead zirconate titanate (PZT), that makes Fe RAM reliable and power-efficient non-volatile memory.
Mechanically Addressed Systems
The mechanically addressed systems use a recording head or a physical interface that connects between the recording apparatus and the storage device to read and write. Compared to electrically addressed systems, they have a large data storage capacity. Here are some examples of mechanically addressed systems:
The optical disk is a flat and circular storage device that stores data in the form of pits and lands on its surface, an aluminum material. It’s mainly used to distribute data offline and for the purpose of long-term data archival. Optical disks are of three types: read-only, recordable, and re-recordable.
A hard disk also referred to as a hard drive, is a non-volatile magnetic storage device made of aluminum or glass coated with magnetic material. It can store large amounts of data, specifically terabytes of data, as individual blocks and can be retrieved in any order even when there’s no power supply. Hard drives are mostly in the form of rectangular boxes.
Holographic memory or 3-D storage is a high-volume data storage device that makes holographic pictures of data instances by using laser beams and storing them. It allows users to store multiple data in the same area. Holographic memory ensures high content security as it follows the write once and read many methods.
Magnetic tape is a magnetic storage device that uses a thin plastic strip coated with magnetic oxide to store data. Although magnetic tapes are slower than other storage devices, they are highly reliable. It can also be used for archival and backup requirements and is suitable for storing large files.
The difference between non-volatile memory and NVMe
NVMe (Non-Volatile Memory Express) uses a PCI Express bus to read solid state drives and give them much closer access to the CPU, putting the stored data closer to main memory. NVMe was created by technology experts who wanted to speed non-volatile memory use. NVMe has more lines through which data can move from storage into main memory, which allows more data to cross that path at once. Though NVMe is a form of non-volatile memory, it’s only one type and is designed to run on solid-state drives, not other hardware.