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Non-volatile memory
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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 disks, and magnetic tape), optical discs, 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.
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Encyclopedia
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 disks, and magnetic tape), optical discs, 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.
A number of conferences are held every year that focus specifically on non-volatile memory. One of the most prominent is the Non-Volatile Memory Technology Symposium ().
Non-volatile data storage can be categorised in electrically addressed systems read only 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 Read Only Memory, is a type of memory integrated circuit that retains its data when its power supply is switched off....
.
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
Mechanically addressed systems basically use a single contact structure ('head') to read and write on a designated storage medium. Since circuitry layout is not a key factor for data density, the amount of storage is typically much larger than for electrically 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.
Organic
There are polymer printed ferroelectric memory (i.e.Thin Film Electronics).
Specifications
| Specification March 2007 | 2.5" HDD | 1" Microdrive | Flash Memory | Optical Disk | Tape | MRAM |
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| Device Model: | | | | Blu Ray | | |
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| Density (GBit/cm2) | 20.3 | 18.4 | 0.8 | 3.8 | 0.047 | 0.0021 |
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| Capacity (GByte) | 160 | 8 | 2 | 50 | 400 | 0.004 |
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| Price per bit (Eur/GByte) | 1.5 | 9.0 | 6.0 | 1.25 | 0.075 | 35000 |
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| Price per unit (Eur) | 110 | 87 | 14 | 635 | 2340 | 17.4 |
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| Price per medium (Eur) | nd | nd | nd | 40 | 30 | nd |
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| Data rate (Mbit/s) | 540 | 80 | 23 | 144 | 640 | 436 |
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| Access time (ms) | 11 | 12 | 0.025 | 180 | 72000 | 0.035 |
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| Power consumption (W) | 1.8 | 0.6 | 0.1 | 25 | 20 | 0.08 |
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Form factor
h x w x d (cm) | 0.95x7x10 | 0.5x3x4 | 0.1x1.2x2 | 4x15x19 | 2x10x10 | 0.1x1x1.8 |
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