Information Technology in Theory By Pelin Aksoy and
Information Technology in Theory By Pelin Aksoy and Laura De. Nardis Chapter 4 Computer Hardware
Objectives • Identify some important historical milestones in the development of computers • Understand logic gates and how computers use them to process information • Identify the fundamental components of a computer • Understand how computer performance depends on factors such as processor speed, chip set, bus width, bus speed, number of CPUs, and instruction set Information Technology in Theory 2
Objectives (continued) • Examine future trends in computing and societal issues related to these advances • Understand the physical principles of data storage and the difference between mechanical, magnetic, optical, and electronic storage Information Technology in Theory 3
A Brief History of Computers • Computing devices trace back at least to the Abacus • Mechanical calculation tools in the 1600 s included William Oughtred’s slide rule, Blaise. Pascal’s arithmetic machine, and Leibniz’s step reckoner • Calculation tools of the 1800 s included Charles Babbage’s Difference and Analytical engines • Augusta Ada Byron, the Countess of Lovelace, worked with Babbage in programming the analytical engine • Herman Hollerith developed the tabulating machine to tabulate the 1890 US census data using punch cards Information Technology in Theory 4
A Brief History of Computers (continued) • The punch card concept is attributed to Joseph Marie Jacquard • The 1940 s spurred big advancements in computing • John Vincent Atanasoff and Clifford Berry at the Iowa State College built the ABC • Konrad Zuse built the Z 3 • Howard Aiken built the Mark I, which was programmed by Grace Murray Hopper • British scientists built the Colossus • John Mauchly and J. Presper Eckert built the ENIAC Information Technology in Theory 5
A Brief History of Computers (continued) • The transistor and integrated circuit enable smaller computers compared to vacuum tubes • Popular Electronics ran a feature story about a home “do-it-yourself” computer kit called the Altair • Radio Shack began to sell the TRS-80 home computer, and Apple Computer Inc. introduced its own line of home computers • These advances, and the introduction of IBM’s personal computer (PC) in 1981, contributed to the explosion of computers in all facets of modern life Information Technology in Theory 6
A Brief History of Computers (continued) • A variety of social and economic factors, along with advances in microprocessors and distributed networks, led to the introduction of mobile, wireless, and handheld computers Information Technology in Theory 7
Digital Logic • A computer has one or more main chips called the microprocessor, which performs most of the processing, and several other supporting chips • Integrated circuits such as microprocessors are built by combining transistors together to create logic gates • These basic building blocks can be combined to create successively larger building blocks, which may subsequently be combined to create complex circuitry and packaged with other components within a carrier to create integrated circuits such as microprocessors Information Technology in Theory 8
Digital Logic (continued) Information Technology in Theory 9
Digital Logic (continued) • Integrated circuits are built using the following types of logic gates • The difference between these logic gates is essentially the way the transistors within them are connected to each other • NOT gate (inverter) • AND gate • NAND (not AND) gate • OR gate • NOR (not OR) gate • Exclusive OR (XOR) gate • XNOR (exclusive NOR) gate Information Technology in Theory 10
Digital Logic (continued) Information Technology in Theory 11
Digital Logic (continued) Information Technology in Theory 12
Digital Logic (continued) Combining log gates to create a 1 -bit binary adder Information Technology in Theory 13
Digital Logic (continued) Information Technology in Theory 14
Fundamental Components of a Computer Information Technology in Theory 15
Fundamental Components of a Computer (continued) • Computer hardware is any physical device associated with a computer, such as a keyboard or monitor • Besides the instructions that users issue through input hardware devices, computers also receive instructions via the software stored in its memory • Software is traditionally defined as a series of instructions written by computer programmers in a language that people can understand that the computer can translate into binary • The computer must interpret and then execute each software instruction, again through the use of its transistors Information Technology in Theory 16
Input/Output Devices • Input and output devices are hardware components that interface a computer to the outside world • Examples of input devices include: – – – – – Mice Trackballs Microphones Web cams Styluses Touch pads Pointing sticks Joysticks Scanners Information Technology in Theory 17
Input/Output Devices (continued) • Examples of output devices include: – – Printers Plotters Speakers Projectors • Some I/O devices, such as keyboards and monitors, are integral parts of computers • However, note that not all computers have input and output devices connected to them • For example, some computers can interface to a user over a network and do not need their own I/O equipment Information Technology in Theory 18
