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Access arm
in magnetic disks, the device where read/write heads are mounted; it's attached to a positioning servo for placing read/write heads on specific tracks
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Access time
the time required to perform one complete read or write operation; a measure of storage device time
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Advanced Intelligent Tape (AIT)
a magnetic tape standard developed by Sony based on Digital Audio Tape; uses helical scanning and an improved tape drive tech to pack more data onto a single tape. AIT includes a small RAM cache in cartridge, which stores directory info. to speed searching and data access.
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Areal density
the surface area allocated to a bit on a storage medium, typically measured in bits, bytes, or tracks per inch
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Average access time
typically expressed as an average of access time for all storage locatiosn
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Block
a generic term for describing secondary storage data transfer units. also refers to a set of logical records grouped on a storage device for efficient processing , storage, or transport as well as a portion of a program that's always executed as a unit.
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Blu-ray Disc (BD)
An update to DVD-ROM, orginally designed for high-definition video discs but has been adapted to data storage
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CD digital audio (CD-DA)
a read-only format for storing and distributing music on a CD
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CD read-only memory (CD-ROM)
a standard 120-mm read-only optical disc; compatible with CD-DA but includes additional formattng info. to store directory and file info.
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Coercivity
the capability of a substance or magnetic storage medium to accept and hold a magnetic charge; directly proportional to mass
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Compact disc (CD)
a technology developed by Sony and Phillips for storing and distributing musicin the CD-DA format on a 120-mm optical disc
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Core memory
in early computers, a technology for implementing primary storage as rings of ferrous materials embedded in a two-dimensional wire mesh
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Cylinder
in magnetic disks, consists of all tracks at an equivalent distance from the edge or spindle on all platter surfaces
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Data transfer rate
the rate at which data is tranmitted through a medium or communication channel; measured in data units per time interval; essentially, it's measure of communicaiton capacity. For storage device, it's computed by dividing 1 by the access time and multiplying the result by the unit of data transfer
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Digital Audio Tape (DAT)
an early magnetic tape tech on which Digital Data Storage standards are based
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Digital Data Storage (DDS)
magnetic tape standards developed by Hewlett-Packard and Sony and based on Digital Audio Tape; DDS drives use helical scanning
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Direct access
see random access
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Disk deframentation
reorganizing data on a disk data so that a file's contents are stored in sequential sectors, tracks, and platters; an OS utility us used to perform this task
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Double data rate (DDR)
a series of techs, each doubling the data transfer rate of the previous synchronous DRAM versions
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Double inline memory module (DIMM)
a small printed circuit board that essentially a SIMM with independent electrica contacts on both sides of the module.
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Drive array
an arrangement of hard drives enclosed in a storage cabinet and accessed as though they're a single storage devices
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Dual inline packages (DIPs)
an early form of packaging for RAM or ROM circuits; had two rows of electrical contact pins
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DVD
An optical disc format for distributing movies and other audiovisual content; stands for both digital video disc and digital versatile disc
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DVD read-only memory (DVD-ROM)
a format for general-purpose read-only data storage on a DVD
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Dynamic RAM (DRAM)
a type of RAM that stores each bit by using a single transistor and capacitor
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Electronically erasable programmable ROM (EEPROM)
a type of nonvolatile memory that can be programmed, erased, and reprogrammed by signals snet from a CPU; the only type of ROM that's currently used
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Erasable programmable ROM (EPROM)
a type of nonvolatile memory that's manufacutured blank, written (programmed) with aspecial EPROM writer, erased by exposure to ultraviolted light
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Firmware
Software, such as system BIOS, stored in nonvolatile memory; can be loaded into main memory at high speeds
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Flash RAM
the most common type of nonvolatile memory; typically used to store firmware and in portable secondary storage systems, such as USB flash drives
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Fragmented
the condition of a hard disk (or other storage drive) with many programs and files scattered across it in noncontiguous storage locations.
