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GLOSSARY
GL – 3
digital magnetic recording
See magnetic recording.
direct access
Access directly to memory location. (See random
access).
direct memory access (DMA)
A mean of data transfer between the device and host
memory without processor intervention.
directory
A listing of files maintained by the disk operation system
(DOS) or a data base management system to enable a
user to quickly access data files.
disk
A flat, circular piece of metal (usually aluminum) or plastic
(usually mylar) with a magnetic coating upon which
information can be recorded. (See, for example, floppy
disk or Winchester disk)
disk drive or disk memory device
The total electromechanical storage device containing
disks and read/write heads, head positioning mechanism,
drive motor, and electronics.
disk pack
A number of metal disks packaged in a canister for
removal from the disk drive (predecessor of Winchester
technology).
disk operating system (DOS)
The master computer system program that schedules
tasks, allocates the computer system resources, controls
accesses to mass storage devices, manages files, and so
forth. Typical disk operating systems include CP/M, MS-
DOS, and UNIX.
disk storage
Auxiliary memory system containing disk drives.
disk transfer rate
The rate that digital data is transferred from one point to
another. Expressed in either bits/second or bytes/
second.
double frequency encoding
Another name for FM encoding. This is because all
possible data combinations will result in only two
possible temporal displacements of adjacent data bits,
specifically “1F” and 2F.”
EE
EE
E
early window
A data window that has been intentionally shifted in time
in an early direction.
embedded servo
A servo technique used for track following. Position
information is prerecorded between data areas in a track
so that a data head, and proper additional circuitry, can
determine the data head location with respect to the
center position of the track (or cylinder) in question.
erase
A process by which a signal recorded on a medium is
removed and the medium made ready for rerecording.
error correction code (ECC)
A mathematical algorithm that can detect and correct
errors in a data field. This is accomplished with the aid of
Check Bits added to the raw data.
error free
A recording surface that has no defects.
error rate
The number of errors (type must be specified) that occur
in a specified number of bits read.
error recovery procedure
The process that occurs in response to a data error. In a
drive without ECC, this would include re-calibration and
re-seeking to the specified track and rereading the
specified data.
extra pulse
Term used in surface certification. It is when a flux field
discontinuity remains after the recording surface is
erased, thereby producing an electrical output of a read
head passing over the area with the discontinuity. An
extra pulse occurs when the electrical output is larger
than a specified threshold.
FF
FF
F
feedback
In a closed-loop system, the output signal (from the
servo head) is used to modify the input signal (to the
positioner).
fetch
A read operation and its related data transfer operations.
file allocation table (FAT)
Allocates space on the disk for files, one cluster at a
time; locks out unusable clusters; identifies unused (free)
area; and lists a file’s location. With two FAT’s present,
the second copy ensures consistency and protects
against loss of data if one of the sectors on the first FAT
is damaged.
flux changes per inch
Synonymous with frpi (flux reversals per inch). Only in
MFM recording does 1 fci equal 1 bpi (bit per inch). In
run-length-limited encoding schemes, generally 1 fci
equals 1.5 bpi.
format
In a disk drive, the arrangement of data on a storage
media. A standard 5.25-inch disk format consists of 17,
26, or 36 sectors per track, and 512 bytes of data per
sector, plus identification, error correction, and other
bytes necessary for accessing and synchronizing data.
formatted capacity
The actual capacity available to store data in a mass
storage device. The formatted capacity is the gross
capacity, less the capacity taken up by the overhead data
used in formatting the sectors.
frequency modulation
A recording code. A flux reversal at the beginning of a
cell time represents clock bit; a “1” bit is a flux reversal at
the center of the cell time, and a “0” bit is an absence of
a flux reversal.