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of SDRAM mounted in a Personal Computer ]]

Computer storage, '''computer memory''', and often casually '''memory''' refer to Computer components, devices and Recording Media that retain digital Data used for computing for some interval of time. Computer storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a Central Processing Unit (CPU), implements the basic Von Neumann computer model used since the 1940s.

In contemporary usage, ''memory'' usually refers to a form of Solid State storage known as Random Access Memory (RAM) and sometimes other forms of fast but temporary storage. Similarly, ''storage'' more commonly refers to Mass Storage - Optical Disc s, forms of Magnetic Storage like Hard Disk s, and other types of storage which are slower than RAM, but of a more permanent nature. Historically, these were respectively called ''primary storage'' and ''secondary storage''.

The contemporary distinctions are helpful, because they are also fundamental to the architecture of computers in general. As well, they reflect an important and significant technical difference between memory and mass storage devices, which has been blurred by the historical usage of the term ''storage''. Nevertheless, this article uses the traditional nomenclature.


PURPOSES OF STORAGE


The fundamental components of a general-purpose computer are Arithmetic And Logic Unit , Control Circuitry , storage space, and input/output devices. If storage is removed, a computer can only function as a Digital Signal Processing device (e.g. Calculator , Media Player ). The ability to store Instruction s that form a computer program, and the information that the instructions manipulate, is what makes Stored Program Architecture computers versatile enough for general purposes.

A Digital Computer represents information using the Binary Numeral System . Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of Bit s, or binary digits, each of which has a value of 1 or 0. The most common unit of storage is the Byte , equal to 8 bits. A piece of information can be manipulated by any computer whose storage space is large enough to accommodate the corresponding Data , or ''the binary representation of the piece of information''. For example, using eight million bits, or about one Megabyte , a typical computer could store a small Novel .

. Additionally, common technology and capacity found in Home Computers of 2005 is indicated next to some items.]] Various forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose, as shown in the diagram.


Primary storage


Primary Storage is directly connected to the Central Processing Unit of the computer. It must be present for the CPU to function correctly. As shown in the diagram, primary storage typically consists of three kinds of storage:
  • Processor Register s are internal to the central processing unit. Registers contain information that the arithmetic and logic unit needs to carry out the current instruction. They are technically the fastest of all forms of computer storage, being switching transistors integrated on the CPU's Silicon Chip , and functioning as electronic "flip-flops".

  • Cache Memory is a special type of internal memory used by many central processing units to increase their performance or "throughput". Some of the information in the main memory is duplicated in the cache memory, which is slightly slower but of much greater capacity than the processor registers, and faster but much smaller than main memory. Multi-level cache memory is also commonly used—"primary cache" being smallest, fastest and closest to the processing device; "secondary cache" being larger and slower, but still faster and much smaller than main memory.

  • Main Memory contains the programs that are currently being run and the data the programs are operating on. In modern computers, the main memory is the electronic solid-state Random Access Memory . It is directly connected to the CPU via a "memory bus" (shown in the diagram) and a "data bus". The arithmetic and logic unit can very quickly transfer information between a processor register and locations in Main Storage , also known as a "memory addresses". The memory bus is also called an Address Bus or Front Side Bus and both busses are high-speed digital "superhighways". Access methods and speed are two of the fundamental technical differences between memory and mass storage devices. (Note that all memory sizes and storage capacities shown in the diagram will inevitably be exceeded with advances in technology over time.)



Secondary storage


''' and DVD drives, are typically even slower than hard disks, although their access speeds are likely to improve with advances in technology. Therefore, the use of virtual memory, which is millions of times slower than "real" memory, significantly degrades the performance of any computer. Virtual memory is implemented by many operating systems using terms like Swap File or "cache file". The main historical advantage of virtual memory was that it was much less expensive than real memory. That advantage is less relevant today, yet most operating systems continue to implement it, despite the significant performance penalties.


Tertiary storage


Tertiary Storage is a system where a robotic arm will "mount" (connect) or "dismount" off-line mass storage media (see the previous item) according to the computer operating system's demands. Tertiary storage is used in the realms of Enterprise Storage and Scientific Computing on large computer systems and business Computer Network s, and is something a typical Personal Computer user never sees firsthand. Nowadays, it is confusingly referred to as ''secondary storage'' (because original ''secondary storage'' now became just ''storage'').


