A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Notice: The information provided herein is presented as is and without any warranty of any kind, either expressed or implied. In no event will DEW Associates Corporation be liable to you for any damages, including any loss of profit or other money damages, arising out of the use or inability to use the information contained herein. We make no representations or warranties with respect to the contents hereof and specifically disclaims any implied warranties of merchantability or fitness for any particular purpose. Any parameters and specifications provided are subject to change without notice.
1. The condition of two planes or parallel lines. Important in disk drives because a lack of it in mechanical assemblies can result in positioning inaccuracy. More precisely: planes-coplanar; lines-colinear.
2. Is the local variation in disk thickness measured independently of thickness itself.
3. The ability of a multiprocessor computer to allocate more than one processor (CPU) to a computing problem, where each CPU works on a separate problem or separate segment of that problem. Also referred to as parallel processing.
PARITY
A simple method of data error detection 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 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.
PC-CARD (PCC)
Also known as PCMCIA
PCMCIA
Personal Computer Memory Card International Association. PCMCIA is an organization
consisting of some 500 companies that has developed a standard for small, credit card-sized
devices, called PC Cards. Originally designed for adding memory to portable computers, the
PCMCIA standard has been expanded several times and is now suitable for many types of
devices. There are in fact four types of PCMCIA cards. All four have the same rectangular
dimensions, 85.6 by 54 millimeters, but differ in thickness:
| Type | Thickness |
|---|---|
| I | 2mm |
| II | 5mm |
| III | 10.5mm |
| IV | 15mm |
PEAK SHIFT
The shifting in time of the zero-slope portion of a read-back voltage from the values
contained in the write current waveform. Sometimes incorrectly used to describe bit jitter.
PERIPHERAL EQUIPMENT
Auxiliary memory, displays, printers, scanners and other equipment usually attached to a computer
system through either controllers or 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.
|
Hard Disk |
|
CPU |
|
Memory |
- The system specifies the location where the data transfer is to begin, and how many sectors to transfer.
- The system then sends a command to transfer the data.
- The drive sets DRQ (Data ReQuest) when it is ready to transfer the data.
READ: The drive does not set DRQ until it has a sector of data ready. The drive also asserts the INTRQ (INTerrupt ReQuest)line when the data is ready.
WRITE: The drive sets DRQ but does not issue an interrupt until after the data is transferred. - The CPU transfers 256 words of data by asserting IOR- (IO Read) or IOW- (IO Write) 256 times. 5) If more data than one sector was requested, the drive/CPU will repeat steps 3 and 4 until all data has been transferred.
| Sectors per Interrupt |
Total Interrupts |
|---|---|
| 1 | 2048 |
| 2 | 1024 |
| 4 | 512 |
| 8 | 256 |
| 16 | 128 |
| 32 | 64 |
From this table we see that by setting the Multiple Block Mode to 16, we eliminate 1920 (2048-128) interrupts. This shows a significant decrease in the time the drive is waiting on the system. The newer the drive the higher # of Sect/Int it can handle. If you set the blocks higher than the drive is capable of, it is the same as setting it to 1. Cycle Times as per ATA Specs:
| PIO Mode | ||
|---|---|---|
| Mode | Cycle Times | Transfer Rate |
| 0 | 600ns | 3.33MB/s |
| 1 | 383ns | 5.22MB/s |
| 2 | 240ns | 8.33MB/s |
| 3 | 180ns | 11.11MB/s |
| 4 | 150ns | 16.66MB/s |
| Single Word DMA | ||
|---|---|---|
| Mode | Cycle Times | Transfer Rate |
| 0 | 960ns | 2.80MB/s |
| 1 | 480ns | 260MB/s |
| 2 | 260ns | 8.33MB/s |
| Multi-Word DMA | ||
|---|---|---|
| Mode | Cycle Times | Transfer Rate |
| 0 | 480ns | 4.16MB/s |
| 1 | 150ns | 13.33MB/s |
| 2 | 60ns | 16.66MB/s |
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.
PLATTER
Magnetic disk memory media. Each platter (disk) has two surfaces, the top and bottom
surface. Each surface has it's own R/W Head for reading,
writing and erasing data.
PLUG AND PLAY (PnP)
The Plug and Play specification was developed by Microsoft in cooperation with
the Intel Corporation as well as a number of other hardware manufacturers. The
primary focus of Plug and Play is to develop hardware (a computer and its
internal components) and software that will work together to automatically configure
the hardware devices and automatically calculate and assign system resources.
This will allow hardware changes and additions without the need for manual resource
assignment or tweaking. As the name suggests, the goal is to be able to just plug in a new
device and immediately be able to use it, without complicated setup procedures. PnP for PCs
became a reality with the release of Windows 95 and PC hardware designed to work with it.
POST (Power On Self-Test)
In a computer system based on one of Intel's x86 processors or compatibles, program execution starts at memory location F000:FFF0h. This address is part of ROM BIOS and contains a jump command to a series of BIOS routines that test and initialize the hardware
components, the Power-On-Self-Test or POST.
POST consists of various test- and initialization routines for the on-board hardware and expansion cards such as the video adapter, modem and other similar devices. If a failure is detected this can be made known by:
- one or more audible beeps, which are referred to as beep codes;
- an error message that is displayed on the video screen;
- a checkpoint code, which is sent to one of the system's output ports.
PREFIX
A prefix is usually a number or letter at the beginning of a drive model number
or serial number that denotes a special characteristic about the drive.
PROCESSING
The process of the computer handling, manipulating and modifying data such as arithmetic
calculation, file lookup and updating, and word processing.
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.
- - Q -
The quality acceptance rate normally refers to the overall percentage of drives (or other products) successfully installed by users, and/or the number of defective parts per million (DPPM) encountered during the entire installation process.
- - R -
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. Visit our Performance Center for more information on Random Access Memory.
READ
To access a storage location and obtain previously recorded data. This occurs
with respect to memory as well as drives and other forms of media. On drives,
this involves the sensing of or 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 level 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 used for permanent information storage. Computer control programs are
often stored in ROM.
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 string of zero bits
in order to compress information being stored on disks.
RUN-LENGTH LIMITED ENCODING
A recording code. Various types exist with different efficiencies. 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.
R/W MULTIPLE MODE
This mode is implemented per ANSI ATA/ATAPI-4 specification. Read/Write Multiple allows the
host to transfer a set number of sectors without an interrupt request between them, reducing
transfer process overhead and improving host performance.
