Harddisk Glossary
ACCESS
Refers to the process of obtaining data from, or placing data into a
disc storage device, register, or RAM. (i.e. accessing a
memory location).
ACCESS TIME
Time required to perform an ACCESS. Usages, e.g.: 1) seek to
location on a disc, 2) amount of time to read or write to a memory
location, 3) the time to position to the correct location in a disc
drive and carry out a read or write operation. ACCESS TIME is often
defined as the time from the leading edge of the first step pulse
received to SEEK COMPLETE (including settling).
ACTUATOR
See HEAD POSITIONER. The two basic types of actuators are
steppers and voice coils. Open-loop steppers generally cannot
achieve tracks per inch (TPI) as high as the closed-loop system
because of the lack of feedback on track positioning accuracy.
In open-loop stepper drives mechanical tolerances are one of the
most significant factors in limiting TPI enhancement.
ADDRESS
(physical) A specific location in memory where a unit record, or
sector, of data is stored. To return to the same area on the disc,
each area is given a unique address consisting of three components:
cylinder, sector, and head. CYLINDER ADDRESSING is accomplished by
assigning numbers to the disc's surface concentric circles (cylinders).
The cylinder number specifies the radial address component of the data
area. SECTOR ADDRESSING is accomplished by numbering the data records
(sectors) from an index that defines the reference angular position of
the discs. Index records are then counted by reading their ADDRESS
MARKS. Finally, HEAD ADDRESSING is accomplished by vertically numbering
the disc surfaces, usually starting with the bottom-most disc data
surface. For example, the controller might send the binary equivalent
of the decimal number 610150 to instruct the drive to access data at
cylinder 610, sector 15, and head 0.
ADDRESS MARK
Two byte address at the beginning of both the ID field and the data
field of the track format. The first byte is the "A1" data pattern,
the second byte is used to specify either an ID field or a data
field.
ADJUSTABLE INTERLEAVE
Interleaving permits access to more than one memory module, e.g., if
one memory module contains odd-numbered address and another
even-numbered address, they can both be accessed simultaneously for
storage. If the interleave is adjustable, the user may select which
ranges or areas are to be accessed each time.
ANSI
American National Standards Institute
APPLICATION PROGRAM
A sequence of programmed instructions that tell the computer how to
perform an end use task (i.e. accounting, word processing or other
work for the computer system user). To use a program, it must first
be loaded into MAIN MEMORY from some AUXILIARY MEMORY such as a floppy
diskette or hard disk.
AREAL DENSITY
Bit density (bits per inch, or BPI) multiplied by track density
(tracks per inch, or TPI), or bits per square inch of the disc
surface. Bit density is measured around a track (circumferential on
the disc), and track density is radially measured.
ASCII
American Standard for Coded Information Interchange.
ASME
American Society of Mechanical Engineers
ASYNCHRONOUS DATA
Data sent usually in parallel mode without a clock pulse. Time
intervals between transmitted bits may be of unequal lengths.
AUTOMATIC BACK UP OF FILES
This gives a user the security to make changes to a file without
worrying about accidently destroying it; there is always another copy.
One weakness of this method is that files take up twice the room on a
disc.
AUXILIARY MEMORY
Memory other than main memory; generally a mass storage subsystem, it
can include disc drives, backup tape drives, controllers and buffer
memory. Typically, AUXILIARY MEMORY is non-volatile.
AUXILIARY STORAGE DEVICE
Devices, generally magnetic tape and magnetic disk, on which data can
be stored for use by computer programs. Also known as secondary
storage.
AVERAGE ACCESS TIME
The average track access time, calculated from the end of the
CONTROLLER commands to access a drive, to drive "seek complete" time
averaged over all possible track locations at the start of ACCESS,
and over all possible data track ADDRESSES. Typically, the minimum
average access time including carriage settling for open loop
actuators is less than 85 ms and for voice coil disc drives is less
than 40 ms. As technology improves these times will continue to
decrease.
AZIMUTH
The angular distance in the horizontal plane, usually measured as an
angle from true track location.
BACKUP DEVICE
Disc or tape drive
used with a fixed
copies of files or other data for off line storage, distribution or
protection against
accidental data deletion from the
drive, or against drive failure.