Central Processing Unit • The most crucial component of any computer is a chip called the microprocessor or central processing unit (CPU) • The microprocessor controls the major functionality of the computer and is often called the “brains” of the computer • The microprocessor’s many tasks include fetching a wealth of software instructions, interpreting and executing these instructions, and storing and outputting results Information Technology in Theory 19
Central Processing Unit (continued) Information Technology in Theory 20
Central Processing Unit (continued) • Key component of the CPU is the control unit, where instructions are interpreted and control signals are generated • Based on the result of the interpretation, the control unit supplies crucial control signals to other parts of the computer • If the instruction involves the addition of two numbers, then the control unit issues control signals to a unit called the ALU, which is responsible for arithmetic and logical operations Information Technology in Theory 21
Central Processing Unit (continued) • The registers within the CPU are small areas for temporarily storing information, such as instructions that are supplied to the control unit and the results of addition and comparison operations implemented by the ALU • Software instructions that the CPU interprets and data that it processes are fetched from a module called main memory that resides outside the CPU • Unlike main memory, cache memory is internal to the CPU Information Technology in Theory 22
Central Processing Unit (continued) • Cache memory holds frequently used instructions and data • Because some software instructions and data are needed more often than others by the CPU, they can be stored inside the cache memory and retrieved when necessary instead of having to be fetched from the distant main memory every time they are needed • Saving frequently used instructions and data in close proximity saves considerable time and energy Information Technology in Theory 23
Main Memory • Main memory of a computer consists of two types of memory: random access memory (RAM) and readonly memory (ROM) • RAM is much more prominent; it retains its contents as long as power is supplied to the computer and loses its contents when the power is switched off • RAM is also called temporary memory or volatile memory • ROM, on the other hand, retains its contents even after power to the computer Information Technology in Theory 24
Main Memory (continued) • Essentially, two types of RAM are used to store information as long as power is supplied to the computer: static RAM (SRAM) and dynamic RAM (DRAM) • The contents of dynamic RAM must be refreshed several times per second, because the electrical charges representing the bits of DRAM leak out and must be replenished • There is no need to refresh the contents of SRAM • DRAMs include transistors and other electronic components called capacitors that store electricity, whereas SRAMs are based on transistors Information Technology in Theory 25
Main Memory (continued) • Although capacitors can store electricity, they cannot do so indefinitely • Electricity eventually leaks out of them, which is why it is necessary to refresh the contents of DRAM • Memory chips that correspond to the main memory are mostly DRAM • Cache memory within the CPU is typically SRAM • Due to fundamental differences in how the two types of RAM are constructed, they differ in speed, size, and cost Information Technology in Theory 26
Main Memory (continued) • It is faster to save and retrieve the contents of SRAM, but SRAM is also more expensive and larger than DRAM • Because cache memory has to be fast, it is better to use SRAM to build cache memory • DRAM is used within the main memory because it is the main holding area of the computer and must have a large holding capacity • Main memory also must be cheap to minimize the cost of the computer Information Technology in Theory 27
Main Memory (continued) • RAM chips retain ones and zeros that correspond to instructions and data within small areas called cells • Each RAM cell can hold one bit of information, regardless of whether it is DRAM or SRAM • Cells within RAM chips are arranged in rows and columns • Saving information to RAM is referred to as “writing to RAM, ” and retrieving RAM contents is called “reading the RAM” • Information is written to and read from the RAM by addressing the RAM cells using addresses Information Technology in Theory 28
Main Memory (continued) Information Technology in Theory 29
Main Memory (continued) Information Technology in Theory 30
Main Memory (continued) • Although a major part of the instructions that the computer executes are supplied from RAM, the computer also relies on some operations based on instructions stored in ROM chips • ROM chips and RAM chips physically reside at different places within a computer, but together they constitute the main memory of the computer • The BIOS chip is an example of a ROM chip found inside a computer Information Technology in Theory 31
Main Memory (continued) • The major variations of ROMs have historically included the following: – Programmable ROM (PROM) – Erasable programmable ROM (EPROM) – Electrically erasable programmable ROM (EEPROM) • A device that is similar to EEPROM is flash memory Information Technology in Theory 32
Storage • Storage and retrieval of digital information is one of the most critical components of an IT system • Without storage, we would have no Web servers, electronic banking, e-mail, or almost any other digital application • Storage technologies are discussed later in the chapter Information Technology in Theory 33
