Report on Computer Storage and Memory.txt

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UNIVERSITY OF SOUTHEASTERN PHILIPPINESREPORT ON MEMORY AND STORAGE

REVIEW OF RELATED LITERATUREGROUP 2BS GEOLOGY 1-2

STORAGEStorage, or more rightly termed as computer data storage, is the technol

ogy that consists the different components in the different kinds of computer and even recording media used to retain different amounts of digital data and information. It may serve as one of the core functions and also one of the fundamental components of the computers we use. The data that is sent to the storage areaof the computers is being controlled and manipulated by the computers central processing unit, commonly abbreviated as the CPU. The principle of having a storage hierarchy, done either from the lowest to the highest or the highest to the lowest, is followed and practiced in most computers nowadays. Fast yet very costlysmall storage unit options are being placed and installed closer to the centralprocessing unit. The larger, easier to maintain units are being placed fartheraway from the central processing unit since it is being considered as one of thecheapest ones available in the market.

MEMORYMemory, or computer memory, is a temporary storage area. It holds the da

ta and instructions that the central processing unit needs. Before a program canbe run, the program is loaded from some storage medium into the memory. This allows the central processing unit direct access to the program. The term "memory", meaning primary memory is often associated with addressable semiconductor memory, i.e. integrated circuits consisting of silicon-based transistors, used for example as primary memory but also other purposes in computers and other digitalelectronic devices. Nearly everything a computer programmer does requires him orher to consider how to manage memory. Even storing a number in memory requiresthe programmer to specify how the memory should store it.

HISTORY OF COMPUTER STORAGE AND MEMORY DEVICESMagnetic Tape

The magnetic tape served as one of the breakthrough technologies in the

development of computer data storage. It was developed in the year 1928 by German-Austrian engineer Fritz Pfleumer. It is one of the earliest forms of media made for magnetic tape recordings. It was made using thin magnetized coatings of long, narrow strips of plastic films.

It was based upon the invention of magnetic wire tape recording, createdby Valdemar Poulsen in the year 1898. An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device thatrecords and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage. In its present day form, it records a fluctuating signal by moving the tape across a tape head that polarizes the magnetic domains in the tape in proportion to the audio signal.The use of magnetic tape for sound recording originated around 1930. Poulsen inv

olved the usage of media that moves in a constant velocity past a recorder, therefore producing an electrical signal, which is related to the sound that is to be recorded, including a pattern of magnetic fields similar to the said signal. Since some early refinements improved the fidelity of the reproduced sound, magnetic tape has been the highest quality analogue sound recording medium available.Prior to the development of magnetic tape, magnetic wire recorders had successfully demonstrated the concept of magnetic recording, but they never offered audio quality comparable to the other recording and broadcast standards of the time.Pfleumer then had the idea of using ferric oxide coated long strips of paper touse in the magnetic tape, making it as a base for the designs used in modern rec


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ording machines.

Magnetic DrumA magnetic drum was invented by Austrian Gustav Tauschek in Austria in t

he year 1932. It was considered as a form of an obsolete magnetic data storage device. It is a large metal cylinder that is coated on the outside surface with aferromagnetic recording material. It could be considered the precursor to the hard disk platter, but in the form of a drum rather than a flat disk. In many cases a row of fixed read-write heads runs along the long axis of the drum, one foreach track.

In many cases of creating machines, a drum is the one responsible for forming the main working memory of the said machine. Different data, programs, andinformation can be loaded on to or off the drum using different media inventedin the past like that of the punched cards system created by Joseph Marie Jacquard. These were commonly used for the main computer memory that can be referred to as magnetic drum machines.

Tauscheks magnetic drums original capability of storing data and information can be estimated to be at an approximate value of five hundred thousand (500,000) bits, or it can be converted to 62.5 kilobytes. It was used as late as 1980in PDP-11/45 machines which used Tauscheks invention for changing and swapping data.

Williams TubeProfessor Frederick C. Williams and colleagues at Manchester University

in the United Kingdom developed the first random access computer storage, through using electrostatic cathode-ray display tubes as digital stores. By 1948, storage of 1024 bits was successfully implemented. William's colleague Tom Kilburn made improvements that increased the capacity to 2048 bits. The Williams-Kilburntubes (commonly known as Williams tubes) were used on several of the early stored program computers, including the Manchester 'Baby' (1948) and the Manchester Mark I which became operational in 1949, and the Institute of Advanced Study (IAS) machine spearheaded by von Neumann at Princeton, finally completed in 1951.

The Williams tube depends on an effect called secondary emission. When adot is drawn on a cathode ray tube, the area of the dot becomes slightly positively charged and the area immediately around it becomes slightly negatively charged, creating a charge well. The charge well remains on the surface of the tubefor a fraction of a second, allowing the device to act as a computer memory. The

lifetime of the charge well depends on the electrical resistance of the insideof the tube. The dot can be erased by drawing a second dot immediately next to the first one, thus filling the charge well. Most systems did this by drawing a short dash starting at the dot position, so that the extension of the dash erasedthe charge initially stored at the starting point. Information is read from thetube by means of a metal pickup plate that covers the face of the tube. Each time a dot is created or erased, the change in electrical charge induces a voltagepulse in the pickup plate. Since this operation is synchronised with whicheverlocation on the screen is being targeted at that moment, it effectively reads the data stored there. Because the electron beam is essentially inertia-free, andthus can be steered from location to location very quickly, there is no practical restriction in the order of positions so accessed, hence the so-called ?random-access? nature of the lookup.

Williams tubes differ in the material they are made from. Others were made from phosphor coatings; others were not. This material difference did not affect the outcome of the tubes performance in the operating machines.

Selectron TubeThe Selectron tube was an early form of computer storage developed by Radio Corporation of America.Like the Williams-Kilburn tube, the Selectron was also a random access storage device. Development started in 1946 with a planned production of 200 by the end of the year, but production problems meant that they were still not available by


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the middle of 1948. By that time their primary customer, John von Neumann's IASmachine, was forced to switch to the Williams-Kilburn tube for storage, and RCAeventually had to scale down the Selectron from storing 4096 bits, to 256. Thissmaller version saw use in a number of IAS-related machines, but finally RCA gave up on the concept.The original 4096-bit Selectron was a large (5 inch by 3 inch) vacuum tube witha cathode running up the middle, surrounded by two separate sets of wires forming a cylindrical grid, a dielectric material outside of the grid, and finally a cylinder of metal conductor outside the dielectric, called the signal plate.The two sets of orthogonal grid wires were normally "biased" slightly positive,so that the electrons from the cathode could flow through the grid and reach thedielectric. The continuous flow of electrons allowed the stored charge to be continuously regenerated by the secondary emission of electrons. To select a bit to be read from or written to, all but two adjacent wires on each of the two grids were biased negative, allowing current to flow to the dielectric at one location only.

A cross-section of a selectron tube

Delay Line MemoryThe basic concept of a delay-line memory consists of inserting an inform

ation pattern into a path which contains delay. If the end of the delay path isconnected back to the beginning through amplifying and timing circuits, a closed

loop is formed allowing for recirculation of the information pattern. A delay-line memory resembles the human device of repeating a telephone number to one's self from the time it is found in the directory until it has been dialed. The delay medium should slow the propagation rate of the information sufficiently so that the size of the storage equipment for a large number of pulses is within reason.The first such systems consisted of a column of mercury with piezo crystal transducers (a combination of speaker and microphone) at either end. Data from the computer was sent to the piezo at one end of the tube, and the piezo would pulse and generate a small wave in the mercury. The wave would quickly travel to the far end of the tube, where it would be read back out by the other piezo and sent back to the computer.To form a memory, additional circuity was added at the receiving end to send the

signal back to the input. In this way the pattern of waves sent into the systemby the computer could be kept circulating as long as the power was applied. Thecomputer would count the pulses by comparing to a master clock to find the particular bit it was looking for.Mercury was used because the acoustic impedance of mercury is almost exactly thesame as that of the piezo-electric quartz crystals; this minimized the energy loss and the echoes when the signal was transmitted from crystal to medium and back again. The high speed of sound in mercury (1450 m/s) meant that the time needed to wait for a pulse to arrive at the receiving end was less than it would have been a slower medium, such as air, but also that the total number of pulses that could be stored in any reasonably sized column of mercury was limited. Othertechnical drawbacks of mercury included its weight, its cost, and its toxicity.Moreover, to get the acoustic impedances to match as closely as possible, the me

rcury had to be kept at a temperature of 40 degrees Celsius, which made servicing the tubes hot and uncomfortable work. Use of a delay line for a computer memory was invented by J. Presper Eckert in the mid-1940s for use in computers such as the EDVAC and the UNIVAC I. Eckert and John Mauchly applied for a patent for adelay line memory system on October 31, 1947; the patent was issued in 1953. This patent focused on mercury delay lines, but it also discussed delay lines madeof strings of inductors and capacitors, magnetostrictive delay lines, and delaylines built using rotating disks to transfer data from a read head at one pointon the circumference to a write head elsewhere around the circumference.


