Systems Architecture ch.5

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.

access time

The time required to perform one complete read or write operation; a measure of storage device time.

areal density

The surface area allocated to a bit on a storage medium, typically measured in bits, bytes, or tracks per inch; also called "recording density" or "bit density."

average access time

Typically expressed as an average of access times for all storage locations.

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.

Blu-ray disc (BD)

An update to DVD-ROM, originally designed for high-definition video discs but has been adapted to data storage.

CD digital audio (CD-DA)

A read-only format for storing and distributing music on a CD.

CD read-only memory (CD-ROM)

A standard 120-mm read-only optical disc; compatible with CD-DA but includes additional formatting information to store directory and file information.

coercivity

The capability of substance or magnetic storage medium to accept and hold a magnetic charge; directly proportional to mass.

compact disc (CD)

A technology developed by Sony and Phillips for storing and distributing music in the CD-DA format on a 120-mm optical disc.

cylinder

In magnetic disks, consists of all tracks at an equivalent distance from the edge or spindle on all platter surfaces.

data transfer rate

The rate at which data is transmitted through a medium or communication channel; measured in data units per time interval; essentially, it's a measure of communication capacity. For a storage device, it's computed by dividing 1 by the access time and multiplying the result by the unit of data transfer.

Digital Audio Tape (DAT)

An early magnetic tape technology on which Digital Data Storage standards are based.

Digital Data Storage (DDS)

Magnetic tape standards developed by Hewlett-Packard and Sony and based on Digital Audio Tape; DDS drives use helical scanning.

direct access

see Random Access

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 perform this task.

double data rate (DDR)

A series of technologies, each doubling the data transfer rate of the previous synchronous DRAM version.

double inline memory module (DIMM)

A small printed circuit board that's essentially a SIMM with independent electrical contacts on both sides of the module.

drive array

An arrangement of hard drives enclosed in a storage cabinet and access as though they're a single storage device.

dual inline packages (DIPS)

An early form of packaging for RAM or ROM circuits; had two rows of electrical contact pins.

DVD

An optical disc format for distributing movies and other audiovisual content; stands for for both "digital video disc" and "digital versatile disc."

DVD read-only memory (DVD-ROM)

A format for general-purpose read-only data storage on DVD.

dynamic RAM (DRAM)

A type of RAM stores each bit by using a single transistor and capacitor.

firmware

Software, such as system BIOS, stored in nonvolatile memory; can be loaded into main memory at high speeds.

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.

fragmented

The condition of a hard disk (or other storage drive) with many programs and files scattered across it in noncontiguous storage locations.

hard disk

A magnetic disk medium with a rigid metal base (substrate) where data is recorded as patterns of magnetic charge.

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.

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.

magnetic leakage

The reduction in strength of a stored magnetic charge because of interference from adjacent magnetic charges of opposite polarity.

magnetic tape

A ribbon of plastic with a coercible (usually metallic oxide) coating, used to store data.

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.

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.

nonvolatile

A term describing storage devices that hold data without loss for long periods; secondary storage is usually nonvolatile.

nonvolatile memory (NVM)

A generic term for memory devices with long-term or permanent data retention.

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.

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.

platters

In magnetic disk media, they're flat, circular disks with metallic coatings that are rotated beneath read/write heads; data is normally recorded on both sides.

Quarter Inch Committee (QIC)

A committee that develops open standards for magnetic tape drives on smaller computers.

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.

read-only memory

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.

read/write head

A mechanism in a storage device that reads and writes data to and from the storage medium; also referred to as a "read/write mechanism."

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.

rotational delay

The time a hard disk controller must wait for the right sector to rotate beneath read/write heads.

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.

sequential access time

The time required to read the second of two adjacent sectors on the same track and platter of a hard disk.

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.

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.

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 gradually replace magnetic disks.

static RAM (SRAM)

A type of RAM that's implemented entirely with transistors; the basic storage unit is a flip-flop circuit.

storage medium

A device or substance in a storage device that actually holds data.

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.

tape drive

A slow serial access device containing motors that wind and unwind tapes and read/write heads to access tape content.

track

One concentric circle of a platter; the surface area that passes under a read/write head when its position is fixed.

track-to-track (TTT) seek time

The average time needed to move a read/write head between two adjacent tracks; typically measured in milliseconds

volatile

A term describing storage devices that can't hold data for long periods; primary storage is usually volatile.