BACKUP FILE
File copies made on another removable media device (disc, tape or
sometimes a remote hard dsic system) and kept to ensure recovery of
data lost due to equipment failure, human errors, updates, disasters
and the like.
BAUD RATE
A variable unit of data transmission speed equal to one bit per
second.
BDOS
The Basic Disk Operating System (BDOS) controls the organization of
data on a disk. BDOS is usually pronounced "B-DOS".
BIDIRECTIONAL BUS
A buss that may carry information in either direction but not in both
simultaneously.
BINARY
A number system like the decimal numbers, but using 2 as its base and
having only the two digits 0 (zero) and 1 (one). It is used in
computers because digital logic can only determine one of two states
- "OFF" and "ON." Digital data is equivalent to a binary number.
BIOS
(BASIC INPUT OUTPUT SYSTEM) A collection of information (firmware)
that controls communication between the Central Processor and its
peripherals.
BIT
The smallest unit of data. Consists of a single binary digit that can
take the value of 0 or 1.
BIT CELL LENGTH
Physical dimension of the bit cell in direction of recording along
the disc circumference of a track.
BIT CELL TIME
The time required to pass one bit of information between the
controller and the drive. Cell time is the inverse of the drive's
data rate; nominally 200 nsec for 5 Mhz drives.
BIT DENSITY
Expressed as "BPI" (for bits per inch), bit density defines how many
bits can be written onto one inch of a track on a disc surface. It is
usually specified for "worst case", which is the inner track. Data is
the densest in the inner tracks where track circumferences are the
smallest.
BIT JITTER
The time difference between the leading edge of read and the center of
the data window.
BIT SHIFT
A data recording effect, which results when adjacent 1's written on
magnetic discs repel each other. The "worst case" is at the inner
cylinder where bits are closest together. BIT SHIFT is also called
pulse crowding.
BLOCK
A group of BYTES handled, stored and accessed as a logical data unit,
such as an individual file record. Typically, one block of data is
stored as one physical sector of data on a disc drive.
BOOT
(Short for bootstrap). Transfer of a disc operating system program
from storage on diskette or hard disc drive to computer's working
memory.
BUFFER
A temporary data storage area that compensates for a difference in
data transfer rates and/or data processing rates between sender and
receiver.
BUFFERED SEEK
A feature of the ST412 INTERFACE. In buffered mode head motion is
postponed until a string of step pulses can be sent to the drive.
These pulses represent the number of tracks that the head is to be
stepped over and are sent much faster than the heads can move. The
pulses are saved or buffered then the optimum head movement to the
correct track is performed.
BUS
A length of parallel conductors that forms a major interconnection
route between the computer system CPU and its peripheral subsystems.
Depending on its design, a bus may carry data to and from
peripheral's addresses, power, and other related signals.
BYTE
A sequence of adjacent BINARY digits or BITS considered as a unit, 8
bits in length. One byte is sufficient to define all the alphanumeric
characters. There are 8 BITS in 1 BYTE. The storage capacity of a disc
drive is commonly measured in MEGABYTES, which is the total number of
bits storable, divided by eight million.
CACHE MEMORY
Cache Memory allows the system to load bytes of data from the hard
disc to memory. The system may then refer to memory for information
instead of going back to the hard disc, thereby increasing the
processing speed.
CAPACITY
Amount of memory (measured in megabytes) which can be stored in a
disc drive. Usually given as formatted (see FORMAT OPERATION).
CARRIAGE ASSEMBLY
Assembly which holds read/write heads and roller bearings. It is
used to position the heads radially by the actuator, in order to
access a track of data.
CENTRAL PROCESSOR UNIT
(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, fast main memory for
temporary (VOLATILE) storage of an application program and its data.
CHARACTER
An information symbol used to denote a number, letter, symbol or
punctuation mark stored by a computer. In a computer a character can
be represented in one (1) byte or eight (8) bits of data. There are
256 different one-byte binary numbers, sufficient for 26 lower case
alphas, 26 upper case alphas, 10 decimal digits, control codes and
error checks.
CHIP
An integrated circuit fabricated on a chip of silicon or other
semiconductor material, e.g., a CHIP is an integrated circuit, a
microprocessor, memory device, or a digital logic device.