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A diagram showing how delay line memory storage works

Magnetic Core MemoryMagnetic core memory was the most widely used form of digital computer m

emory from its birth in the early 1950s until the era of integrated-circuit memory began in the early 1970s. Aside from being extremely reliable, magnetic corememory is an appealing technology because it is based on a very simple idea.It was the predominant form of random-access computer memory for 20 years (from19551975). It uses tiny magnetic rings, the cores, through which wires are threaded to write and read information. Each core represents one bit of information. The cores can be magnetized in two different ways (clockwise or counter clockwise) and the bit stored in a core is zero or one depending on that core's magnetization direction. The wires are arranged to allow an individual core to be set toeither a "one" or a "zero", and for its magnetization to be changed, by sendingappropriate electric current pulses through selected wires. The process of reading the core causes the core to be reset to a "zero", thus erasing it.A magnetic core is a ring of ferrite material. It can be permanently magnetisedeither clockwise or anti-clockwise about its axis just as a vertical bar magnetcan be magnetised up or down. We can then turn a magnetic core into a bit of digital memory by letting these two magnetisation states correspond to 0 and 1.The core needs no power to retain its data. In other words, core memory is a form of non-volatile storage like modern hard disk drives, although in its day it fulfilled the high-speedrole of modern RAM. The performance of early core memoriescan be characterized in today's terms as being very roughly comparable to a clo

ck rate of 1 MHz (equivalent to early 1980s home computers, like the Apple II and Commodore 64). Early core memory systems had cycle times of about 6 s, which had fallen to 1.2 s by the early 1970s, and by the mid-70s it was down to 600 ns (0.6 s). Some designs had substantially higher performance: the CDC 6600 had a memory cycle time of 1.0 s in 1964, using cores that required a half-select current of 200 mA. Everything possible was done in order to decrease access times and increase data rates (bandwidth), including the simultaneous use of multiple grids of core, each storing one bit of a data word. For instance, a machine might use 32 grids of core with a single bit of the 32-bit word in each one, and the controller could access the entire 32-bit word in a single read/write cycle.As expected, the cores were much larger physically than those of read-write memory. This type of memory was exceptionally reliable.

The first magnetic core memoryThe IBM version of the magnetic core memory

Hard DiskThe hard disk drive, simply abbreviated as the HDD, is a data storage de

vice used for storing and retrieving digital information using rapidly rotatingdisks (platters) coated with magnetic material. An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocksof data can be stored or retrieved in any order rather than sequentially. An HDD consists of one or more rigid, or very hard way, manners in rapidly rotating disks, may be referred to as platters, with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.

The hard disk uses rigid rotating platters. It stores and retrieves digital data from a planar magnetic surface. Information is written to the disk by transmitting an electromagnetic flux through an antenna or write head that is very close to a magnetic material, which in turn changes its polarization due to the flux. The information can be read back in a reverse manner, as the magnetic fields cause electrical change in the coil or read head that passes over it.A typical hard disk drive design consists of a central axis or spindle upon which the platters spin at a constant speed. Moving along and between the platters on a common armature are the read-write heads, with one head for each platter face. The armature moves the heads in a radiant manner across the platters as they


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spin, allowing each head access to the entirety of the platter.The first computer with a hard disk drive as standard was the IBM 350 Disk File,introduced in 1956 with the IBM 305 computer. This drive had fifty 24 inch platters, with a total capacity of five million characters. In 1952, an IBM engineernamed Reynold Johnson developed a massive hard disk consisting of fifty platters, each two feet wide, which rotated on a spindle at 1200 rpm with read/write heads for the first database running RCAs Bismark computer. The storage capacity of the 305's 50 two-foot diameter disks was 5 megabytes of data.The primary characteristics of an HDD are its capacity and performance. Capacityis specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion bytes). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Performance is specified by thetime to move the heads to a file (Average Access Time) plus the time it takes for the file to move under its head (average latency, a function of the physical rotational speed in revolutions per minute) and the speed at which the file is transmitted (data rate).According t Wikipedia, mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machinesor designed to fit a 19" rack. In 1962, IBM introduced its model 1311 disk, which used 14 inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years. Such large platters were never used with microprocessor-based systems.

With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Thus HDD Form factors initially followed those of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Because there were no smaller floppy disk drives, smaller HDD form factors developed from product offerings or industry standards.These are as follows:8 inch9.5 in 4.624 in 14.25 in (241.3 mm 117.5 mm 362 mm). In 1979, Shugart Associate' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8" FDD.5.25 inch5.75 in 3.25 in 8 in (146.1 mm 82.55 mm 203 mm). This smaller form factor, firsused in an HDD by Seagate in 1980, was the same size as full-height 5 1/4-inch-

diameter (130 mm) FDD, 3.25-inches high. This is twice as high as "half height";i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks(DVD, CD) use the half height 5" dimension, but it fell out of fashion for HDDs.The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (25 mm) and "ultra-low-profile" (20 mm) high versions.3.5 inch4 in 1 in 5.75 in (101.6 mm 25.4 mm 146 mm) = 376.77344 cm. This smaller form tor is similar to that used in an HDD by Rodime in 1983,[72] which was the samesize as the "half height" 3" FDD, i.e., 1.63 inches high. Today, the 1-inch high("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops.2.5 inch2.75 in 0.2750.59 in 3.945 in (69.85 mm 715 mm 100 mm) = 48.895104.775 cm3.

aller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It came to be widely used for HDDs in mobile devices (laptops, music players, etc.) and for solid-state drives (SSDs), by 2008 replacing some 3.5 inch enterprise-class drives. It is also used in the PlayStation 3 and Xbox 360 video game consoles. Drives 9.5 mm high became an unofficial standard for all except the largest-capacity laptop drives (usually having two platters inside); 12.5 mm-high drives, typically with three platters, are used for maximum capacity, but will not fit most laptop computers. Enterprise-class drives can have a height up to 15 mm. Seagate released a 7 mm drive aimed at entry level laptops and high endnetbooks in December 2009. Western Digital released on April 23, 2013 a hard dr


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ive 5 mm in height specifically aimed at UltraBooks.1.8 inch54 mm 8 mm 71 mm = 30.672 cm. This form factor, originally introduced by IntegralPeripherals in 1993, evolved into the ATA-7 LIF with dimensions as stated. Fora time it was increasingly used in digital audio players and subnotebooks, but its popularity decreased to the point where this form factor is increasingly rareand only a small percentage of the overall market.1 inch42.8 mm 5 mm 36.4 mm. This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch"drive in its product literature.0.85 inch24 mm 5 mm 32 mm. Toshiba announced this form factor in January 2004 for use inmobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba manufactured a 4 GB (MK4001MTD) and an 8 GB (MK8003MTD) versionCompact Audio or Cassette Tapes

The compact audio cassette audio storage medium was introduced by Philips in 1963. The compact cassette had originally been intended for use in dictation machines, but soon became, and remains, a popular medium for distributing pre-recorded music. Starting in 1979, Sony's Walkman helped the format become widelyused and popular.The cassette was a great step forward in convenience from reel-to-reel audio tape recording, though because of the limitations of the cassette's size and speed,

it compared poorly in quality. Unlike the open reel format, the two stereo tracks lie adjacent to each other rather than a 1/3 and 2/4 arrangement. These permitted monaural cassette players to play stereo recordings "summed" as mono tracksand permitted stereo players to play mono recordings through both speakers. Thetape is 1/8 inch (3.175 mm) wide, with each stereo track being 1/32 inch (0.79mm) wide and moves at 17/8 inches per second (47.625 mm/s). For comparison, thetypical open reel format was inch (6.35 mm) wide, each stereo track being 1/16 inch (1.5875 mm) wide, and running at either 3 or 7 inches per second (95.25 or 190.5 mm/s). Some machines did use 17/8 inches per second (47.625 mm/s) but the quality was poor.The original magnetic material was based on ferrite (Fe2O3), but then chromium dioxide (CrO2) and more exotic materials were used in order to improve sound quality to try to match or exceed that of vinyl records. Cobalt doped ferrite was in

troduced by TDK and proved very successful. Sony tried a dual layer tape with both ferrite and chrome dioxide. Finally pure metal particles as opposed to oxideformulations were used. These each had different bias and equalization requirements requiring specialized settings. Ferrite tapes use 120 S equalization (known as Type 1), while chrome and cobalt doped tape types require 70 S equalization (Type 2). In practice the cassette shell was modified with indents to automaticallyselect the proper bias and equalization on compatible cassette decks.Many home computers of the 1980s, notably the TRS-80, Commodore 64, ZX Spectrum,Amstrad CPC and BBC Micro, used cassettes as a cheap alternative to floppy disks as a storage medium for programs and data. Data rates were typically 500 to 2000 bit/s, although some games used special faster loading routines, up to around4000 bit/s. A rate of 2000 bit/s equates to a capacity of around 660 kilobytesper side of a 90 minute tape. In 1935, decades before the introduction of the Co

mpact Cassette, AEG released the first reel-to-reel tape recorder (in German: Tonbandgert), with the commercial name "Magnetophon", based on the invention of themagnetic tape (1928) by Fritz Pfleumer, which used similar technology but withopen reels (for which the tape was manufactured by BASF). These instruments werestill very expensive and relatively difficult to use and were therefore used mostly by professionals in radio stations and recording studios. For private use the (reel-to-reel) tape recorder was not very common and only slowly took off from about the 1950s; with prices between 700 and 1,500 DM (which would now be about 1600 to 3400) such machines were still far too expensive for the mass market and their vacuum tube construction made them very bulky. In the early 1960s, howe