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Germanium, antimony, and tellurium (GST)
a glasslike compound, used in phase-change memory, that can change between amorphous and crystalline states
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Hard disk
a magnetic disk medium with a rigid metal base (substrate) where data is recorded as patterns of magnetic storage
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Head-to-head (HTH) switching time
the time needed to switch a hard drive's read/write circuitry to the correct read/write head before accessing a sector
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Helical scanning
a geometric approach to recording data on a tape surface in which data is read and written by rotating the read/write head at an angle and moving from tape edgeto tape edge
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Linear recording
a geometric approach to recording data on a tape surface in which bits are placed along parallel tracks that run along the tape's entire length
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Linear Tape Open (LTO)
a magnetic tape standard developed by Hewlett-Packard, IBM, and Seagate; uses linear recording and has tech improvements in tape cartridges, coercible materials, read/write heads, and tape control
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Magnetic decay
the tendency of magnetically charged particles to lose their charge over time; it's constant over time and proportional to the power of the charge
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Magnetic leakage
the reduction in strength of a stored magnetic charge because of interference from adjacent magnetic charges of opposite polarity
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Magnetic tape
a ribbon of plastic with a coercible (usually metallic oxide) coating, used to store data
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Magneto-optical (MO) drive
a secondary storage device that uses a laser and reflected light to sense magnetically recorded bit values; data reading is based on the polarity of the reflected laser light, which is determined by the polarity of the magnetic charge
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Magnetoresistive RAM (MRAM)
a type of nonvolatile memory under development that stores bit values by using two magnetic elements, one with fixed polarity and the other with polarity that changes when a bit is written; has better longetivity than conventional flash RAM
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Mammoth
a magnetic tape standard, developed by Exabyte, based on Digital Audion Tape; uses helical scanning and an improved tape drive tech. to pack more data onto a single tape
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Nonvolatile
a term describing storage devices that hold data without loss for long periods; secondary storage is usually nonvolatile
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Nonvolatile memory (NVM)
a generic term for memory devices with long-term or permanent data retention
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Parallel access
an access method that can access multiple storage locations simultaneously; can also be achieved by subdividing data items and storing the component pieces on multiple storage devices
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Phase-change memory (PCM)
a type of nonvolatile memory under development that uses a GST compound capable of switching between amorphous and crystalline states; has fast write times and high longevity
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Platters
in magnetic disk media, they're flat, circular disks with metallic coatings that rotated beneath read/write heads; data is normally recorded on both sides
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Quarter Inch Committee (QIC)
a committee that develops open standards for magnetic tape drives on smaller computers.
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Random access
an access method that can access any storage location directly and in any order; primary storage devices and disk storage devices use random access
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read-only memory (ROM)
the earliest type of nonvolatile memory, which data content written permanently during manufacturer; this primary storage device can be read, but no further data can be written
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Read/write head
a mechanism in a storage device that reads and writes data to and from the storage medium
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Refresh cycle
in dynamic RAM, the period during which circuitry supplies fresh infusions of power automatically; read and write operations can't be performed during this cycle. Also refers to the transfer of a full screen of data from the display generator to the monitor
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Rotational delay
the time a hard disk controller must wait for the right sector to rotate beneath read/write heads
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Sector
the data transfer unit for magnetic disk and optical disc drives; the size is generally stated in bytes and can vary from one device to another. Also refers to a fractional portion of a track on magnetic disk media.
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Sequential access time
the time required to read the second of the two adjacent sectors on the same track and platter of a hard disk
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Serial access
an access method that stores and retrieves data items in a linear (sequential) order; mainly used to hold backup copies of data stored on other storage devices
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Single inline memory module (SIMM)
a small printed circuit board that incorporates mulitiple DIPs and has a row of electrical contacts on the edge; the entire package is designed to lock onto a SIMM slot on a motherboard
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Solid-state drive (SSD)
a storage device that mimics the behavior of a magnetic disk but uses flash RAM or other nonvolatile memory devices as the storage medium and read/write mechanism; expected to replace magnetic disks gradually
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Static RAM (SRAM)
a type of RAM that's implemented entirely with transistors; the basic storage unit is a flip-flop circuit
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Storage medium
a device or substance in a storage device that actually holds data
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Super Digital Linear Tape (SDLT)
a magnetic tape standard developed by Quantum Corporation; the cartridge has only one reel, and the device records in parallel linear tracks in an end-to-end format
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Sustained data transfer rate
the maximum data transfer rata a storage device or communication channel can sustain during lenthly data transfer operations
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Synchronous DRAM (SDRAM)
a read-ahead RAM that uses the same clock pulse as the system bus; read and write operations are broken into simple steps that can be completed in one bus clock cycle
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Tape drive
a slow serial access device containing motors that wind and unwind tapes and read/write heads to access tape content
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Track
one oncentric circle of a platter; the surface area that passes under a read/write head when its position is fixed
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Track-to-track (TTT) seek time
the average time needed to move a read/write head between two adjacent tracks; typically measured in milliseconds
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Volatile
a term describing storage devices that can't hold data for long periods; primary storage is usually volatile
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