Interconnection System • The physical system that connects I/O devices, main memory, CPU, storage, and other components is called the interconnection system • This system is actually a set of wires; they are often grouped together as a set of parallel wires that transfer signals corresponding to data, instructions, and control information in an arrangement called a parallel bus • Some connections do not rely on a set of parallel wires • Instead, they use a main connection that transfers bits one after the other, or serially, from one point to another • Such connections are called a serial bus Information Technology in Theory 34
Interconnection System (continued) Information Technology in Theory 35
Interconnection System (continued) • The number of parallel lines within a parallel bus and the transfer speed of bits across each line dictate how many overall bits the bus can carry • The larger the bus width, the quicker the bits can reach their destination; an example would be the delivery of bits from the main memory to the CPU Information Technology in Theory 36
Interconnection System (continued) • Types of computer buses include the following: – System bus—A parallel bus that connects the CPU and main memory; the system bus is also called the front side bus – Peripheral Component Interconnect (PCI) bus and PCI Express (PCI-E) bus—The PCI bus and its newer and faster version, PCI-E bus, connect expansion cards such as network interface cards and sound cards/adapters – Accelerated Graphics Port (AGP) bus—This bus connects expansion cards called graphics cards/adapters to the CPU; this circuitry supports computer graphics capabilities Information Technology in Theory 37
Interconnection System (continued) • Expansion cards are connected to the CPU by buses • They are attached to the buses by being plugged into special components called expansion slots on the computer’s motherboard Information Technology in Theory 38
Factors That Affect Computer Performance • Some of the key factors that affect computer performance are shown in the following list: – – – – Word length Bus width and bus speed Memory size and memory access speed Processor speed Instruction set Number of CPUs Chip set Information Technology in Theory 39
Word Length • Word length is expressed in terms of bits and corresponds to the maximum number of bits of information that a computer can process at one time • The larger the word length is, the faster the computer • Engineers are constantly striving to design computers with larger word lengths to achieve high performance Information Technology in Theory 40
Word Length (continued) Information Technology in Theory 41
Bus Width and Bus Speed • Another factor that affects performance is the bus size, or parallel bus width • Like word length, bus size is measured in terms of bits • A computer with a large bus width can carry more bits at a time between computer components, such as between main memory and the CPU, making it faster than a computer with a small bus width • Because the buses carry important instructions, data, addresses, and control signals, the speed you can gain by using a larger bus width is important Information Technology in Theory 42
Bus Width and Bus Speed (continued) • The bus size determines how many physical wires are constructed within the bus • The more wires that designers can incorporate within a bus, the larger the bus width becomes, meaning that the bus can carry more bits at one time • Bus speed, which is measured in hertz (Hz), also affects computing performance • Even if large bus widths can enable large number of bits to move between computer components in parallel, each bus line must also carry bits as quickly as possible Information Technology in Theory 43
Memory Size and Memory Access Speed • Computers need large amounts of memory for high performance and multitasking • Because every program occupies some amount of space in RAM, a large amount of RAM is essential to operate with multiple programs • Furthermore, RAM size significantly affects computer speed because information is frequently written to and erased from RAM • If there is insufficient RAM space, the computer must frequently resort to using its hard disk, which results in diminished performance Information Technology in Theory 44
Memory Size and Memory Access Speed (continued) • Other than RAM, the size of the cache memory (expressed in bytes) is also a consideration • The more cache memory the CPU has, the better the CPU performs • The speed at which RAM and cache memory contents are written and retrieved helps to determine performance • The higher the read/write speed, the faster the computer is • Typical RAM access speeds are on the order of nanoseconds (ns), or billionths of a second Information Technology in Theory 45
Processor Speed • The on and off switching of transistors within the integrated circuits of a computer is managed by a central digital electrical signal called a clock • Computer clock speeds are measured in frequency units called hertz • Just as multipliers are used in the IT world to express large numbers of bits, multipliers are used to express large frequency values: – 1 k. Hz = 1000 Hz – 1 MHz = 1, 000 Hz – 1 GHz = 1, 000, 000 Hz Information Technology in Theory 46
Processor Speed (continued) • The higher the clock frequency, the faster the computer usually is • Typical clock frequencies for modern computers range from hundreds of megahertz to a few gigahertz Information Technology in Theory 47
Processor Speed (continued) Information Technology in Theory 48