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ver, the weights and the prices dropped when vacuum tubes were replaced by transistors. Reel-to-reel tape recorders then became more common in household use, though never but a small fraction of the number of homes using long playing recorddisc players.In 1958, following four years of development, RCA Victor introduced the stereo,quarter-inch, reversible, reel-to-reel RCA tape cartridge. It was a cassette, big (5" 7"), but offered few pre-recorded tapes; despite multiple versions, it failed.In 1962, Philips invented the Compact Cassette medium for audio storage, introducing it in Europe on 30 August 1963 (at the Berlin Radio Show), and in the United States (under the Norelco brand) in November 1964, with the trademark name Compact Cassette. The team at Philip's was led by Lou Ottens.Although there were other magnetic tape cartridge systems, Philips' Compact Cassette became dominant as a result of Philips' decision in the face of pressure from Sony to license the format free of charge. Philips also released the NorelcoCarry-Corder 150 recorder/player in the U.S. in November 1964. By 1966 over 250,000 recorders had been sold in the US alone and Japan soon became the major source of recorders. By 1968, 85 manufacturers had sold over 2.4 million players.In the early years, sound quality was mediocre, but it improved dramatically bythe early 1970s when it caught up with the quality of 8-track tape and kept improving. The Compact Cassette went on to become a popular (and re-recordable) alternative to the 12-inch vinyl LP during the late 1970s.The Hewlett Packard HP 9830 was one of the first desktop computers in the early1970s to use automatically controlled cassette tapes for storage. It could save

and find files by number, using a clear leader to detect the end of tape. Thesewould be replaced by specialized cartridges, such as the 3M DC-series. Many of the earliest microcomputers implemented the Kansas City standard for digital datastorage. Most home computers of the late 1970s and early 1980s could use cassettes for data storage as a cheaper alternative to floppy disks, though users often had to manually stop and start a cassette recorder. Even the first version ofthe IBM PC of 1981 had a cassette port and a command in its ROM BASIC programming language to use it. However, IBM cassette tape was seldom used, as by 1981 floppy drives had become commonplace in high-end machines.The typical encoding method for computer data was simple FSK, which resulted indata rates of typically 500 to 2000 bit/s, although some games used special, faster-loading routines, up to around 4000 bit/s. A rate of 2000 bit/s equates to acapacity of around 660 kilobytes per side of a 90-minute tape.

Among home computers that used primarily data cassettes for storage in the late1970s were Commodore PET (early models of which had a cassette drive built-in),TRS-80 and Apple II, until the introduction of floppy disk drives and hard drives in the early 1980s made cassettes virtually obsolete for day-to-day use in theUS. However, they remained in use on some portable systems such as the TRS-80 Model 100 line - often in microcassette form - until the early 1990s.Floppy disk storage had become the standard data storage medium in the United States by the mid-1980s; for example, by 1983 the majority of software sold by Atari Program Exchange was on floppy. Cassette remained more popular for 8-bit computers such as the Commodore 64, ZX Spectrum, MSX, and Amstrad CPC 464 in many countries such as the United Kingdom (where 8-bit software was mostly sold on cassette until that market disappeared altogether in the early 1990s.) Reliability of cassettes for data storage is inconsistent, with gamers recalling repeated att

empts to load video games. In some countries, including the United Kingdom, Poland, Hungary, and the Netherlands, cassette data storage was so popular that someradio stations would broadcast computer programs that listeners could record onto cassette and then load into their computer.The use of better modulation techniques, such as those used in modern modems, combined with the improved bandwidth and signal to noise ratio of newer cassette tapes, allowed much greater capacities (up to 60 MB) and data transfer speeds of10 to 17 kB/s on each cassette. They found use during the 1980s in data loggersfor scientific and industrial equipment. The cassette was adapted into what is called a streamer cassette, a version dedicated solely for data storage, and used


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chiefly for hard disk backups and other types of data. Streamer cassettes lookalmost exactly the same as a standard cassette, with the exception of having a notch about 1/4 inch wide and deep situated slightly off-center at the top edge of the cassette. Streamer cassettes also have a re-usable write-protect tab on only one side of the top edge of the cassette, with the other side of the top edgehaving either only an open rectangular hole, or no hole at all. This is due tothe whole 1/8 inch width of the tape loaded inside being used by a streamer cassette drive for the writing and reading of data, hence only one side of the cassette being used. Streamer cassettes can hold anywhere from 50 to 160 megabytes ofdata.One of the first German cassettes to be sold for personal computer purposes in 1980

DRAMIn 1966 Robert H. Dennard invented Dynamic Random Access Memory (DRAM) c

ells, one-transistor memory cells that store each single bit of information as an electrical charge in an electronic circuit. This technology permitted major increases in memory density.

DRAM is a type of random access memory that stores each bit of data in a separate capacitor. The number of electrons stored in the capacitor determines whetherthe bit is considered 1 or 0. As the capacitor leaks electrons, the informationgets lost eventually, unless the charge is refreshed periodically.

To store data using this kind of memory, a row is opened and a given column's sense amplifier is temporarily forced to the desired high or low voltage state, thus causing the bit-line to charge or discharge the cell storage capacitor to thedesired value. Due to the sense amplifier's positive feedback configuration, itwill hold a bit-line at stable voltage even after the forcing voltage is removed. During a write to a particular cell, all the columns in a row are sensed simultaneously just as during reading, so although only a single column's storage-cell capacitor charge is changed, the entire row is refreshed (written back in), as illustrated in the figure to the right.Subsequently, DRAM needs to be refreshed once in a while. Typically, manufacturers specify that each row must have its storage cell capacitors refreshed every 64 ms or less, as defined by the JEDEC (Foundation for developing Semiconductor Standards) standard. Refresh logic is provided in a DRAM controller which automat

es the periodic refresh, that is no software or other hardware has to perform it. This makes the controller's logic circuit more complicated, but this drawbackis outweighed by the fact that DRAM is much cheaper per storage cell and becauseeach storage cell is very simple, DRAM has much greater capacity per unit of surface than SRAM.Some systems refresh every row in a burst of activity involving all rows every 64 ms. Other systems refresh one row at a time staggered throughout the 64 ms interval. For example, a system with 213 = 8192 rows would require a staggered refresh rate of one row every 7.8 s which is 64 ms divided by 8192 rows. A few real-time systems refresh a portion of memory at a time determined by an external timer function that governs the operation of the rest of a system, such as the vertical blanking interval that occurs every 1020 ms in video equipment. All methods require some sort of counter to keep track of which row is the next to be refresh

ed. Most DRAM chips include that counter. Older types require external refresh logic to hold the counter.Under some conditions, most of the data in DRAM can be recovered even if the DRAM has not been refreshed for several minutes.

Twistor MemoryTwistor memory is, similar to core memory, formed by wrapping magnetic t

ape around a current-carrying wire. It was developed at Bell Labs, but it was used for only a brief time in the marketplace between 1968 and the mid-1970s. It w


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as often replaced by RAM chips.Twistor memory used the same concept as core memory, but instead of small circular magnets, it used magnetic tape to store the patterns. The tape was wrapped around one set of the wires, say X, in such a way that it formed a 45 degree helix. The Y wires were replaced by solenoids wrapping a number of twistor wires. Selection of a particular bit was the same as in core, with one X and Y line beingpowered, generating a field at 45 degrees. The magentic tape was specifically selected to only allow magnetization along the length of the tape, so only a single point of the twistor would have the right direction of field to become magnetized.In core memory, small ring-shaped magnets - the cores - are threaded by two crossed wires, X and Y, to make a matrix known as a plane. When one X and one Y wireare powered, a magnetic field is generated at a 45-degree angle to the wires. The core magnets sit on the wires at a 45-degree angle, so the single core wrapped around the crossing point of the powered X and Y wires will pick up the induced field.The materials used for the core magnets were specially chosen to have a very "square" magnetic hysteresis pattern. This meant that fields just below a certain threshold will do nothing, but those just above this threshold will cause the core to pick up that magnetic field. The square pattern and sharp flipping states ensures that a single core can be addressed within a grid; nearby cores will seea slightly different field, and not be selected.The basic operation in a core memory is writing. This is accomplished by powering a selected X and Y wire both to the current level that will, by itself, create

the critical magnetic field. This will cause the field at the crossing point tobe greater than the core's saturation point, and the core will pick up the external field. Ones and zeros are represented by the direction of the field, whichcan be set simply by changing the direction of the current flow in one of the two wires.In core memory, a third wire - the sense/inhibit line - is needed to write or read a bit. Reading uses the process of writing; the X and Y lines are powered inthe same fashion that they would be to write a "0" to the selected core. If thatcore held a "1" at that time, a short pulse of electricity is induced into thesense/inhibit line. If no pulse is seen, the core held a "0". This process is distractive; if the core did hold a "1", that pattern is destroyed during the read, and has to be re-set in a subsequent operation.The sense/inhibit line is shared by all of the cores in a particular plane, mean

ing that only one bit can be read (or written) at once. Core planes were typically stacked in order to store one bit of a word per plane, and a word could be read or written in a single operation by working all of the planes at once.A figure showing how a twistor memory looks upclose

Introduced in 1957, the first commercial use was in their 1ESS switch which wentinto operation in 1965. Twistor was used only briefly in the late 1960s and early 1970s, when semiconductor memory devices replaced almost all earlier memory systems. The basic ideas behind twistor also led to the development of bubble memory, although this had a similarly short commercial lifespan.

