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Non-volatile memory retains data and information when the power is turned off. Common types of non-volatile memory include read-only memory (ROM), flash memory, ferroelectric RAM (FRAM), magnetic storage like hard drives, optical discs, and early computer storage methods like paper tape and punched cards.
In computing, memory and storage devices are categorized as either volatile or non-volatile. Volatile memory only retains data and information while powered on, while non-volatile memory can retain data for extended periods of time without power.
When a computing device like a personal computer is turned off, any information stored in volatile random access memory (RAM) is lost. However, information written to non-volatile storage will persist through being powered off and on again. This makes non-volatile memory critical for long-term information storage in computing devices.
There are many different types of non-volatile memory and storage, ranging from ancient methods like paper all the way to modern flash memory. Each has advantages and disadvantages in terms of speed, cost, density, longevity, and durability. Selecting the best non-volatile storage technology depends on the specific use case.
Types of Non-Volatile Memory
Read Only Memory (ROM)
Read Only Memory (ROM) is a type of storage media that permanently stores data on integrated circuits. It is non-volatile memory, as the data remains even when power is removed. Once data has been written to a ROM chip, it cannot be modified or erased.
Some key attributes of ROM:
- Data cannot be electrically modified after the manufacturing process
- Slower access speeds than volatile RAM
- Lower cost per bit than volatile RAM
- Used for permanent data storage, like firmware or operating systems
There are several types of ROM, including:
- PROM (Programmable Read-Only Memory) – Can only be written to once, during manufacturing
- EPROM (Erasable Programmable Read-Only Memory) – Can be erased with ultraviolet light exposure
- EEPROM (Electrically Erasable Programmable Read-Only Memory) – Can be electrically erased for reuse
Flash Memory
Flash memory is an advanced form of EEPROM designed for high speed and efficiency. It gets its name from its ability to be erased at high speeds using electrical signals. It is widely used for storage in consumer devices like cameras, phones, USB drives, etc.
Advantages of flash memory include:
- High read/write speeds compared to other non-volatile memory
- Shock resistance
- Low power consumption
- High storage density capabilities
There are two main types of flash memory:
- NAND – Very high densities, used for mass storage. Slower speeds.
- NOR – Higher read speeds, executes code better. Used when random reads are required.
Ferroelectric RAM (FRAM)
Ferroelectric RAM, or FRAM, uses a ferroelectric film capacitor to store data. It offers non-volatile storage like flash memory, but can write data much faster. Read and write speeds are comparable to volatile DRAM.
Other advantages of FRAM include:
- Extremely high write endurance – 10^14 to 10^16 cycles
- Very low power usage
- Immunity to radiation effects
FRAM is used in applications like embedded systems, security systems, electricity meters, and networking hardware.
Magnetic Storage
Magnetic storage takes advantage of the ability of certain materials to have their magnetic polarity changed when in proximity to a magnetic field. By changing the magnetic polarity of a given area on the media, binary data can be written. The polarity can later be read to retrieve the written information.
Common examples of magnetic storage include:
- Hard Disk Drives – Common in computers, uses circular platters coated with magnetic film to store data. Data is accessed using a read/write head.
- Floppy Disk – A flexible disk coated with magnetic oxide with a read/write head. Portable storage used on older computers.
- Magnetic Tape – Polyester film tape with a magnetic coating. Used for offline, archival, and backup storage.
- Magnetic Stripes – Specialized magnetic tape, often used on credit cards and ID cards.
Key attributes of magnetic storage include:
- Non-volatile storage persists after power is removed
- Relatively inexpensive per megabyte compared to other storage
- Moderately high data read/write speeds
Optical Discs
Optical storage uses laser light to read and write data. By changing the reflectivity of a point on the storage media, binary 1s and 0s can be represented. Types of optical media include:
- CD – Compact Disc; stores up to 700MB of data
- DVD – Digital Versatile Disc; stores up to 4.7GB per layer
- Blu-ray – Laser with shorter wavelength allows for higher densities, up to 25GB per layer
Advantages of optical discs include:
- Inexpensive per megabyte
- Long shelf life; estimated to be 50+ years
- Good for archival storage
- Highly portable and removable
However, they suffer from slower random access times compared to magnetic hard drives and flash memory. Optical discs like CDs and DVDs have largely been replaced by flash drives and internet storage in consumer devices.
Early Computer Storage
Early computing predated solid state memory and magnetic disks. Some of the first methods of non-volatile storage include:
- Paper Tape – Long strips of paper encoded with holes representing data. Used with mainframe computers.
- Punch Cards – Cards with holes punched in specific positions to store data and program instructions.
- Delay Line Memory – Data written as sound waves propagating through a medium like mercury. Could store about 72 bits.
These methods relied on mechanical systems to read and write data. Although incredibly slow by modern standards, they allowed early computers to store information outside of temporary memory for the first time.