CLOCK RATE
The rate at which bits or words are transferred between internal
elements of a computer or to another computer.
CLOSED
A control system consisting of one or more feedback control loops in
which functions of the controlled signals are combined with functions
of the command to maintain prescribed relationships between the
commands and the controlled signals.
This control technique allows the head actuator system to detect and
correct off-track errors. The actual head position is monitored and
compared to the ideal track position, by reference information either
recorded on a dedicated servo surface, or embedded in the inter-sector
gaps. A position error is used to produce a correction signal
(FEEDBACK) to the actuator to correct the error. See TRACK FOLLOWING
SERVO.
CLUSTER SIZE
Purely an operating system function or term describing the number of
sectors that the operating system allocates each time disc space is
needed.
CODE
A set of unambiguous rules specifying the way which digital data is
represented physically, as magnetized bits, on a disc drive. One of
the objectives of coding is to add timing data for use in data
reading. See DATA SEPARATOR, MFM and RLL.
COERCIVITY
A measurement in units of orsteads of the amount of magnetic energy
to switch or "coerce" the flux change (di-pole) in the magnetic
recording media.
COMMAND
1) An instruction sent by the central processor unit (CPU) to a
controller for execution. 2) English-like commands entered by users
to select computer programs or functions. 3) A CPU command, which is
a single instruction such as "add two binary numbers" or "output a
byte to the display screen."
CONSOLE
(also called CRT or Terminal) A device from which a computer can be
operated; often includes a monitor and keyboard.
CONTROLLER
A controller is a printed circuit board required to interpret data
access commands from host computer (via a BUS), and send track
seeking, read/write, and other control signals to a disc drive. The
computer is free to perform other tasks until the controller signals
DATA READY for transfer via the CPU BUS.
CORE
Originally a computer's main memory was made of ferrite rings (CORES)
that could be magnetized to contain one bit of data each. CORE
MEMORY is synonymous with MAIN MEMORY. Main memory today is
fabricated from CHIPS.
CPU
See CENTRAL PROCESSOR UNIT
CRASH
A malfunction in the computer hardware or software, usually causing
loss of data.
CYCLIC-REDUNDANCY-CHECK
(CRC). Used to verify data block integrity. In a typical scheme,
2 CRC bytes are added to each user data block. The 2 bytes are
computed from the user data, by digital logical chips. The
mathematical model is polynomials with binary coefficients. When
reading back data, the CRC bytes are read and compared to new CRC
bytes computed from the read back block to detect a read error.
The read back error check process is mathematically equivalent to
dividing the read block, including its CRC, by a binomial
polynomial. If the division remainder is zero, the data is error
free.
CYLINDER
The cylindrical surface formed by identical track numbers on
vertically stacked discs. At any location of the head positioning
arm, all tracks under all heads are the cylinder. Cylinder number is
one of the three address components required to find a specific
ADDRESS, the other two being head number and sector number.
DAISY CHAIN
A way of connecting multiple drives to one controller. The
controller drive select signal is routed serially through the drives,
and is intercepted by the drive whose number matches. The disc drives
have switches or jumpers on them which allow the user to select the
drive number desired.
DATA
Information processed by a computer, stored in memory, or fed into a
computer.
DATA ACCESS
When the controller has specified all three components of the sector
address to the drive, the ID field of the sector brought under the
head by the drive is read and compared with the address of the target
sector. A match enables access to the data field of the sector.
DATA ADDRESS
To return to the same area on the disc, each area is given a
unique address consisting of the three components: cylinder, head
and sector. HORIZONTAL: accomplished by assigning numbers to the
concentric circles (cylinders) mapped out by the heads as the
positioning arm is stepped radially across the surface, starting
with 0 for the outermost circle. By specifying the cylinder
number the controller specifies a horizontal or radial address
component of the data area. ROTATIONAL: once a head and cylinder
have been addressed, the desired sector around the selected track
of the selected surface is found by counting address marks from
the index pulse of the track. Remember that each track starts
with an index pulse and each sector starts with an address mark.