Bubble Memory

Bubble Memory uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles, which each store one bit of data. Andrew Bobeckinvented the Bubble Memory in 1970. His development of the magnetic core memoryand the development of the twistor memory put him in a good position for the development of Bubble Memory.

In 1967, Bobeck joined a team at Bell Labs and started work on improvingtwistor. He thought that, if he could find a material that allowed the movementof the fields easily in only one direction, a strip of such material could havea number of read/write heads positioned along its edge instead of only one. Patterns would be introduced at one edge of the material and pushed along just as i


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n twistor, but since they could be moved in one direction only, they would naturally form "tracks" across the surface, increasing the areal density. This wouldproduce a sort of "2D twistor". Paul Charles Michaelis working with permalloy magnetic thin films discovered that it was possible to propagate magnetic domainsin orthogonal directions within the film. This seminal work led to a patent application.

The memory device and method of propagation were described in a paper presented at the 13th Annual Conference on Magnetism and Magnetic Materials, Boston, Massachusetts, September 15, 1967. The device used anisotropic thin magneticfilms that required different magnetic pulse combinations for orthogonal propagation directions. The propagation velocity was also dependent on the hard and easy magnetic axes. This difference suggested that an isotropic magnetic medium would be desirable.Starting work extending this concept using orthoferrite, Bobeck noticed an additional interesting effect. With the magnetic tape materials used in twistor the data had to be stored on relatively large patches known as "domains". Attempts tomagnetize smaller areas would fail. With orthoferrite, if the patch was writtenand then a magnetic field was applied to the entire material, the patch would shrink down into a tiny circle, which he called a bubble. These bubbles were muchsmaller than the "domains" of normal media like tape, which suggested that veryhigh area densities were possible.It is conceptually a stationary disk with spinning bits. The unit, only a coupleof square inches in size, contains a thin film magnetic recording layer. Globular-shaped bubbles (bits) are electromagnetically generated in circular strings i

nside this layer. In order to read or write the bubbles, they are rotated past the equivalent of a read/write head.According to the authorities from Bell Lab, five (5) discoveries took place, namely:

1. The controlled two-dimensional motion of single wall domains in permalloy films2. The application of orthoferrites3. The discovery of the stable cylindrical domain4. The invention of the field access mode of operation5. The discovery of growth-induced uniaxial anisotropy in the garnet systemand the realization that garnets would be a practical material

The bubble system cannot be described by any single invention, but in terms of the above discoveries. Andy Bobeck was the sole discoverer of (4) and (5) and co-discoverer of (2) and (3); (1) was performed in P. Bonyhard's group. At one point, over 60 scientists were working on the project at Bell Labs, many of whom have earned recognition in this field. For instance, in September 1974, H.E.D. Scovil, P.C. Michaelis and Bobeck were awarded the IEEE Morris N. Liebmann MemorialAward by the IEEE with the following citation: For the concept and development of single-walled magnetic domains (magnetic bubbles), and for recognition of their importance to memory technology.It took some time to find the perfect material, but they discovered that garnetturned out to have the right properties. Bubbles would easily form in the material and could be pushed along it fairly easily. The next problem was to make themmove to the proper location where they could be read back out twistor was a wir

e and there was only one place to go, but in a 2D sheet things would not be so easy. Unlike the original experiments, the garnet did not constrain the bubbles to move only in one direction, but its bubble properties were too advantageous toignore.The solution was to imprint a pattern of tiny magnetic bars onto the surface ofthe garnet. When a small magnetic field was applied, they would become magnetized, and the bubbles would "stick" to one end. By then reversing the field they would be attracted to the far end, moving down the surface. Another reversal wouldpop them off the end of the bar to the next bar in the line.A memory device is formed by lining up tiny electromagnets at one end with detec


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tors at the other end. Bubbles written in would be slowly pushed to the other, forming a sheet of twistors lined up beside each other. Attaching the output fromthe detector back to the electromagnets turns the sheet into a series of loops,which can hold the information as long as needed.Bubble memory is a non-volatile memory. Even when power was removed, the bubblesremained, just as the patterns do on the surface of a disk drive. Better yet, bubble memory devices needed no moving parts: the field that pushed the bubbles along the surface was generated electrically, whereas media like tape and disk drives required mechanical movement. Finally, because of the small size of the bubbles, the density was in theory much higher than existing magnetic storage devices. The only downside was performance; the bubbles had to cycle to the far end of the sheet before they could be read.One of the limitations of bubble memory was the slow access. A lagre bubble memory would requure large loops, so accessing a bit require cycling through a hugenumber of other bits first. This is why it was considered something that reallydid not totally work out. A conceptual drawing of how a bubble memory works

8Floppy DiskA floppy disk is a data storage device that is composed of a circular pi

ece of thin, flexible (i.e. "floppy") magnetic storage medium encased in a square or rectangular plastic wallet. Floppy disks are read and written by a floppy disk drive.In 1967 IBM started developing a simple and inexpensive system for loading micro

code into their System/370 mainframes. It should be faster and more purpose built than tape drives that could also be used to send out updates to customers for$5. The result of this work was a read-only, 8-inch (20 cm) floppy they called the "memory disk", holding 80 kilobytes in 1971.So the first disks were designed for loading microcodes into the controller of the Merlin (IBM 3330) disk pack file (a 100 MB storage device). So, in effect, the first floppies were used to fill another type of data storage device. Overnight, additional uses for the floppy were discovered, making it the "new" program and file storage medium.The first floppy disk was 8 inches in diameter, and was protected by a flexibleplastic jacket. IBM used this size as a way of loading microcode into mainframeprocessors, and the original 8 inch disk was not field-writeable. Rewriteable disks and drives became useful. Early microcomputers used for engineering, busines

s, or word processing often used one or more 8 inch disk drives for removable storage; the CP/M operating system was developed for microcomputers with 8 inch drives.An 8-inch disk could store about a megabyte; many microcomputer applications didn't need that much capacity on one disk, so a smaller size disk with lower-costmedia and drives was feasible. The 5 inch drive succeeded the 8 inch size in manyapplications, and developed to about the same storage capacity as the original8 inch size, using higher-density media and recording techniques.

5.25Floppy DiskAnother floppy disk size variant was developed.

In 1976 Alan Shugart developed a new floppy disk. The main reason for this development was that the normal 8 inch floppy disk was to make it large for using it

in desktop computers. So the new 5.25 inch floppy disk was born. Its storage capability was 110 kilobytes. These new floppy disk drives were cheaper than the ones for 8 inch floppy disks and replaced them very quickly.At this time only one side of the floppy disk was used. So in 1978 a double-sided drive for reading 5.25 inch floppy was introduced. So the storage capability was increased again to 360 kilobytes.The head gap of an 80-track high-density (1.2 MB in the MFM format) 5 1/4-inch drive is smaller than that of a 40-track double-density (360 KB) drive but can format, read and write 40-track disks well provided the controller supports doublestepping or has a switch to do such a process. A blank 40-track disk formatted


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and written on an 80-track drive can be taken to its native drive without problems, and a disk formatted on a 40-track drive can be used on an 80-track drive. Disks written on a 40-track drive and then updated on an 80 track drive become unreadable on any 40-track drives due to track width incompatibility.Single sided disks were coated on both sides, despite the availability of more expensive double sided disks. The reason usually given for the higher cost was that double sided disks were certified error-free on both sides of the media. Architectural differences among computer platforms negated this claim, however, withRadioShack TRS-80 Model I computers using one side and the Apple II machines the other. Double-sided disks could be used in drives for single-sided disks, oneside at a time, by turning them over (flippy disks); more expensive dual-head drives which could read both sides without turning over were later produced, and later became used universally.

