volatile memory

Non-volatile memory

Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy disk drives, and magnetic tape), optical disc drives, and early computer storage methods such as paper tape and punch cards.

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 (RAM), meaning that when the computer is shut down, anything contained in RAM is lost. Unfortunately, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory.

Several companies are working on developing non-volatile memory systems comparable in speed and capacity to volatile RAM. For instance, IBM is currently developing MRAM (Magnetic RAM). Not only would such technology save energy, but it would allow for computers that could be turned on and off almost instantly, bypassing the slow start-up and shutdown sequence.

Non-volatile data storage can be categorised in electrically addressed systems random access memory and mechanically addressed systems hard disks, optical disc, magnetic tape, Holographic memory and such. Electrically addressed systems are expensive, but fast, whereas mechanically addressed systems have a low price per bit, but are slow. Non-volatile memory may one day eliminate the need for comparatively slow forms of secondary storage systems, which include hard disks.

Electrically addressed

Electrically addressed non-volatile memories based on charge storage can be categorised according to their write mechanism:

Mask-programmed ROM

One of the earliest forms of non-volatile read-only memory, the mask-programmed ROM was prewired at the design stage to contain specific data; once the mask was used to manufacture the integrated circuits, the data was cast in stone (silicon, actually) and could not be changed.

The mask ROM was therefore useful only for large-volume production, such as for read-only memories containing the startup code in early microcomputers. This program was often referred to as the "bootstrap", as in pulling oneself up by one's own bootstraps.

Due to the very high initial cost and inability to make revisions, the mask ROM is rarely if ever used in new designs.

Programmable ROM

See main article Programmable read-only memory.

The next approach was to create a chip which was initially blank; the programmable ROM originally contained silicon or metal fuses, which would be selectively "blown" or destroyed by a device programmer or PROM programmer in order to change 0s to 1s. Once the bits were changed, there was no way to restore them to their original condition. Non-volatile but still somewhat inflexible.

Early PAL programmable array logic chips used a similar programming approach to that used in the fuse-based PROMs.

In most new designs, eraseable memories or one-time programmable chips have replaced the old fuse PROMs.

Erasable PROMs

See main article EPROM.

There are two classes of non-volatile memory chips based on EPROM technology.

UV-erase EPROM

The original erasable non-volatile memories were EPROM's; these could be readily identified by the distinctive quartz window in the centre of the chip package. These operated by trapping an electrical charge on the gate of a field-effect transistor in order to change a 1 to a 0 in memory. To remove the charge, one would place the chip under an intense short-wavelength fluorescent ultraviolet lamp for 20-30 minutes, returning the entire chip to its original blank (all ones) state.

OTP (one-time programmable) ROM

An OTP is electrically an EPROM, but with the quartz window physically missing. Like the fuse PROM it can be written once, but cannot be erased. It has largely replaced PROM chips in electronic production as an EPROM with no window is inexpensive to manufacture and can be programmed using identical equipment to that used to write to the UV-window EPROM.

Electrically erasable PROM

See main article EEPROM.

Electrically erasable PROM's have the advantage of being able to selectively erase any part of the chip without the need to erase the entire chip and without the need to remove the chip from the circuit. While an erase and rewrite of a location appears nearly instantaneous to the user, the write process is slightly slower than the read process; the chip can be read at full system speeds.

The limited number of times a single location can be rewritten is usually in the 10000-100000 range; the capacity of an EEPROM also tends to be smaller than that of other non-volatile memories. Nonetheless, EEPROMs are useful for storing settings or configuration for devices ranging from dial-up modems to satellite receivers.

Flash memory

See main article Flash memory.

The flash memory chip is a close relative to the EEPROM; it differs in that it can only be erased one block or "page" at a time. Capacity is substantially larger than that of an EEPROM, making these chips a popular choice for digital cameras and desktop PC BIOS chips.

Mechanically addressed systems

Tape

See main article Magnetic tape.

Hard disk

See main article Hard disk.

Optical disk

See main article Optical disc.

Nanodrive

See main article IBM Millipede.

Holographic storage

See main article Holographic memory.

Specifications

Specification March 2007 2.5" HDD 1" Microdrive Flash Memory Optical Disk Tape MRAM
Device Model: Hitachi Travelstar 5k160 Hitachi Microdrive 3k8 Hynix HY27UH08AG5M Blu Ray HP Ultrium 960 Freescale MR2A16A
Density (GBit/cm2) 20.3 18.4 0.8 3.8 0.047 0.0021
Capacity (GByte) 160 8 2 50 400 0.004
Price per bit (Eur/GByte) 1.5 9.0 6.0 1.25 0.075 35000
Price per unit (Eur) 110 87 14 635 2340 17.4
Price per medium (Eur) nd nd nd 40 30 nd
Data rate (Mbit/s) 540 80 23 144 640 436
Access time (ms) 11 12 0.025 180 72000 0.035
Power consumption (W) 1.8 0.6 0.1 25 20 0.08
Form factor
h x w x d (cm)
0.95x7x10 0.5x3x4 0.1x1.2x2 4x15x19 2x10x10 0.1x1x1.8

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