VERTICAL: assume a disc pack with six surfaces, each with its own
read/write head, vertical addressing is accomplished by assigning
the numbers 00 through XX to the heads, in consecutive order. By
specifying the head number, the controller specifies the vertical
address component of the data area.
DATA BASE
An organized collection of data stored in DISC FILES, often shared by
multiple users., e.g., the Official Airline Guide, which contains
up-to-date schedules for all airlines.
DATA BASE MANAGEMENT SYSTEM
(DBMS) Application program used to manage, access and update files in a
data base.
DATA ENCODING
To use a code such as GCR, MFM, RLL, NZR, etc. to represent
characters for memory storage.
DATA FIELD
The portion of a sector used to store the user's DIGITAL data. Other
fields in each sector include ID, SYNC and CRC which are used to locate
the correct data field.
DATA SEPARATOR
Controller circuitry takes the CODED playback pulses and uses the
timing information added by the CODE during the write process to
reconstruct the original user data record. See NRZ, MFM, and RLL.
DATA TRACK
Any of the circular tracks magnetized by the recording head during
data storage.
DATA TRANSFER RATE
(DTR). Speed at which bits are sent: In a disc storage system, the
communication is between CPU and controller, plus controller and the
disc drive. Typical units are bits per second (BPS), or bytes per
second, e.g., ST506/412 INTERFACE allows 5 Mbits/sec. transfer rate.
DECREASE THE FLYING HEIGHT
Since the head core is closer to the media surface, the lines of flux
magnetize a smaller area. Thus, more bits can be recorded in a given
distance, and higher BPI (bits per inch) is achievable.
DEDICATED SERVO SYSTEM
A complete disc surface is dedicated for servo data.
DEFAULT
A particular value of a variable which is used by a computer unless
specifically changed, usually via an entry made through a software
program.
DENSITY
Generally, bit recording density. SEE AREAL, BIT and STORAGE
DENSITY.
DIGITAL
Any system that processes digital binary signals having only the
values of a 1 or 0. An example of a non-digital signal is an analog
signal which continuously varies, e.g., TV or audio.
DIGITAL MAGNETIC RECORDING
See MAGNETIC RECORDING
DIRECT ACCESS
Generally refers to an AUXILIARY MEMORY device, having all data
on-line. E.G., a tape drive without a tape mounted is not direct
access, but a
DIRECTORY
A special disc storage area (usually cylinder zero) that is read by a
computer operating system to determine the ADDRESSES of the data
records that form a DISC FILE.
DISC FILE
A file of user data, e.g. the company employee list, with all names
and information. The data in the file is stored in a set of disc
SECTORS (records).
DISC OPERATING SYSTEM
(DOS). A computer program which continuously runs and mediates
between the computer user and the APPLICATION PROGRAM, and allows
access to disc data by DISC FILE names.
DISC PACK
A number of metal discs packaged in a canister for removal from the
disc drive. WINCHESTER DRIVES do not have disc packs.
DISC/PLATTER
For rigid discs, a flat, circular aluminum disc substrate, coated on
both sides with a magnetic substance (iron oxide or thin film metal
media) for non-VOLATILE data storage. The substrate may consist of
metal, plastic, or even glass. Surfaces of discs are usually
lubricated to minimize wear during drive start-up or power down.
DISC STORAGE
Auxiliary memory system containing disc drives.
DISKETTE
A floppy disc. A plastic (mylar) substrate, coated with magnetic
iron oxide, enclosed in a protective jacket.
DRIVE
A computer memory device with moving storage MEDIA (disc or tape).
DRIVE SELECT
An ADDRESS component that selects among a string of drives attached
to a disc controller. In the ST 506/412 interface standard, a
drive's select code is physically set in the drive to a value between
0 and 3. When the controller activates one of the four drive select
code lines in the J1 cable, the selected drive is enabled to respond
to access commands from the controller.
DROP-IN/DROP-OUT
Types of disc media defects usually caused by a pin-hole in the disc
coating. If the coating is interrupted, the magnetic flux between
medium and head is zero. A large interruption will induce two
extraneous pulses, one at the beginning and one at the end of the
pin-hole (2 DROP-INs). A small coating interruption will result in no
playback from a recorded bit (a DROP-OUT).