Compact DiscsA compact disc (or CD) is an optical disc used to store digital data, or

iginally developed for storing digital audio.A standard compact disc, often known as an audio CD to differentiate it from later variants, stores audio data is in a format compliant with the red book standard. An audio CD consists of several stereo tracks stored using 16-bit PCM codingat a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120mm, though 80mm versions exist in circular and "business-card" forms. The 120mm discs can hold 74 minutes of audio, and versions holding 80 or even 90 minutes have been introduced. The 80mm discs are used as "CD-singles" or novelty "business-

card CDs". They hold about 20 minutes of audio.Red Book is the standard for audio CDs (Compact Disc Digital Audio system, or CDDA). It is named after one of a set of colour-bound books that contain the technical specifications for all CD and CD-ROM formats. The Red Book was released bySony and Philips in 1980, but the idea of the CD is older. In the 1960s James T.Russell had the idea to use light for recording and replaying music. So he invented in 1970 an optical digital television recording and playback machine, but the world did not jump on. In 1975 representatives of Philips visited Russell athis lab. They discounted his invention but they put million of dollars in development of the CD and presented it together with Sony in 1980. So James T. Russellwas the original inventor of the idea of the CD.The first CD player was called Sony CDP-101. It was presented on the 1st October1982 and was able to play audio CDs. The price was 625 US dollar.

A CD is made from 1.2 millimetres (0.047 in) thick, polycarbonate plastic and weighs 1520 grams. From the center outward, components are: the center spindle hole(15 mm), the first-transition area (clamping ring), the clamping area (stackingring), the second-transition area (mirror band), the program (data) area, and the rim. The inner program area occupies a radius from 25 to 58 mm.A thin layer of aluminium or, more rarely, gold is applied to the surface makingit reflective. The metal is protected by a film of lacquer normally spin coateddirectly on the reflective layer. The label is printed on the lacquer layer, usually by screen printing or offset printing.CD data is represented as tiny indentations known as "pits", encoded in a spiraltrack moulded into the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 m in length. The distance between the tracks, the pitch,

is 1.5 m.Scanning velocity is 1.21.4 m/s (constant linear velocity) equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge. (A disc played from beginning to end slows down during playback.)The program area is 86.05 cm2 and the length of the recordable spiral is (86.05cm2 / 1.6 m) = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74minutes, or 650 MB of data on a CD-ROM. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a narrower track pitch of 1.5 m increases the playing timeto 80 minutes, and data capacity to 700 MB.


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The pits in a CD are 500 nm wide, between 830 nm and 3,000 nm long and 150 nm deepA CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laserthrough the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photodiode, the data can be read from the disc.The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, non-return-to-zero, inverted encoding is used: a change frompit to land or land to pit indicates a one, while no change indicates a seriesof zeros. There must be at least two and no more than ten zeros between each one, which is defined by the length of the pit. This in turn is decoded by reversing the eight-to-fourteen modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw data stored on the disc. These encoding techniques (defined in the Red Book) were originally designed for the CD Digital Audio, but they later became a standard for almost all CD formats (such as CD-ROM).CDs are susceptible to damage during handling and from environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disc. Scratches on the clear side can be repaired by refilling them with similar refractive plastic, or bycareful polishing. The edges of CDs are sometimes incompletely sealed, allowinggases and liquids to corrode the metal reflective layer and to interfere with the focus of the laser on the pits.

The digital data on a CD begins at the center of the disc and proceeds toward the edge, which allows adaptation to the different size formats available. Standard CDs are available in two sizes. By far, the most common is 120 millimetres (4.7 in) in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB (737,280,000 bytes) data capacity. This capacity was reportedly specified by Sonyexecutive Norio Ohga so as to be able to contain the entirety of London Philharmonic Orchestra's recording of Beethoven's Ninth Symphony on one disc. This diameter has been adopted by subsequent formats, including Super Audio CD, DVD, HD DVD, and Blu-ray Disc. 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 24 minutes of music or 210 MB of data but never becamepopular. Today, nearly every single is released on a 120 mm CD, called a Maxi single.Novelty CDs are also available in numerous shapes and sizes, and are used chiefl

y for marketing. A common variant is the "business card" CD, a single with portions removed at the top and bottom making the disk resemble a business card.Physical size Audio Capacity CD-ROM Data Capacity Definition120 mm 7480 min 650700 MB Standard size80 mm 2124 min 185210 MB Mini-CD size80x54 mm 80x64 mm ~6 min 10-65 MB "Business card" size

3.5Floppy DiskIn the early 1980s, a number of manufacturers introduced smaller floppy

drives and media in various formats. A consortium of 21 companies eventually settled on a 3 1/2-inch floppy disk (actually 90 mm wide), similar to a Sony design, but improved to support both single-sided and double-sided media, with formatted capacities of 360 KB and 720 KB respectively. Single-sided drives shipped in

1983, and double sided in 1984. What became the most common format, the double-sided, high-density (HD) 1.44 MB disk drive, shipped in 1986.The first Macintosh computers used single-sided 3.5 inch floppy disks, but with400 KB formatted capacity. These were followed in 1986 by double-sided 800 KB floppies. The higher capacity was achieved at the same recording density by varying the disk rotation speed with arm position so that the linear speed of the headwas closer to constant. Later Macs could also read and write 1.44 MB HD disks in PC format with fixed rotation speed.All 3 1/2-inch disks have a rectangular hole in one corner which, if obstructed,write-enabled the disk. The HD 1.44 MB disks have a second, unobstructed hole i


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n the opposite corner which identifies them as being of that capacity.In IBM-compatible PCs, the three densities of 3 1/2-inch floppy disks are backwards-compatible: higher density drives can read, write and format lower density media. It is physically possible to format a disk at the wrong density, althoughthe resulting disk will not work properly. Fresh disks manufactured as high density can theoretically be formatted at double density only if no information hasbeen written on the disk in high density, or the disk has been thoroughly demagnetized with a bulk eraser, as the magnetic strength of a high density record isstronger and overrides lower density, remaining on the disk and causing problems.The 3,5'' disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being much better protected against unintended physical contact with the disk surface when the disk was handled outside the disk drive. When the disk was inserted, a part inside the drive moved the metal cover aside, giving the drive's read/write heads the necessary access to the magnetic recording surfaces. Adding the slide mechanism resulted in a slight departure fromthe previous square outline. The rectangular shape had the additional merit thatit made it impossible to insert the disk sideways by mistake, as had indeed been possible with earlier formats.Like the 5'', the 3,5'' disk underwent an evolution of its own. They were originally offered in a 360 KB single-sided and 720 KB double-sided double-density format. A newer "high-density" format, displayed as "HD" on the disks themselves andstoring 1440 KB of data, was introduced in the mid-80s. IBM used it on their PS/2 series introduced in 1987.

CD-ROMThe CD-ROM, an abbreviation for Compact Disk Read-Only-Memory, is an optic

al data storage medium using the same physical format as the audio compact discs. Digital information is encoded at near-microscopic size, allowing a large amount of information to be stored. CDs record binary data as tiny pits (or non-pits) pressed into the lower surface of the plastic disc; a semiconductor laser beamin the player reads these. Most CDs cannot be written with a laser, but CD-R discs have coloured dyes that can be burned(written to) once, and CD-RW (rewritable) discs contain phase-change material that can be written and overwritten several times. The standard CD-ROM can hold approximately 650-700 megabytes of data, although data compression technology allows larger capacities. The yellow-book standard for the CD-ROM was first established in 1985 by Sony and Philips.

It may also be known as a pre-pressed compact disc which contains data.Computers can read CD-ROMs, but cannot write on them. CD-ROMs are popularly usedto distribute computer software, including video games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video)is only usable on a computer.

CD-ROMs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. Themost common size of CD-ROM is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes

(e.g., business card-sized media) are also available.Data is stored on the disc as a series of microscopic indentations. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "lands"). Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of thelaser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data.Data stored on CD-ROMs follows the standard CD data encoding techniques describe


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d in the Red Book specification (originally defined for audio CD only). This includes cross-interleaved ReedSolomon coding (CIRC), eight-to-fourteen modulation (EFM), and the use of pits and lands for coding the bits into the physical surface of the CD.The data structures used to group data on a CD-ROM are also derived from the RedBook. Like audio CDs (CD-DA), a CD-ROM sector contains 2,352 bytes of user data, divided into 98 24-byte frames. Unlike audio CDs, the data stored in these sectors corresponds to any type of digital data, not audio samples encoded according to the audio CD specification. In order to structure, address and protect thisdata, the CD-ROM standard further defines two sector modes, Mode 1 and Mode 2,which describe two different layouts for the data inside a sector. A track (a group of sectors) inside a CD-ROM only contains sectors in the same mode, but if multiple tracks are present in a CD-ROM, each track can have its sectors in a different mode from the rest of the tracks. They can also coexist with audio CD tracks as well, which is the case of mixed mode CDs.Both Mode 1 and 2 sectors use the first 16 bytes for header information, but differ in the remaining 2,336 bytes due to the use of error correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM adds a 32-bit cyclic redundancy check (CRC) code for error detection, a and a third layer of ReedSolomon error correction using a Reed-Solomon Product-like Code (RSPC). Mode 1, used mostlyfor digital data, contains 288 bytes per sector for error detection and correction, leaving 2,048 bytes per sector available for data. Mode 2, which is more app

ropriate for image or video data, contains no error detection or correction bytes, having therefore 2,336 available data bytes per sector. Note that both modes,like audio CDs, still benefit from the lower layers of error correction at theframe level.Before being stored on a disc with the techniques described above, each CD-ROM sector is scrambled to prevent some problematic patterns from showing up. These scrambled sectors then follow the same encoding process described in the Red Bookin order to be finally stored on a CD.According to the guidelines on transfer rates:Transfer speed KiB/sMbit/sMiB/s [10]RPM