How Non-Volatile Memory Works
There are a few key principles that allow non-volatile memory to persist even when power is removed:
- Electronic Storage – ROM and flash memory use transistors and gates to permanently store charge, encoding a binary 0 or 1.
- Magnetization – Magnetic drives and tape have their polarity adjusted by a write head to encode data.
- Optical Reflection – Discs have their reflectivity adjusted by a laser to store data.
- Mechanical Representations – Paper/punch tape/cards encode data in physical holes and patches.
In every case, the storage medium is physically altered to encode the 0s and 1s representing data. When power is lost, these physical representations remain in place, allowing the data to be read again later.
Volatile memory like DRAM instead stores data in the electrical charge of capacitors. These capacitors must be periodically refreshed by reading and rewriting the data. If power is lost, the charge dissipates and the stored data is lost.
Choosing the Right Non-Volatile Storage
There are many factors to consider when selecting an appropriate type of non-volatile storage. The optimal choice depends on parameters like:
- Read and write speed performance
- Ability to be rewritten
- Storage capacity
- Physical size and portability
- Power consumption
- Cost per megabyte
For example, flash drives provide a good balance of speed, rewritability, size, and cost for portable consumer storage needs. But for high capacity archival storage, magnetic tape drives are much more affordable.
Speed and Performance
If storage speed is critical, flash memory provides the best performance, with speeds nearly as fast as volatile RAM. It offers much faster random reads and writes compared to hard drives and optical media.
Hard disk drives are moderately fast, while optical discs are generally the slowest due to the physical motion required by their drives. Older options like paper tape were glacially slow by modern standards.
Ability to Rewrite
Most non-volatile storage can be repeatedly overwritten, but some types like PROM and paper tape can only be written once. Re writable ability allows the media to be reused after deleting old data.
Flash memory and hard drives can generally be rewritten hundreds of thousands of times for consumer models, and millions of times for high endurance models.
Capacity
Hard disk drives offer the highest data density and overall capacity for a single drive, ranging from gigabytes to tens of terabytes. Online cloud storage can offer effectively unlimited capacity.
At the lower end, paper tape and punch cards stored only dozens of kilobytes. Capacity must be considered based on application needs.
Physical Size
Size is an important factor for mobile and consumer uses. Flash memory has the advantage of being extremely compact while also offering high capacities up to terabytes for SSDs (solid state drives).
Optical discs are generally larger in diameter but very thin and light. Older media like tape is bulky by comparison.
Power
Power consumption should be minimized for battery-powered devices. Flash memory consumes less power than magnetic hard drives. Optical drives also tend to use less energy when not actively reading or writing.
Newer storage technologies focus on lowering energy use for green computing.
Cost
Magnetic tape is generally the lowest cost per gigabyte, followed by hard drives. Online cloud backup can also offer relatively inexpensive capacity with easier scalability.
Rewritable flash drives and SSDs are moderately priced but offer superior speed. Optical discs are slowing becoming obsolete as higher density flash memory drops rapidly in cost.
The Future of Non-Volatile Memory
Non-volatile storage has made incredible progress over the decades, evolving from paper punch cards to high speed terabyte SSDs. Computing devices would be severely limited without the ability to persist data when powered off.
Ongoing research is focused on improving various aspects of non-volatile memory:
- Increasing read/write speeds – New technologies like Magnetoresistive RAM aim for DRAM-like speed.
- Raising maximum capacities – Storing more data in smaller spaces through greater density.
- Faster startup times – Consumer devices with non-volatile memory can boot instantly.
- Lower power – Reducing electricity consumption for green efficiency.
- Increased longevity – Media capable of lasting decades without decay.
There are many promising technologies emerging from R&D labs that could replace conventional hard drives, flash memory, and optical discs in the future:
- Magnetoresistive RAM (MRAM) – Combines non-volatility with DRAM-like speeds. High costs currently limit applications.
- Ferroelectric FETs – Transistors with ferroelectric properties for fast non-volatile memory.
- Racetrack memory – Magnetic domains shifted using spin currents for high density storage.
- Carbon nanotube RAM – SWCNTs used as electrode material for high performance RAM.
Software and operating systems will also continue adapting to best take advantage of emerging storage hardware improvements. The rapidly evolving landscape of non-volatile memory technologies will shape computing for decades to come.
Conclusion
Non-volatile memory encompasses a wide range of technologies united by their ability to retain data when powered off. Finding the right non-volatile storage involves balancing factors like capacity, speed, power, longevity, and cost for the target application.
While volatile RAM still dominates peak data access performance thanks to raw speed, non-volatile memory offers the persistent storage critical for long term data storage. Ongoing research aims to close this gap by improving non-volatile technology.
From humble beginnings like paper tape and punch cards, non-volatile memory has become an indispensable cornerstone of the computing world. Both portable consumer devices and large-scale data centers rely on non-volatile memory today and will continue leveraging its future evolution for persistence and performance.