DRUM
An early form of rotating magnetic storage, utilizing a rotating
cylindrical drum and a multiplicity of heads (one per track). Discs
stack more compactly than drums.
ECC
ERROR CORRECTION CODE: The ECC hardware in the controller used to
interface the drive to the system can typically correct a single burst
error of 11 bits or less. This maximum error burst correction length
is function of the controller. With some controllers the user is
allowed to the select this length. The most common selection is 11.
ELECTRO-STATIC DISCHARGE
(ESD) An integrated circuit (CHIP) failure mechanism. Since the
circuitry of CHIPs are microscopic in size, they can be damaged or
destroyed by small static discharges. People handling electronic
equipment should always ground themselves before touching the
equipment. Electronic equipment should always be handled by the
chassis or frame. Components, printed circuit board edge connectors
should never be touched.
EMBEDDED SERVO SYSTEM
Servo data is embedded or superimposed along with data on every
cylinder.
ERASE
To remove previously recorded data from magnetic storage media.
ERROR
See HARD ERROR and SOFT ERROR.
ESDI
ENHANCED SMALL DEVICE INTERFACE. A set of specifications for the
drives. See also SCSI.
EXECUTE
To perform a data processing operation described by an instruction or
a program in a computer.
FCI
(FLUX CHANGES PER INCH): Synonymous with FRPI (flux reversals per
inch). In MFM recording 1 FCI equals 1 BPI (bit per inch). In RLL
encoding schemes, 1 FCI generally equals 1.5 BPI.
FEEDBACK
A closed-loop control system, using the head-to-track positioning
signal (from the servo head) to modify the HEAD POSITIONER signal
(to correctly position the head on the track).
FETCH
A CPU read operation from MAIN MEMORY and its related data transfer
operations.
FIELDS
1. SOFTWARE: Storage units grouped together to make a record are
considered to be a field; e.g., a record might be a company's
address; a field in the record might be the company's ZIP code.
FILE
See DISC FILE. 1. SOFTWARE: A file consists of a group of
logically related records that, in turn, are made up of groups of
logically related fields.
FILE ALLOCATION TABLE
FAT: What the operating systems uses to keep track of which clusters
are allocated to which files and which are available for use. FAT is
usually stored on Track-0.
FILE NAME
Each file has a name, just like the name on the tab of a file
folder. When you want DOS to find a file, you give DOS the file
name.
FIRMWARE
A computer program written into a storage medium which cannot be
accidentally
erased, e.g.,
containing such programs.
FIXED DISC
A disc drive with discs that cannot be removed from the drive by
the user, e.g.,
FLOPPY DISC
A flexible plastic disc coated with magnetic media and packaged in
a stiff envelope. Comes in 8-inch, 5-1/4-inch, and various sub-4
inch sizes. FLOPPY DISCS generally exhibit slow ACCESS TIME and
smaller CAPACITY compared to WINCHESTER DRIVES, but feature
removable diskettes.
FLUX CHANGE
Location on the data track, where the direction of magnetization
reverses in order to define a 1 or 0 bit.
FLUX CHANGES PER INCH
(FCI). Linear recording density defined as the number of flux
changes per inch of data track.
FM
Frequency modulation CODE scheme, superceded by MFM, which is being
superceded by RLL.
FORMAT
The purpose of a format is to record "header" data that organize the
tracks into sequential sectors on the disc surfaces. This
information is never altered during normal read/write operations.
Header information identifies the sector number and also contains
the head and cylinder ADDRESS in order to detect an ADDRESS ACCESS
error.
FORMATTED CAPACITY
Actual capacity available to store user data. The formatted capacity
is the gross capacity, less the capacity taken up by the overhead data
used in formatting the discs. While the unformatted size may be 24 M
bytes, only 20 M bytes of storage may actually be available to the user
after formatting.
FPI
(flux changes per inch), also FRPI, the number of Flux Reversals per
inch.
FRICTION
Resistance to relative motion between two bodies in contact; e.g.,
there is sliding friction between head and disc during drive power
up/down.
FULL
mini-floppy drive (called the full-height form factor).
