How cumbersome it would be if one had to re-enter all data on hard drive whenever they used a computer: all types of nonvolatile memory storage are designed to ensure that you don’t have to face this problem. Non-volatile memory or NVM is any memory which can store your data and keep it safe regardless of power. Unlike Random Access Memory (RAM) which loses any data when computer is turned off, NVM is most suitable for long term persistent storage.
There are two broad types of nonvolatile memory. One is electrically addressed systems. These types of nonvolatile memory can be categorized on the basis of their write mechanism, and includes read-mostly devices, flash memory, ferroelectric RAM (FRAM) and magnetoresistive RAM (MRAM). The other type is mechanically addressed systems which are characterized by a contact structure for reading and writing on a designated storage location.
An erasable programmable read-only memory or, EPROM is a ROM on which data can be changed more than once. It requires a special programmer circuit to write new data. The old data can be erased using ultraviolet rays because of the quartz window that the EPROM has. However, the whole device is cleared at one time. A One-Time Programmable (OTP) device also uses EPROM chip but removes the quartz window making it cheaper.
An electrically erasable programmable read-only memory (EEPROM) makes use of electrical signals for erasing data. But this ROM requires much more time to erase data and to write new data.
These types of nonvolatile memory are very similar to EEPROM. One major difference is that it can erase only one block or page at a time. Also, its storage capacity is much larger than an EEPROM. This makes it used popularly in digital cameras and also in desktop PC BIOS chip.
These devices use two different types of logical technologies to map data: NOR and NAND. NAND flash reads and writes at a high speed but it is slower than NOR when on read. However, NAND flash reads faster than it writes. NAND flash is less expensive than NOR flash at high densities and also provides much higher capacity for the same size of silicon.
Ferroelectric RAM (FRAM)
These types of nonvolatile memory are similar to DRAM devices in construction but contains a thin ferroelectric film of lead zirconatetitanate (PZT). Because of this PZT layer, FRAM retains the data in its memory even when power is turned off unlike the usual random access memory (RAM). It has high endurance, extremely low power consumption and also gamma radiation tolerance.
Magnetoresistive RAM (MRAM)
These types of nonvolatile memory are the result of newest approaches to NVMs designed to encompass all desirable qualities of other types of memory devices. These include the feature of non-volatility, high endurance of data, high speed of reading and writing, low cost. MRAMs store data in magnetic tunnel junctions (MTJs).
Mechanically addressed systems
These include hard disks, magnetic tape, optical disks, holographic memory etc. These are cheaper than electrically addressed systems but very slow and may be eliminated in future.