Instruction Set • A computer’s CPU is typically based on a complex instruction set computer (CISC) architecture or a reduced instruction set computer (RISC) architecture • The difference between them is the number of clock cycles it takes to execute a single instruction • Some computer architectures require more cycles to execute the same instruction than others Information Technology in Theory 49
Number of CPUs • Incorporating more than one microprocessor increases a computer’s processing power and speed because it can use more transistors and perform multitasking • Some computers have dual-core processors and quadcore processors • Besides multicore computers, multiprocessor computers are also available Information Technology in Theory 50
Chip Set • The term chip set refers to a group of chips that support the microprocessor by controlling the flow of information between it and other components, such as the memory chips, graphics and sound cards, disk drives, and I/O devices • Various manufacturers produce different types of chip sets, which can significantly affect computer performance, due to the limitations that chip sets may impose on memory size, number of processors, and bus speed Information Technology in Theory 51
Inside a Typical Computer • Some essential components of a modern computer include: – – – – – The motherboard DVD drive Hard drive Graphics card Sound card Network card Ports Power supply Cooling system Information Technology in Theory 52
Inside a Typical Computer (continued) Information Technology in Theory 53
The Motherboard • The motherboard is a printed circuit board (PCB)— a planar structure made of resin or other materials that contains the microprocessor and many other chips • It also includes the chip set and components that support the operation of a computer, such as capacitors and mechanical switches • A PCB supports and interconnects components through metallic traces called buses printed on the board Information Technology in Theory 54
The Motherboard (continued) • Other components on the motherboard include: – – – The BIOS chip The chip set Slots for connecting RAM modules to the microprocessor Default circuitry for supporting sound and graphics capabilities Expansion slots for connecting external graphics cards, sound cards, and network cards – Connectors for attaching hard drives, CD drives, and other disk drives to the motherboard – Ports for connecting I/O devices such as the keyboard and mouse Information Technology in Theory 55
The Motherboard (continued) • Boards that carry the RAM chips are called memory modules, and are connected to the microprocessor through memory slots on the motherboard • Modern motherboards also come with extra memory slots to allow users to expand the RAM for faster processing Information Technology in Theory 56
The Motherboard (continued) • Circuitry for supporting enhanced graphics may also be incorporated on separate PCBs called graphics cards or graphics adapters • These types of external graphics adapters connect to the motherboard via a special expansion slot, such as the AGP slot • Sound adapters enable a computer to output high-quality sound • Like graphics adapters, external sound adapters can be connected to the motherboard via one of its expansion slots, such as the PCI/PCI-E slots Information Technology in Theory 57
The Motherboard (continued) • Computer networking is all around us, so motherboards also come with circuitry that supports networking • Alternatively, network adapter cards may be plugged into expansion slots on the motherboard • The motherboard also contains connectors and ports for hard drives, I/O devices such as the keyboard and mouse, and other devices Information Technology in Theory 58
The Motherboard (continued) Information Technology in Theory 59
Drives • Drives are devices that can read and write large amounts of information to and from various types of magnetic, optical, or electrical devices, such as hard disks, CDs, DVDs, and flash memory • If drives are installed in the computer case, they are called internal drives • If they are connected to the computer externally, they are called external drives • These drives attach to the motherboard using its specifically reserved connectors Information Technology in Theory 60
Drives (continued) • One common type of hard drive is the Advanced Technology Attachment (ATA) drive, which is also called a parallel ATA drive, an Integrated Device Electronics (IDE) drive, or an Enhanced IDE (EIDE) drive • These drives include an integrated component called a controller, and are attached to the motherboard via special connectors • Other hard drives include serial ATA (SATA) and small computer system interface (SCSI) drives, which also attach to the motherboard via special connectors Information Technology in Theory 61
Drives (continued) • SATA connections have two advantages over parallel ATA: they can transfer data at a faster rate between the motherboard and the drive, and they are more physically compact • SCSI drives are faster than ATA drives, but because they are more complex, SCSI drives are not a standard feature of most computers • While internal hard drives connect to the motherboard with special connectors, external hard drives attach to the motherboard via Universal Serial Bus (USB) or Fire. Wire ports outside the computer casing Information Technology in Theory 62
Computer Ports • Peripheral devices may be connected to a computer through different types of ports that vary depending on the type of computer • These ports in turn connect to the motherboard itself • Some examples of ports include serial ports, parallel ports, video ports, S-video ports, USB ports, Fire. Wire ports, sound ports, keyboard ports, mouse ports, network ports, and telephone ports Information Technology in Theory 63