1 150 1.2288 0.146 2005002 300 2.4576 0.293 400-1,0004 600 4.9152 0.586 8002,0008 1,200 9.8304 1.17 1,6004,00010 1,500 12.288 1.46 2,0005,00012 1,800 14.7456 1.76 2,4006,00020 1,2003,000 up to 24.576 up to 2.93 4,000 (CAV)32 1,9204,800 up to 39.3216 up to 4.69 4,800 (CAV)36 2,1605,400 up to 44.2368 up to 5.27 7,200 (CAV)40 2,4006,000 up to 49.152 up to 5.86 8,000 (CAV)48 2,8807,200 up to 58.9824 up to 7.03 9,600 (CAV)52 3,1207,800 up to 63.8976 up to 7.62 10,400 (CAV)56 3,3608,400 up to 68.8128 up to 8.20 11,200 (CAV)

72 6,75010,800 up to 88.4736 up to 10.5 2,000 (multi-beam)

Digital Audio TapeDigital Audio Tape (DAT or R-DAT) is a signal recording and playback med

ium introduced by Sony in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughlyhalf the size at 73 mm 54 mm 10.5 mm. As the name suggests the recording is digital rather than analog, DAT converting and recording at higher equal or lower sampling rates than a CD (48, 44.1 or 32 kHz sampling rate, and 16 bits quantization) without data compression. This means that the entire input signal is retain


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S tape machines. However, most DDS tape drives cannot retrieve the audio storedon a DAT cartridge.

DDS uses tape with a width of 3.8mm, with the exception of the latest formats, DAT 160 and DAT 320, which are 8mm wide. Initially, the tape was 60 meters (197 feet) or 90 meters (295 ft.) long. Advancements in materials technology have allowed the length to be increased significantly in successive versions. A DDS tape drive uses helical scanning for recording, the same process used by a video cassette recorder (VCR). If errors are present, the write heads rewrite thedata.

There are several generations for the digital data storage:DDS-1 - Stores up to 1.3 GB uncompressed (2.6 GB compressed) on a 60 m c

artridge, 2 GB uncompressed (4 GB compressed) on a 90 m cartridge. The DDS-1 Cartridge often does not have the -1 designation. It can often be recognized by having 4 vertical bars separated from DDS by the words e "Digital Data Storage".

DDS-2 - Stores up to 4 GB uncompressed (8 GB compressed) on a 120 m cartridge.

DDS-3 - Stores up to 12 GB uncompressed (24 GB compressed) on a 125 m cartridge. DDS-3 uses PRML (Partial Response Maximum Likelihood). PRML minimizes electronic noise for a cleaner data recording.

DDS-4 - DDS-4 stores up to 20 GB uncompressed (40 GB compressed) on a 150 m cartridge. This format is also called DAT 40.

DAT 72 - DAT 72 stores up to 36 GB uncompressed (72 GB compressed) on a170 m cartridge. The DAT 72 standard was developed by HP and Certance. It has the same form-factor as DDS-3 and -4 and is sometimes referred to as DDS-5.

DAT-160 - DAT 160 was launched in June 2007 by HP, stores up to 80 GB uncompressed (160 GB compressed). A major change from the previous generations isthe width of the tape. DAT 160 uses 8 mm wide tape in a slightly thicker cartridge while all prior versions use 3.81 mm wide tape. Despite the difference in tape widths, DAT 160 drives are backwards compatible with DAT 72 and DAT 40 (DDS-4)tapes. Native capacity is 80 GB and native transfer rate was raised to 6.9 MB/s, mostly due to prolonging head/tape contact to 180 (compared to 90 previously).DAT-320 - In November 2009 HP launched the new DAT 320 which stores up to 160 GBuncompressed (marketed as 320 GB assuming 2:1 compression).FUTURE GEN 8 - It is expected to store approximately 300 GB uncompressed.

Digital Versatile DiscDVD is the new generation of optical disc storage technology. DVD is ess

entially a bigger, faster CD that can hold cinema-like video, better-than-CD audio, still photos, and computer data. DVD aims to encompass home entertainment, computers, and business information with a single digital format. It has replacedlaserdisc, is well on the way to replacing videotape and video game cartridges,and could eventually replace audio CD and CD-ROM. DVD has widespread support from all major electronics companies, all major computer hardware companies, and all major movie and music studios. With this unprecedented support, DVD became the most successful consumer electronics product of all time in less than three years of its introduction.

According to Wikipedia, the basic types of DVD (12 cm diameter, single-sided or homogeneous double-sided) are referred to by a rough approximation of their capacity in gigabytes. In draft versions of the specification, DVD-5 indeedheld five gigabytes, but some parameters were changed later on as explained abov

e, so the capacity decreased. Other formats, those with 8 cm diameter and hybridvariants, acquired similar numeric names with even larger deviation.The 12 cm type is a standard DVD, and the 8 cm variety is known as a MiniDVD. These are the same sizes as a standard CD and a mini-CD, respectively. The capacity by surface (MiB/cm2) varies from 6.92 MiB/cm2 in the DVD-1 to 18.0 MiB/cm2 inthe DVD-18.As with hard disk drives, in the DVD realm, gigabyte and the symbol GB are usually used in the SI sense (i.e., 109, or 1,000,000,000 bytes). For distinction, gibibyte (with symbol GiB) is used (i.e., 10243 (230), or 1,073,741,824 bytes).Each DVD sector contains 2,418 bytes of data, 2,048 bytes of which are user data


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. There is a small difference in storage space between + and - (hyphen) formats:Designation Sides Layers(total) Diameter(cm) Capacity

(GB)(GiB)DVD-1[27]SS SL 1 1 8 1.46 1.36DVD-2 SS DL 1 2 8 2.66 2.47DVD-3 DS SL 2 2 8 2.92 2.72DVD-4 DS DL 2 4 8 5.32 4.95DVD-5 SS SL 1 1 12 4.70 4.37DVD-9 SS DL 1 2 12 8.54 7.95DVD-10 DS SL 2 2 12 9.40 8.75DVD-14[28]DS SL+DL 2 3 12 13.24 12.33DVD-18 DS DL 2 4 12 17.08 15.90

It can be interpreted in a way that SS means that the disc is single-sided; DS means double-sided; SL means single layered; and DL means dual-layered.

The following table is now for the re-writable discs:Designation Sides Layers(total) Diameter(cm) Capacity

(GB)(GiB)DVD-R SS SL (1.0) 1 1 12 3.95 3.68DVD-R SS SL (2.0) 1 1 12 4.70 4.37DVD-RW SS SL 1 1 12 4.70 4.37DVD+R SS SL 1 1 12 4.70 4.37DVD+RW SS SL 1 1 12 4.70 4.37DVD-R DS SL 2 2 12 8.54 7.96DVD-RW DS SL 2 2 12 8.54 7.96DVD+R DS SL 2 2 12 8.55 7.96DVD+RW DS SL 2 2 12 8.55 7.96DVD-RAM SS SL 1 1 8 1.46 1.36*DVD-RAM DS SL 2 2 8 2.65 2.47*

DVD-RAM SS SL (1.0) 1 1 12 2.58 2.40DVD-RAM SS SL (2.0) 1 1 12 4.70 4.37DVD-RAM DS SL (1.0) 2 2 12 5.16 4.80DVD-RAM DS SL (2.0) 2 2 12 9.40 8.75

The following table is for the capacity differences of the writable DVDs:Type Sectors Bytes kBMBGBKiB

MiBGiBDVD-R SL 2,298,496 4,707,319,808 4,707,319.808 4,707.3204.707 4,596,992 4,489.250 4.384DVD+R SL 2,295,104 4,700,372,992 4,700,372.992 4,700.3734.700 4,590,208 4,482.625 4.378DVD-R DL 4,171,712 8,543,666,176 8,543,666.176 8,543.6668.544 8,343,424 8,147.875 7.957DVD+R DL 4,173,824 8,547,991,552 8,547,991.552 8,547.9928.548 8,347,648 8,152.000 7.961


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The following table shows the transfer rates of the different DVDs:Drive speed Data rate ~Write time (minutes)

