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.”
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.
GLOSSARY
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
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.
F
feedback
In a
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
format
In a disk drive, the arrangement of data on a storage media. A standard
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.
GL – 3