Chapter 5
storage medium
a device or substance in a storage device that actually holds data
hybrid disk drive
a drive that combines flash RAM and magnetic disk storage
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
nonvolatile memory (NVM)
a generic term for memory devices with long-term or permanent data retention
germanium, antimony, and tellurium (GST)
a glasslike compound used in phase-change memory, that can change between amorphous and crystalline states
read/write head
a mechanism in a storage device that reads abd writes data to and from the storage medium ; also refereed to as a "read/write mechanism"
synchronous DRAM (SDRAM)
a read-ahead RAM that uses the same clock pulse as the system bus; read and write operations are broken up into simpler steps that can be completed within one bus clock cycle
magnetic tape
a ribbon of plastic with a coercible (usually metallic oxide) coating, used to store data
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 i determined by the polarity of the magnetic charge
double data rate (DDR)
a series of technologies, each doubling the data transfer rate of the previous synchronous DRAM version
disk drive
a single deice with rigid platters, access arms and read/write heads, a motor to spin platters, a servo to move the access arms, and all of the electronics needed to control the drive, read and write sectors and communicate with the system bus
tape drive
a slow serial access device containing motors that wind and unwind tapes and read/write heads to access tape content
single inline memory module (SIMM)
a small printed circuit board that incorporates multiple DIPs and has a row of electrical contacts on the edge; the entire package is designed to lock into a SIMM slot on a motherboard
double inline memory module (DIMM)
a small printed circuit board that's essentially a SIMM with independent electrical contacts on sides of the module
solid-state drive (SSD)
a storage device that mimics the behavior of magnetic disk but uses flash RAM and other nonvolatile memory devices as the storage medium and read/write mechanism; expected to replace magnetic disks gradually
read threshold
a successful read operation requires a magnetic charge above a certain threshold
compact disc (CD)
a technology developed by Sony and Phillips for storing and distributing music in CD-DA (digital audio) format on a n optical disc
volatile
a term describing storage devices that cant hold data for long periods; primary storage is usually volatile
nonvolatile
a term describing storage devices that hold data without loss for long periods; secondary storage is usually nonvolatile
static RAM (SRAM)
a type of RAM that is implemented entirely with transistors; the basic storage unit is a flip flop circuit
dynamic RAM (DRAM)
a type of RAM that stores each bit by using a single transistor and capacitor
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 longevity than conventional flash RAM
phase-change memory
a type of nonvolatile memory under development that uses a GST compound; has fast write times and high longevity
random access
an access method that can access any random storage location directly in any order; primary storage devices and disk storage devices use random access
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
serial access
an access method that stores and retrieves data items in a linear (sequential) order;mainly used to hold back up copies of data stored on other storage devices
drive array
an arrangement of hard drives enclosed in a storage cabinet and accessed as though they're a single storage device
DVD
an optical disc format for distributing movies and other audiovisual content; stands for "digital video disc" and "digital versatile disc"
single-level cell (SLC)
cells that store single bit value
refresh cycle
in dynamic RAM, the period during which circuitry supplies fresh infusions of power automatically; read and write operations cant be preformed during this cycle. also refers to the transfer of a full screen of data from the display generator to the monitor
platter
in magnetic disk media they are flat circular disc with a metallic coatings that are rotated beneath read/write heads; data is usually recorded in both sides
cylinder
in magnetic disks, consists of all tracks at equivalent distance from the edge or spindle on all platter surfaces
access arm
in magnetic disks, the device where read/write heads are mounted; its attached to a positioning servo for placing read/write heads on specific tasks
track
one concentric circle of a platter; the surface area that passes under read/write head when its position is fixed
disk defragmentation
reorganizing data on a disk drive so that a file's contents are stored in sequential sectors, tracks and platters; an OS utility is used to preform this task
random access memory
semiconductor devices used to implement primary storage; they don't provide permanent storage because RAM's contents are erased when system power is turned off
firmware
software such as system BIOS, stored in nonvolatile memory; can be loaded into main memory at high speeds
track-to-track (TTT) seek time
the average time needed to move a read/write head between two adjacent tracks; typically measured in milliseconds
coercivity
the capability of a substance or magnetic storage medium to accept and hold a magnetic charge
flash RAM
the common type of nonvolatile memory; typically used to store firmware and in portable secondary storage systems, such as USB flash drives
fragmented
the condition of a hard disk (or other storage drive) with many programs and files scattered across it in non-contiguous storage locations
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
read-only memory (ROM)
the earliest type of nonvolatile memory, with data content written permanently during manufacture; this primary storage device can be read, but no further data can be written
sustained data transfer rate
the max data transfer rate a storage device or communication channel can sustain during lengthy data transfer operations
data transfer rate
the rate at which data is transmitted through a medium or communication channel; measured in data units per time interval; essentially its a measure of communication capacity. For storage device its computed by dividing 1 by the access time and multiplying the result by unit of data transfer
magnetic leakage
the reduction inn strength of a stored magnetic charge because of interference from adjacent magnetic charges of opposite polarity
areal density
the surface area allocated to a bit on a storage medium, typically measured in bits, bytes, tracks per inch; also called "recording density" or "bit density"
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
rotational delay
the time a hard disk controller must wait for the right sector to rotate beneath read/write heads
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
access time
the time required to preform one completed read or write operation; a measurement of storage device time
sequential access time
the time required to read the second of two adjacent sectors on the same track and platter of a hard disk