Mbit/sMB/sSingle-Layer Dual-Layer1 11.08 1.39 57 1032 22.16 2.77 28 512.4 26.59 3.32 24 432.6 28.81 3.60 22 404 44.32 5.54 14 266 66.48 8.31 9 178 88.64 11.08 7 1310 110.80 13.85 6 1012 132.96 16.62 5 916 177.28 22.16 4 618 199.44 24.93 3 620 221.60 27.70 3 522 243.76 30.47 3 524 265.92 33.24 2 4

Multimedia CardThe Multimedia Card (MMC) is a flash memory card standard. Unveiled in 1

997 by Siemens and SanDisk, it is based on Toshiba's NAND-based flash memory, an

d is therefore much smaller than earlier systems based on Intel NOR-based memorysuch as Compact Flash. MMC is about the size of a postage stamp: 24mm x 32mm x1.5mm. MMC originally used a 1-bit serial interface, but newer versions of the specification allow transfers of 4 or sometimes even 8 bits at a time. They havebeen more or less superseded by SD cards, but still see significant use becauseMMC cards can be used in any device which supports SD cards.Typically, an MMC card is used as storage media for a portable device, in a formthat can easily be removed for access by a PC. MMC cards are currently available in sizes up to and including 2 GB, and are used in almost every context in which memory cards are used, like cell phones, mp3 players, digital cameras, and PDAs. Since the introduction of Secure Digital few companies build MMC slots intotheir devices, but the slightly-thinner, pin-compatible MMC cards can be used inany device that supports SD cards.

In 2004, the Reduced-Size MultiMediaCard (RS-MMC) was introduced as a smaller form factor of the MMC, about half the size: 24 mm 18 mm 1.4 mm. The RS-MMC uses asimple mechanical adapter to elongate the card so it can be used in any MMC (orSD) slot. RS-MMCs are currently available in sizes up to and including 2 GB.The modern continuation of an RS-MMC is commonly known as MiniDrive (MD-MMC). AMiniDrive is generally a microSD card adapter in the RS-MMC form factor. This allows a user to take advantage of the wider range of modern MMCs available to exceed the historic 2 GB limitations of older chip technology.The only significant hardware implementations of RS-MMCs were Nokia and Siemens,who used to use RS-MMC in their Series 60 Symbian smartphones, the Nokia 770 Internet Tablet, and generations 65 and 75 (Siemens). However, since 2006 all of Nokia's new devices with card slots have used miniSD or microSD cards, with the company appearing to abandon the MMC standard in its products. Siemens exited the

mobile phone business completely in 2006. Siemens continue to use MMC for somePLC storage.The Dual-Voltage MultimediaCard (DV-MMC) is one of the first substantial changesin MMC was the introduction of dual-voltage cards that support operations at 1.8 V in addition to 3.3 V. Running at lower voltages reduces the card's energy consumption, which is important in mobile devices. However, simple dual-voltage parts quickly went out of production in favour of MMCplus and MMCmobile which offer capabilities in addition to dual-voltage support.The version 4.x of the MMC standard, introduced in 2005, brought in two very significant changes to compete against SD cards: support for running at higher spee


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ds (26 MHz and 52 MHz) than the original MMC (20 MHz) or SD (25 MHz, 50 MHz) anda four- or eight-bit-wide data bus. Version 4.x full-size cards and reduced-size cards can be marketed as MMCplus and MMCmobile respectively. Version 4.x cardsare fully backward compatible with existing readers but require updated hardware/software to use their new capabilities; even though the four-bit-wide bus andhigh-speed modes of operation are deliberately electrically compatible with SD,the initialization protocol is different, so firmware/software updates are required to support these features in an SD reader.MMCmicro is a micro-size version of MMC. With dimensions of 14 mm 12 mm 1.1 mm,it is even smaller and thinner than RS-MMC. Like MMCmobile, MMCmicro supports dual voltage, is backward compatible with MMC, and can be used in full-size MMC and SD slots with a mechanical adapter. MMCmicro cards have the high-speed and four-bit-bus features of the 4.x spec but not the eight-bit bus, due to the absenceof the extra pins. It was formerly known as S-card when introduced by Samsung on December 13, 2004. It was later adapted and introduced in 2005 by the MultiMediaCard Association (MMCA) as the third form factor memory card in the MultiMediaCard family. MMCmicro appears very similar to microSD but the two formats are not physically compatible and have incompatible pinouts.The MiCard is a backward-compatible extension of the MMC standard with a theoretical maximum size of 2048 GB (2 TB) announced on June 2, 2007. The card is composed of two detachable parts, much like a microSD card with an SD adapter. The small memory card fits directly in a USB port while it also has MMC-compatible electrical contacts, which with an included electromechanical adapter fits in traditional MMC and SD card readers. To date, only one manufacturer has produced card

s in this format. Developed by Industrial Technology Research Institute of Taiwan, as of the announcement 12 Taiwanese companies (including A-DATA Technology, Asustek, BenQ, Carry Computer Eng. Co., C-One Technology, DBTel, Power Digital Card Co., and RiCHIP) had signed on to manufacture the new memory card. However, as of June 2011 none of the listed companies has released any such cards, and norhave any further announcements been made about plans for the format. The card was announced to be available starting in the third quarter of 2007. It was expected to save the 12 Taiwanese companies who plan to manufacture the product and related hardware up to USD 40 million in licensing fees that presumably would otherwise be paid to owners of competing flash memory formats. The initial card wasto have a capacity of 8 GB, while the standard would support sizes up to 2048 GB. It was stated to have data transfer speeds of 480 Mbit/s (60 Mbyte/s), with plans to increase data throughput over time.

Memory StickA Memory Stick is a removable memory card format, launched by Sony in Oc

tober 1998.Typically, a Memory Stick is used as storage media for a portable device, in a form that can easily be removed for access by a PC. For example, Sony digital cameras use Memory Sticks for storing image files. With a Memory Stick reader a user could copy the information from the stick to the PC.The original Memory Sticks were approximately the size and thickness of a stickof chewing gum, and came in sizes from 4 MB up to and including 128 MB. This size limitation became limiting fairly quickly, so Sony introduced the now-uncommonMemory Stick Select, which was similar in concept (if not in execution) to theway in which 5.25" floppy disks used both sides of a disk. A Memory Stick Select

was two (or rarely four) separate 128 MB partitions which the user could switchbetween using a switch on the card. This solution was fairly unpopular, but didallow for users with older Memory Stick devices to use higher-capacity flash memory. The 256 MB Memory Stick Select is still being manufactured by Lexar.The original memory stick was launched in October 1998, was available in sizes up to 128 MB, and a sub-version, Memory Stick Select allowed two banks of 128 MBselectable by a slider switch, essentially two cards squeezed into one. The largest capacity Memory Stick currently available is 64 GB. According to Sony, the Memory Stick PRO has a maximum theoretical size of 2 TB.Typically, Memory Sticks are used as storage media for a portable device, in a f


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orm that can easily be removed for access by a personal computer. For example, Sony digital compact cameras use Memory Stick for storing image files. With a Memory Stick-capable Memory card reader a user can copy the pictures taken with theSony digital camera to a computer. Sony typically includes Memory Stick readerhardware in its first party consumer electronics, such as digital cameras, digital music players, PDAs, cellular phones, the VAIO line of laptop computers, andthe PlayStation Portable.Memory Sticks include a wide range of actual formats.The original Memory Stick is approximately the size and thickness of a stick ofchewing gum. It was available in sizes from 4 MB to 128 MB. The original MemoryStick is no longer manufactured.In response to the storage limitations of the original Memory Stick, Sony introduced the Memory Stick Select. The Memory Stick Select was two separate 128 MB partitions which the user could switch between using a (physical) switch on the card. This solution was fairly unpopular, but it did give users of older Memory Stick devices more capacity. Its size was still the same as the original Memory Stick.The Memory Stick PRO, introduced in 2003 as a joint effort between Sony and SanDisk, would be the longer-lasting solution to the space problem. Most devices that use the original Memory Sticks support both the original and PRO sticks sinceboth formats have identical form factors. Some readers that were not compatiblecould be upgraded to Memory Stick PRO support via a firmware update. Memory Stick PROs have a marginally higher transfer speed and a maximum theoretical capacity of 32 GB, although it appears capacities higher than 4 GB are only available i

n the PRO Duo form factor. High Speed Memory Stick PROs are available, and newerdevices support this high speed mode, allowing for faster file transfers. All Memory Stick PROs larger than 1 GB support this High Speed mode, and High Speed Memory Stick Pros are backwards-compatible with devices that don't support the High Speed mode. High capacity memory sticks such as the 4 GB versions are expensive compared to other types of flash memory such as SD cards and CompactFlash.The Memory Stick Duo was developed in response to Sony's need for a smaller flash memory card for pocket-sized digital cameras, cell phones and the PlayStationPortable. It is slightly smaller than the competing Secure Digital (SD) format and roughly two thirds the length of the standard Memory Stick form factor, but costs more. Memory Stick Duos are available with the same features as the largerstandard Memory Stick, available with and without high speed mode, and with andwithout MagicGate support. The Memory Stick PRO Duo has replaced the Memory Stic

k Duo due to its 128 MB size limitation, but has kept the same form factor as the Duo.The Memory Stick PRO Duo (MSPD) quickly replaced the Memory Stick Duo due to theDuo's size limitation of 128 MB and slow transfer speed. Memory Stick PRO Duosare available in all the same variants as the larger Memory Stick PRO, with andwithout High Speed mode, and with and without MagicGate support.Sony has released two different versions of Memory Stick PRO Duo. One is a 16 GBversion on March 2008 and other is a 32 GB version on August 21, 2009, In 2009Sony and SanDisk also announced the joint development of an expanded Memory Stick PRO format tentatively named "Memory Stick PRO Format for Extended High Capacity" that would extend capacity to a theoretical maximum of 2 terabytes. Sony hassince finalized the format and released its specification under the new name, Memory Stick XC.