G
A G is a unit of force applied to a body at rest equal to the force
exerted on it by gravity. Hard disc drive shock specifications are
usually called out in Gs. A shock specification of 40 Gs non-operating
means that a drive will not suffer any permanent damage if subjected to
a 40 G shock. This is roughly equivalent to a drop of the drive to a
hard surface from a distance of 1 inch.
GAP
1. FORMAT: Part of the disc format. Allows mechanical compensations
(e.g. spindle motor rotational speed variations) without the last
sector on a track overwriting the first sector. 2. HEAD: An
interruption in the permeable head material, usually a glass bonding
material with high permeability, allowing the flux fields to exit the
head structure to write / read data bits in the form of flux changes on
the recording media.
GAP LENGTH
Narrowing the head gap length achieves higher bit density because the
lines of force magnetize a smaller area where writing data in the form
of flux changes on the recording media.
GAP WIDTH
The narrower the gap width, the closer the tracks can be placed. Closer
track placement results in higher TPI.
GCR
GROUP CODE ENCODING. Data encoding method.
GUARD BAND
1. Non-recorded band between adjacent data tracks, 2. For closed loop
servo drives, extra servo tracks outside the data band preventing the
Carriage Assembly from running into the crash stop.
HALF
A
mini-floppy drive.
HARD
Commonly called
rigid disc drives, or
electromechanical device that can read rigid discs. Though similar to
floppy disc drives, the hard discs have higher bit density and multiple
read/write surfaces.
HARD ERROR
An error that occurs repeatedly at the same location on a disc surface.
Hard errors are caused by imperfections in the disc surface, called
media defects. When formatting hard disc drives, hard error locations,
if known, should be spared out so that data ia not written to these
locations. Most drives come with a hard error map listing the
locations of any hard errors by head, cylinder and BFI (bytes from
index - or how many bytes from the beginning of the cylinder).
HARD ERROR MAP
Also called defect map, bad spot map, media map. Media defects are
avoided by deleting the defective sectors from system use, or assigning
an alternative track (accomplished during format operation). The
defects are found during formatting, and their locations are stored on
a special DOS file on the disc, usually on cylinder 0.
HARDWARE
Computer equipment (as opposed to the computer progrms and software).
HDA
HEAD/DISK
ASSEMBLY: A sealed
heads, filter and actuator assembly.
HEAD
An electromagnetic device that can write (record), read (playback),
or erase data on magnetic media. There are three types:
Head Type BPI TPI Areal density
Monolithic 8000
450 3.6 X
Composition 12000
1000 12 X
Thin-film 25000 1500 37.5 X 10 to 6th
HEAD CRASH
A head landing occurs when the disc drive is turned on or off. This
function normally does not damage the disc as the disc has a very thin
lubricant on it. A head crash occurs when the head and disc damage
each other during landing, handling or because a contaminant particle
gets between them. Head crash is a catastrophic failure condition and
causes permanent damage and loss of data.
HEAD LANDING AND TAKEOFF
In
drive is not powered. During the power up cycle, the disc begins
rotation and an "air bearing" is established as the disc spins up to
full RPM (rotations per minute). This air bearing prevents any
mechanical contact between head and disc.
HEAD LANDING ZONE
An area of the
disc set aside for takeoff and landing of the
heads when the drive is turned on and off.
HEAD POSITIONER
Also known as the ACTUATOR, a mechanism that moves the CARRIAGE
ASSEMBLY to the cylinder being accessed.
HEAD SLAP
Similar to a head crash but occurs while the drive is turned off. It
usually occurs during mishandling or shipping. Head slap can cause
permanent damage to a hard disc drive. See HEAD CRASH.
HEXIDECIMAL
(HEX) A number system based on sixteen, using digits 0 through 9 and
letters A through F to represent each digit of the number. (A = 10, B =
11, C = 12, D = 13, E = 14, F = 15).
ID FIELD
The address portion of a sector. The ID field is written during the
Format operation. It includes the cylinder, head, and sector
number of the current sector. This address information is compared
by the disc controller with the desired head, cylinder, and sector
number before a read or write operation is allowed.
IMAGE-BACKUP MODE
Used with streaming tape, image-backup mode records an exact copy of
the disc, including unused sectors and bad tracks.
INDEX
(PULSE): The Index Pulse is the starting point for each disc track.
