Glossary
A
ACCESS
To obtain data from, or place data into, RAM, a register, or data storage device.
ACCESSTIME
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.
ALTERNATETRACK
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.
APPLICATIONPROGRAM
A sequence of programmed instructions that tell the computer how to perform some end-user task, such as accounting or word processing.
AREALDENSITY
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.
AUXILIARYMEMORY
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).
AVERAGEACCESSTIME
The average time to make all possible length accesses (seeks).
AVERAGESEEKTIME
The average time to make all possible length seeks. A typical measure of performance.
B
BADBLOCK
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.
CENTRALPROCESSINGUNIT(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.
CLOSEDLOOP
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).
CLOSEDLOOPSERVO
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.