On December 11, 2006, Sony, together with SanDisk, announced the Memory Stick PRO-HG Duo. While only serial and 4-bit parallel interfaces are supported in the Memory Stick PRO format, an 8-bit parallel interface was added to the Memory Stick PRO-HG format. Also, the maximum interface clock frequency was increased from40 MHz to 60 MHz. With these enhancements, a theoretical transfer rate of 480 Mbit/s (60 Mbyte/s) is achieved, which is three times faster than the Memory StickPRO format.In a joint venture with SanDisk, Sony released a new Memory Stick format on February 6, 2006. The Memory Stick Micro (M2) measures 15 12.5 1.2 mm (roughly one-quarter the size of the Duo) with 64 MB, 128 MB, 256 MB, 512 MB, 1 GB, 2 GB, 4 GB


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, 8 GB, and 16 GB capacities available. The format has a theoretical limit of 32GB and maximum transfer speed of 160 Mbit/s. However, as with the PRO Duo format, it has been expanded through the XC series as Memory Stick XC Micro and Memory Stick XC-HG Micro, both with the theoretical maximum capacity of 2 TB.On January 7, 2009, SanDisk and Sony announced the Memory Stick XC format (tentatively named "Memory Stick Format Series for Extended High Capacity" at the time). The Memory Stick XC has a maximum 2 TB capacity, 64 times larger than that ofthe Memory Stick PRO which is limited to 32 GB. XC series has the same form factors as PRO series, and supports MagicGate content protection technology as wellas Access Control function as PRO series does. In line with the rest of the industry, the XC series uses the newer exFAT file system due to size and formattinglimitations of FAT/FAT16/FAT32 filesystems used in the PRO series. A maximum transfer speed of 480 Mbit/s (60 Mbyte/s) is achieved through 8-bit parallel datatransfer.Sony announced the release of the Memory Stick PRO-HG Duo HX on May 17th, 2011 which was considered the fastest card ever made by the manufacturer. It measures20 x 31 x 1.6 mm, with 8 GB, 16 GB or 32 GB versions available. Also, the formatoffers a maximum transfer speed of 50 MB per second.

MicrodriveA microdrive is originally a miniaturized hard disk in the format of a C

ompactFlash-Card developed by IBM. The first generation of microdrives had a capacity of 340 MB. This version was already used by the NASA. The next generationwere available with a capacity of 512 MB and 1 GB. Microdrives have a magnetic m

emory with a high capacity and a disc-diameter of 1 inch. These small hard diskscan be easily destroyed by vibrations and too low air pressure. Microdrives are usually used in PDAs and digital cameras.

The Microdrive was developed and launched in 1999 by IBM with a capacityof 170 MB. Capacity expanded to 8 GB by 2006. They weigh about 16 g (~1/2 oz),with dimensions of 42.836.45mm (1.71.4.2in). They were the smallest hard drives in he world at the time. From 1999 to 2003, they were known as IBM Microdrives, andfrom 2003 as Hitachi Microdrives, after Hitachi bought IBM's hard drive division. Microdrive was a registered trademark by IBM and Hitachi for each period.IBM initially released 170 MB and 340 MB models. The next year, 512 MB and 1 GBmodels became available. In December 2002 Hitachi bought IBM's disk drive business, including the Microdrive technology and brand. By 2003, 2GB models were introduced. Over the years, larger sizes have become available.

In 2004, Seagate launched 2.5 and 5 GB models, and tends to refer to them as either 1-inch hard drives, or CompactFlash hard drives due to the trademark issue.These drives are also commonly known as the Seagate ST1. In 2005 Seagate launched an 8 GB model. Seagate also sold a standalone consumer product based on thesedrives with a product known as the Pocket Hard Drive. These devices came in theshape of a hockey puck with an integrated USB2.0 cable.

Universal Serial BusA USB Flash Drive is essentially NAND-type flash memory integrated with

a USB interface used as a small, lightweight, removable data storage device. This hot-swappable, non-volatile, solid-state device is universally compatible withpost-Windows 98 platforms, Macintosh platforms, and most Unix-like platforms.USB Flash Drive are also known as "pen drives", "thumb drives", "flash drives",

"USB keys", "USB memory keys", "USB sticks", "jump drives", "keydrives","vault drives" and many more names. They are also sometimes miscalled memory sticks (a Sony trademark describing a different type of portable memory).A flash drive consists of a small printed circuit board encased with a robust plastic casing, making the drive sturdy enough to be carried around in a pocket, as a keyfob, or on a lanyard. Only the USB connector protrudes from this plasticprotection, and is often covered by a removable plastic cap. Most flash drives feature the larger type-A USB connection, although some feature the smaller "miniUSB" connection.Flash drives are active only when powered by a USB computer connection, and requ


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ire no other external power source or battery power source; key drives are run off the limited supply afforded by the USB connection (5V). To access the data stored in a flash drive, the flash drive must be connected to a computer, either by direct connection to the computer's USB port or via a USB hub.USB flash drives are often used for the same purposes for which floppy disks orCD-ROMs were used, i.e., for storage, back-up and transfer of computer files. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because they have no moving parts. Until about 2005, most desktop and laptop computers were supplied with floppy disk drives in addition to USBports, but floppy disk drives have been abandoned due to their lower capacity compared to USB flash drives.USB flash drives use the USB mass storage standard, supported natively by modernoperating systems such as Linux, OS X, Windows, and other Unix-like systems, aswell as many BIOS boot ROMs. USB drives with USB 2.0 support can store more data and transfer faster than much larger optical disc drives like CD-RW or DVD-RWdrives and can be read by many other systems such as the Xbox 360, PlayStation 3, DVD players and in a number of handheld devices such as smartphones and tabletcomputers.A flash drive consists of a small printed circuit board carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic, metal, or rubberized case which can be carried in a pocket or on a key chain,for example. The USB connector may be protected by a removable cap or by retracting into the body of the drive, although it is not likely to be damaged if unprotected. Most flash drives use a standard type-A USB connection allowing connecti

on with a port on a personal computer, but drives for other interfaces also exist.Trek Technology and IBM began selling the first USB flash drives commercially in2000. Trek Technology sold a model under the brand name "ThumbDrive", and IBM marketed the first such drives in North America with its product named the "DiskOnKey", which was developed and manufactured by M-Systems. IBM's USB flash drivebecame available on December 15, 2000, and had a storage capacity of 8 MB, morethan five times the capacity of the then-common floppy disks.In 2013, most second generation (2nd) USB flash drives have USB 2.0 connectivity. USB 2.0 the Hi-Speed specification has a 480 Megabit per second (Mbit/s) upperbound on the transfer rate, but after protocol overhead, that translates to only 35 Megabytes per second (MB/s) effective throughput. The fastest USB 2.0 flashdrives approach that speed. That is considerably slower than hard disk drive or

Solid-state drive can transfer through a SATA interface.File transfer speeds vary considerably. Speeds may be given in megabytes (Mbyte)per second, megabits per second (Mbit/s) or optical drive multipliers such as "180X" (180 times 150 Kibibyte(KiB) per second). Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that speed. This is about 20 times faster than USB 1.1 "full speed" devices, which are limited to a maximum speed of 12 Mbit/s (1 MB/s with overhead). The speed of the device is significantly affected by the access pattern. For example small writes to random locations are much slower (and cause more wear) than long sequential reads.Like USB 2.0 before it, USB 3.0 offers dramatically improved data transfer ratescompared to its predecessor. It was announced in late 2008, but consumer devices were not available until the beginning of 2010. The USB 3.0 interface specifies transfer rates up to 5 Gbit/s (625 MB/s), compared to USB 2.0's 480 Mbit/s (60

MB/s). All USB 3.0 devices are backward compatible with USB 2.0 ports. Computers with such ports are becoming very popular and common. Many newer laptops and desktops have at least one such port. USB 3.0 port expansion cards are availableto upgrade older systems, and many newer motherboards feature two or more USB 3.0 jacks. Even though the interface allows extremely high data transfer speeds, as of 2011 most USB 3.0 flash drives do not utilize the full speed of the interface due to limitations of their memory controllers (though some four channel memo

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