Computer Ports (continued) Information Technology in Theory 64
Power Supply • A power supply is included within the computer casing (or externally) to convert the electricity levels from the wall outlet into levels that the computer can use • Besides power supplies, other components such as surge protection systems and uninterruptible power supplies (UPS) may be employed to safeguard the computer against sudden surges of electricity caused by lightning strikes, power outages, and other phenomena Information Technology in Theory 65
Cooling System • A significant amount of heat is generated by the many electrical components in a computer, and problems can arise if these components are not sufficiently cooled • The chip that generates the most heat is typically the microprocessor • One popular technique to cool the chip is to install a metallic component called a heat sink • Another way to conduct heat away from the chip is to install a fan on the microprocessor or pass cryogenic materials or other liquids inside tubes in contact with the microprocessor Information Technology in Theory 66
Cooling System (continued) Information Technology in Theory 67
Types of Computers and Their Applications • Types of computers include: – – – Desktop computers Laptops Handhelds Mainframes Supercomputers Servers • Some of these computers may function standalone or as servers and clients • Some types of servers include e-mail servers, file servers, and Web servers Information Technology in Theory 68
Types of Computers and Their Applications (continued) • Servers usually have more than one processor, considerable memory, and extra hardware • One popular type of server is called a blade server • Blade servers can be controlled in a computer network through a remote access card installed in the server casing • They have the advantage of saving a significant amount of space; they can be housed in remote locations such as data centers • These servers generate significant amounts of heat and must be kept properly cooled Information Technology in Theory 69
Types of Computers and Their Applications (continued) • Thin clients are computers that do not have the regular hardware typically found on other computers • They do not have a hard disk or other disk drives, and they have a scaled-down version of an operating system stored on a special memory chip • Thin clients rely on other computers over a network to perform most of their processing functions • Their major advantage is lower cost due to their lack of hardware and low power consumption Information Technology in Theory 70
Types of Computers and Their Applications (continued) Information Technology in Theory 71
The Future of Computers • Although transistors have continued to shrink over the years in accordance with Moore’s Law, the trend will not hold true indefinitely • Similarly, wires that connect transistors to each other continue to diminish in size • This creates a problem as well, because very thin wires can cause a large loss of energy and limit the speed by which signals travel over the wires that constitute computer buses, thereby limiting the computer’s speed Information Technology in Theory 72
The Future of Computers (continued) • Alternative technologies are underway, some of which include: – – – Optical computing Optical interconnects Nanotechnology Carbon nanotubes Quantum computing Biological computing Information Technology in Theory 73
Storage Technologies • Storage technologies are critical for businesses to archive and access data, for governments to store important political information, and for libraries to store electronic information • Storage schemes initiated with paper punch cards and paper tapes • Magnetic tapes and disks were introduced later • Optical and electronic media were introduced last Information Technology in Theory 74
Magnetic Storage • To record bits, a plastic tape, plastic disk, or ceramic disk is coated with a ferromagnetic material—a unique material that can be magnetized • The recording material is positioned close to a writing head that magnetizes small sections of the medium in one of two directions corresponding to ones and zeros • A hard disk stores information using the same principle as magnetic tape, but there are major differences in their recording formats Information Technology in Theory 75
Magnetic Storage (continued) Information Technology in Theory 76
Magnetic Storage (continued) • A hard disk stores bits on a circular disk (also called a platter) made of highly polished glass or another material • A typical computer hard disk has many of these circular disks stacked on top of each other • Each disk stores information on circular tracks and wedge-like sectors Information Technology in Theory 77
Magnetic Storage (continued) Information Technology in Theory 78
Magnetic Storage (continued) • In the case of magnetic tape, information is accessed serially (sequential access), whereas a magnetic disk can provide random access to data • Hard disks are limited mainly by their complex and sensitive mechanics for spinning, writing, and reading data • Floppy disks are external storage devices that are not widely used anymore except for file backups in some legacy systems • Floppies are conceptually similar to hard disks, but they have a much lower storage capacity Information Technology in Theory 79
