GLOSSARY
P
parallelism
1.The condition of two planes or lines being parallel. Important in disk drives because a lack of it in mechanical assemblies can result in positioning inaccuracy. More precisely:
parity
A simple method of data error detections that always makes numbers either odd or even, using an extra bit in which the total number of binary 1s (or 0s) in a byte is always odd or always even; thus, in an odd parity scheme, every byte has eight bits of data and one parity bit. If using odd parity and the number of 1 bits comprising the byte of data is not odd, the ninth or parity bit is set to 1 to create the odd parity. In this way, a byte of data can be checked for accurate transmission by simply counting the bits for an odd parity indication. If the count is ever even, an error is indicated.
partition
A logical section of a disk drive, each of which becomes a logical device with a drive letter.
peak shift
The shifting in time of the
peripheral equipment
Auxiliary memory, displays, printers, and other equipment usually attached to a computer system’s CPU by controllers and cables. (They are often packaged together in a desktop computer.)
phase locked loop (PLL)
A circuit whose output locks onto and tracks the frequency of an input signal. Sometimes incorrectly called a data separator.
phase margin
Measure in degrees of the amount of difference between excursions from the window center where flux reversals can occur and the edge of the data window. Similar to window margin.
physical sector
The smallest grouping of data on the hard disk; always 512 bytes.
PIO
Programmable Input Output. A means of accessing device registers. Also describes one form of data transfers. PIO data transfers are performed by the host processor using PIO register accesses to the data register.
plated thin film media
Magnetic disk memory media having its surface plated with a thin coating of a metallic alloy instead of being coated with oxide.
processing
The process of the computer handling, manipulating and modifying data such as arithmetic calculation, file lookup and updating, and word pressing.
pulse crowding
Modification of playback amplitude due to super- positioning of adjacent flux reversal fields being sensed by the read/write gap.
pulse detect
A digital pulse train in which each leading edge or each edge corresponds to a magnetic transition read from the disk. If transition qualification circuitry exists in the drive, this signal is the output of same. Also known as transition detect.
R
random access memory (RAM)
Memory designed so that any storage location can be accessed randomly, directly and individually. This is contrasted to sequential access devices such as tape drives.
read
To access a storage location and obtain previously recorded data. To sense the presence of flux reversals on magnetic media. Usually implemented such that a dynamic flux amplitude will cause a proportional electrical output from the transducer.
read gate signal
A digital input signal which causes the drive circuitry to recover data.
read only memory (ROM)
A form of memory which cannot be changed in formal operational modes. Many different types are available. RAM is used for permanent information storage.
Computer control programs are often stored in ROM applications.
read/write head
The recording element which writes data to the magnetic media and reads recorded data from the media.
re-calibrate
The action of moving the head of a disk drive to cylinder zero.
recoverable error
A read error, transient or otherwise, falling within the capability of an ECC mechanism to correct, or able to overcome by rereading the data in question.
rotational latency
The amount of delay in obtaining information from a disk drive attributable to the rotation of the disk.
run-length limited
An encoding process that repositions data bits and limits the length of zero bits in order to compress information being stored on disks.
run-length limited encoding
A recording code. Sometimes meant to denote “2.7 RLL” which can signify 1.5 times the bits as MFM, given the same number of flux reversals in a given lineal distance.
GL – 6