Off-line storage


Off-line Storage exists on media physically removed from the storage device. Off-line storage is used for Data Transfer , archival or Disaster Recovery purposes. In modern computers, CDs, DVDs, Memory Card s, Flash Memory devices including " USB drives", Floppy Disk s, Zip Disk s and Magnetic Tapes are commonly used for off-line mass storage purposes. "Hot-pluggable" USB hard disks are also available. Off-line storage media used in the past include Punched Card s, Microform s, drums.


CHARACTERISTICS OF STORAGE


The division to primary, secondary, tertiary and off-line storage is based on Memory Hierarchy , or ''distance from the central processing unit''. There are also other ways to characterize various types of storage.


Volatility of information

  • Volatile Memory requires constant power to maintain the stored information. Volatile memory is typically used only for primary storage. (Primary storage is not necessarily volatile, even though today's most cost-effective primary storage technologies are. Non-volatile technologies have been widely used for primary storage in the past and may again be in the future.)

  • Non-volatile Memory will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information, and therefore used for secondary, tertiary, and off-line storage.

  • Dynamic memory is volatile memory which also requires that stored information is periodically '' Refreshed '', or read and rewritten without modifications.



Ability to access non-contiguous information

  • Random Access means that any location in storage can be accessed at any moment in the same, usually small, amount of time. This makes Random Access Memory well suited for primary storage.

  • Sequential Access means that the accessing a piece of information will take a varying amount of time, depending on which piece of information was accessed last. The device may need to ''seek'' (e.g. to position the Read/write Head correctly), or ''cycle'' (e.g. to wait for the correct location in a revolving medium to appear below the read/write head).



Ability to change information

  • Read/write storage, or '''mutable storage''', allows information to be overwritten at any time. A computer without some amount of read/write storage for primary storage purposes would be useless for many tasks. Modern computers typically use read/write storage also for secondary storage.

  • Read only storage retains the information stored at the time of manufacture, and '''write once storage''' ( WORM ) allows the information to be written only once at some point after manufacture. These are called '''immutable storage'''. Immutable storage is used for tertiary and off-line storage. Examples include CD-R .

  • Slow write, fast read storage is read/write storage which allows information to be overwritten multiple times, but with the write operation being much slower than the read operation. Examples include CD-RW .



Addressability of information

  • In location-addressable storage, each individually accessible unit of information in storage is selected with its numerical Memory Address . In modern computers, location-addressable storage usually limits to primary storage, accessed internally by computer programs, since location-addressability is very efficient, but burdensome for humans.

  • In File System Storage , information is divided into '' Files '' of variable length, and a particular file is selected with Human-readable directory and file names. The underlying device is still location-addressable, but the Operating System of a computer provides the file system Abstraction to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems.

  • In Content-addressable Storage , each individually accessible unit of information is selected with a Hash Value , or a short identifier with number? pertaining to the memory address the information is stored on. Content-addressable storage can be implemented using Software (computer program) or Hardware (computer device), with hardware being faster but more expensive option.



Capacity and performance

  • Storage Capacity is the total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of Bit s or Byte s (e.g. 10.4 Megabyte s).

  • Storage Density refers to the compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume (e.g. 1.2 megabytes per square inch).

  • Latency is the time it takes to access a particular location in storage. The relevant Unit Of Measurement is typically Nanosecond for primary storage, Millisecond for secondary storage, and Second for tertiary storage. It may make sense to separate ''read latency'' and ''write latency'', and in case of sequential access storage, ''minimum'', ''maximum'' and ''average latency''.

  • Throughput is the rate at which information can be read from or written to the storage. In computer storage, throughput is usually expressed in terms of ''megabytes per second'' or ''MB/s'', though Bit Rate may also be used. As with latency, ''read rate'' and ''write rate'' may need to be differentiated.



Network connectivity


A secondary or tertiary storage may connect to a computer utilizing Computer Networks .
This concept does not pertain to the primary storage, which is shared between multiple processors in a much lesser degree.
  • Direct-attached Storage (DAS) is a traditional mass storage, that does not use any network. This is still a most popular approach. This term was coined lately, together with NAS and SAN.

  • Network-attached Storage (NAS) is mass storage attached to a computer which another computer can access at file level over a Local-area Network , a private Wide-area Network , or in the case of Online File Storage , over the Internet .

  • --- Network Computer s are computers that do not contain internal mass storage devices. Instead, all data are stored exclusively on a NAS.