Optical Storage • Types of optical storage devices include: – – – Compact disc-read only memory (CD-ROM) Compact disc-recordable (CD-R) Compact disc-rewritable (CD-RW) Digital versatile disk-read only memory (DVD-ROM) Digital versatile disk-recordable (DVD-R) Digital versatile disk-rewritable (DVD-RW) Information Technology in Theory 80
Optical Storage (continued) • Bits on optical disks are stored by creating small areas or domains with different reflective properties across a long spiral track • Bits are read by shining laser light on these tiny domains and detecting the light reflected back from them • Each domain reflects light in one of two ways, enabling the reader to distinguish between a one and a zero Information Technology in Theory 81
Optical Storage (continued) • Bits on a CD-ROM are recorded by physically creating tiny protrusions, also called lands, on the CD’s clear plastic surface during the manufacturing phase • Areas without lands remain shallow; these areas are called pits Information Technology in Theory 82
Optical Storage (continued) Information Technology in Theory 83
Optical Storage (continued) • CDs can record many types of information • For a device to properly read and save the information stored on a CD, the device must use standard formats • For example, music on an audio CD is recorded following the Red Book standard developed by Philips and Sony • DVDs rely on the same principles as CD technology • DVDs can store much more data than CDs, primarily because the size of lands and pits can be diminished and spaced in closer proximity, and because more than one spiral track may be used to store data due to multiple layers Information Technology in Theory 84
Electronic Storage • Electronic storage, also called semiconductor storage, has become an extremely effective technology for quick and efficient information storage • Electronic storage lets users save data quickly without the relatively lengthy burn process required for optical media such as CDs and DVDs • Electronic storage technologies store bits by trapping electricity within an array of transistors arranged on a silicon chip Information Technology in Theory 85
Electronic Storage (continued) Information Technology in Theory 86
Comparing Various Types of Storage Media • Hard disks provide very fast recording and access to information and can store large amounts of data • Because they can be written over and over again without encountering the limitations of CDs or DVDs, hard disks are currently the primary storage medium for computers • Optical disks are portable, lightweight, and can store information with greater stability than magnetic media • Optical disks are not affected by magnetic fields, so they are considered more robust than magnetic devices, which are prone to data loss over time • Electronic storage media are compact, fast, and versatile because they do not contain moving parts Information Technology in Theory 87
Comparing Various Types of Storage Media (continued) • Manufacturers can achieve greater capacity by reducing the physical area occupied by each bit on the media • However, each storage technology has a limit to its packing density • On magnetic media, the superparamagnetic effect causes random fluctuations toward the magnetic field if bits are packed too closely to each other • The ability to pack these bits closely on optical media is governed by the wavelength of light and the quality of the optics used to focus the laser beam onto the tracks Information Technology in Theory 88
Comparing Various Types of Storage Media (continued) • Integrated circuit fabrication technology limits the density of semiconductor storage devices because the size of transistors is limited • With advancements in chip manufacturing techniques, flash memory devices are now considered to be at the forefront of portable storage technology • The ultimate limitations of all these storage technologies have spurred a large amount of research for alternatives, including holographic storage and molecular storage Information Technology in Theory 89
Summary • Transistors can be combined to create logic gates, which can be used as the basic building blocks for integrated circuits • A computer’s fundamental components include input/output (I/O) devices, a central processing unit, main memory, storage (secondary memory), and an interconnection system • Factors that affect the performance of computers include word length, bus width and bus speed, memory size and memory access speed, processor rate, instruction set, number of processors, and chip set Information Technology in Theory 90
Summary (continued) • Components you can expect to find in a modern computer include the motherboard, CD drive, hard drive, graphics adapter, sound adapter, network adapter, ports, power supply, and a cooling system • Computers come in various types, including desktop computers, laptops, handhelds, mainframes, supercomputers, servers, and thin clients • Future possibilities for computing include nanotechnology, quantum computing, and biological computing Information Technology in Theory 91
Summary (continued) • Magnetic media store digital data by magnetizing a ferromagnetic medium in one of two directions • Optical media store digital data by creating domains with different reflectivities • Electronic storage media save data by storing different levels of electrical charge through transistors • Magnetic tapes are classified as serial access devices; hard disks, floppy disks, CDs, DVDs, and electronic memory are classified as random access devices • Possible alternatives for future storage media include holographic and molecular storage Information Technology in Theory 92
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