GLOSSARY
GL – 1
GLOSSARY
Glossary
A
ACCESS
To obtain data from, or place data into, RAM, a register, or data storage device.
ACCESS TIME
The interval between the issuing of an access command and the instant that the target
data may be read or written. Access time includes seek time, latency and controller
overhead time.
ADDRESS
A number, generally binary, distinguishing a specific member of an ordered set of
locations. In disk engineering, the address may consist of drives (unit address), radial
positions (cylinder address), or circumferential position (sector address).
ALLOCATION
A process of assigning designated areas of the disk to particular files.
ALTERNATE TRACK
A spare track used in the event that a normal track becomes damaged or is unusable.
ANALOG
A signal or system that does not use digital states to convey information. A signal may
have any number of significant states (values), contrasted to digital signals which can
only have two states.
ANSI
American National Standards Institute.
APPLICATION PROGRAM
A sequence of programmed instructions that tell the computer how to perform some
end-user task, such as accounting or word processing.
AREAL DENSITY
Bit density (bits per inch) multiplied by track density (tracks per inch) or bits per square
inch.
ASYMMETRY
A distortion of the readback signal which is shown in different intervals between the
positive and negative voltage peaks.
AUXILIARY MEMORY
Memory other than main memory; generally a mass-storage subsystem containing disk
drives and backup tape drives, controller(s) and buffer memory (also called peripheral
memory).
AVERAGE ACCESS TIME
The average time to make all possible length accesses (seeks).
AVERAGE SEEK TIME
The average time to make all possible length seeks. A typical measure of performance.
B
BAD BLOCK
A block that cannot store data because of a media flaw.
BIT
An abbreviation for binary digit, of which there are two (0 and 1). A bit is the basic
data unit of most digital computers. A bit is usually part of a data byte or word, but bits
may be used singly to control or read logic “on-off” functions. The fundamental unit
information, often used loosely to refer to a circuit or magnetization state at a particular
instant in time.
BIOS
Acronym for Basic Input/Output System. The firmware area of a CPU that controls
operations through the system bus and to the attached cards and peripheral devices.
BPI
Acronym for bits per inch. See bit density.
BLOCK
A group of bytes handled, stored, and accessed as a logical data unit, such as an
individual file record.
BUFFER
A temporary data storage area that compensates for a difference in data transfer rates
and/or data processing rates between sender and receiver.
BUS
A collection of functionally parallel conductors that forms an interconnection between
functional blocks in a digital device. A length of parallel conductors that forms a major
interconnection route between the computer system CPU (central processing unit) and
its peripheral subsystems. Depending on its design, a bus may carry data, addresses,
power, and more.
BYTE
An ordered collection of bits treated as a unit. Most often, a byte is understood to
consist of eight bits. One byte is necessary to define an alphanumeric character.
C
CACHE
Random access memory (RAM) used as a buffer between the CPU and the disk drive.
CAPACITY
The amount of data, usually expressed in bytes, which can be stored in a given device
or portion of same.
CENTRAL PROCESSING UNIT (CPU)
The heart of the computer system that executes programmed instructions. It includes
the arithmetic logic unit (ALU) for performing all math and logic operations, a control
section for interpreting and executing instructions, internal memory for temporary
storage of program variables and other functions.
CHANNEL
A collection of electronic circuits used in the process of writing and reading
information to and from magnetic media.
CHARACTER
An ordered collection of bits representing one of a set of predefined symbols. Often the
term is used interchangeably with byte, but this is inexact.
CLOSED LOOP
A control technique that enables the positioning system to correct off-track errors in
real time. The actual head position is monitored and compared to the ideal track
position to determine any position error that might be occurring. This information is
then used to produce a correction signal (feedback) that goes to the positioner to correct
the error. (See also track following servo).
CLOSED LOOP SERVO
A servo control technique that uses position feedback to correct off-track errors. See
Track Following Servo.
CLUSTER
The smallest allocatable unit of disk storage allowed by MS-DOS; each FAT entry
represents one cluster.
CONTROLLER
An electronic device for connecting one or more mass storage peripherals (rigid disk
drives, tape drives, and optical disk drives) to the input/output circuits of a host
computer. Controllers vary in complexity, with more sophisticated units able to buffer
and schedule commands, correct data errors, and bypass media defects without host
intervention.
CONTROLLER
A miniature CPU dedicated to controlling a peripheral device, such as a disk drive,
tape drive, video display terminal, or printer. The controller executes commands from
the central processing unit and reissues commands to the peripheral device.
CORRECTABLE ERROR
An error that can be overcome by the use of Error Detection and Correction.