  • Storage Area Network (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN is the former presents and manages file systems to client computers, whilst a latter provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity.



TECHNOLOGIES, DEVICES AND MEDIA


Magnetic storage

Magnetic Storage uses different patterns of Magnetization on a Magnetically coated surface to store information. Magnetic storage is ''non-volatile''. The information is accessed using one or more Read/write Head s. Since the read/write head only covers a part of the surface, magnetic storage is ''sequential access'' and must seek, cycle or both. In modern computers, the magnetic surface will take these forms:

In early computers, magnetic storage was also used for primary storage in a form of Magnetic Drum , or Core Memory , Core Rope Memory , Thin Film Memory , Twistor Memory or Bubble Memory . Also unlike today, magnetic tape was often used for secondary storage.


Semiconductor storage

Semiconductor Memory uses Semiconductor -based Integrated Circuit s to store information. A semiconductor memory chip may contain millions of tiny Transistor s or Capacitor s. Both ''volatile'' and ''non-volatile'' forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or Dynamic Random Access Memory . Since the turn of the century, a type of non-volatile semiconductor memory known as Flash Memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers.


Optical disc storage

Optical Disc Storage uses tiny pits etched on the surface of a circular disc to store information, and reads this information by illuminating the surface with a Laser Diode and observing the reflection. Optical disc storage is ''non-volatile'' and ''sequential access''. The following forms are currently in common use:

The following form have also been proposed:


Magneto-optical disc storage

Magneto-optical Disc Storage is optical disc storage where the magnetic state on a Ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is ''non-volatile'', ''sequential access'', slow write, fast read storage used for tertiary and off-line storage.


Ultra Density Optical disc storage

Ultra Density Optical Disc Storage An Ultra Density Optical disc or UDO is a 5.25" ISO cartridge optical disc encased in a dust-proof caddy which can store up to 30 GB of data. Utilising a design based on a Magneto-optical disc, but utilising Phase Change technology combined with a blue violet laser, a UDO disc can store substantially more data than a Magneto-optical disc or MO, because of the shorter Wavelength (405 Nm ) of the blue-violet laser employed. MOs use a 650-nm-wavelength red laser. Because its beam width is shorter when burning to a disc than a red-laser for MO, a blue-violet laser allows more information to be stored digitally in the same amount of space.


Optical Jukebox storage

Optical Jukebox Storage is a robotic storage device that utilizes optical disk device and can automatically load and unload optical disks and provide terabytes of near-line information. The devices are often called optical disk libraries, robotic drives, or autochangers. Jukebox devices may have up to 1,000 slots for disks, and usually have a picking device that traverses the slots and drives. The arrangement of the slots and picking devices affects performance, depending on the space between a disk and the picking device. Seek times and transfer rates vary depending upon the optical technology.
Jukeboxes are used in high-capacity archive storage environments such as imaging, medical, and video. HSM is a strategy that moves little-used or unused files from fast magnetic storage to optical jukebox devices in a process called migration. If the files are needed, they are migrated back to magnetic disk.


Other early methods

Paper Tape and ''' Punch Card s''' have been used to store information for automatic processing since the 1890s , long before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium, and was read by electrically (or, later, optically) sensing whether a particular location on the medium was solid or contained a hole.

Williams Tube used a Cathode Ray Tube , and ''' Selectron Tube ''' used a large Vacuum Tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and Selectron tube was expensive.

Delay Line Memory used Sound Wave s in a substance such as Mercury to store information. Delay line memory was ''dynamic volatile'', ''cycle sequential'' read/write storage, and was used for primary storage.


Other proposed methods

Phase-change Memory uses different mechanical phases of Phase Change Material to store information, and reads the information by observing the varying Electric Resistance of the material. Phase-change memory would be ''non-volatile'', ''random access'' read/write storage, and might be used for primary, secondary and off-line storage.

Holographic Storage stores information optically inside Crystal s or Photopolymer s. Holographic storage can utilize the whole volume of the storage medium, unlike optical disc storage which is limited to a small number of surface layers. Holographic storage would be ''non-volatile'', ''sequential access'', and either write once or read/write storage. It might be used for secondary and off-line storage.

Molecular Memory stores information in Polymer s that can store electric charge. Molecular memory might be especially suited for primary storage.


Primary storage topics



Secondary, tertiary and off-line storage topics



Data storage conferences



REFERENCES