The index pulse provides initial synchronization for sector addressing
on each individual track.
INDEX TIME
The time interval between similar edges of the index pulse, which
measures the time for the disc to make one revolution. This
information is used by a disc drive to verify correct rotational speed
of the media.
INPUT
1. Data entered into the computer to be processed. 2. User commands
or queries.
INPUT/OUTPUT
The process of entering data into or removing data from a computer
system.
INTELLIGENT PERIPHERAL
A peripheral device that contains a processor or microprocessor to
enable it to interpret and execute commands, thus relieving the
computer for other tasks.
INTERFACE
The protocol data transmitters, data receivers, logic and wiring that
link one piece of computer equipment to another, such as a disc drive
to a controller or a controller to a system bus. Protocol means
a set of rules for operating the physical interface, e.g., don't
read or write before SEEK COMPLETE is true.
INTERFACE STANDARD
The interface specifications agreed to by various manufacturers to
promote industry-wide interchangeability of products such as disc
drives and controllers. An interface standard generally reduces
product costs, allows buyers to purchase from more than one source,
and allows faster market acceptance of new products. (See ST-506/412,
SCSI, ESDI)
INTERLEAVE FACTOR
The ratio of physical disc sectors skipped for every sector actually
written.
INTERLEAVING
The interleave value tells the controller where the next logical
sector is located in relation to the current sector. For example, an
interleave value of one (1) specifies that the next logical sector is
physically the next sector on the track. Interleave of two (2)
specifies every other physical sector, three (3) every third sector
and so on. Interleaving is used to improve the system throughout
based on overhead time of the host software, the disc drive and the
controller; e.g., if an APPLICATION PROGRAM is processing sequential
logical records of a DISC FILE in a CPU time of more than one second
but less than two, then an interleave factor of 3 will prevent wasting
an entire disc revolution between ACCESSES.
INTERRUPT
A signal, usually from a peripheral device to a CPU, to signify that
a commanded operation has been completed or cannot be completed.
I/O PROCESSOR
Intelligent processor or controller that handles the input/output
operations of a computer.
KILOBYTE
(KBYTE). 1) 1024 bytes (two to the tenth power); 2) 1000 bytes;
1024 bytes is the normal definition.
LAN
Local Area Network
LANDING ZONE
The landing zone is where the read/write head sits when it is not
active. If the system features a dedicated landing zone, the head will
rest on the same track each time.
LATENCY
(ROTATIONAL) The time for the disc to rotate the accessed sector
under the head for read or write. On the average, latency is the
time for half of a disc revolution.
LOGIC
Electronic circuitry that switches on and off ("1" and "0") to
perform digital operations.
LOOKUP
The action of obtaining and displaying data in a file.
LOW LEVEL FORMAT
The first step in preparing a drive to store information after
physical installation is complete. The process sets up the
"handshake" between the drive and the controller. In an XT system,
the low level format is usually done using DOS's debug utility. In an
AT system, AT advanced diagnostics is typically used. Other third
party software may also be used to do low level format on both XTs and
ATs.
LUN
Logical Unit Number
MAGNETIC MEDIA
A disc or tape with a surface layer containing particles of metal, or
metallic oxides that can be magnetized in different directions to
represent bits of data, sounds or other information.
MAGNETIC RECORDING
The use of a head, recording head, recording media (tape or disc),
and associated electronic circuitry for storing data or sound or
video.
MAINFRAME COMPUTER
A large computer generally found in data processing centers. See
MINICOMPUTER AND MICROCOMPUTER.
MAIN MEMORY
Random-access memory used by the CPU for storing program instructions
and data currently being processed by those instructions. See
RANDOM-ACCESS MEMORY.
MEAN TIME BEFORE FAILURE
(MTBF). The average time before a failure will occur. This is not a
warranty measurement. MTBF is a calculation taking into consideration
the MTBF of each component in a system and is the statistical average
operation time between the start of a unit's lifetime and its time of
a failure. After a product has been in the field for a few years, the
MTBF can become a field proven statistic.
MEAN TIME TO REPAIR
(MTTR) The average time to repair a given unit. Limited to a
qualified technician with proper equipment.
MEDIA
The magnetic layers of a disc or tape. See DISC/PLATTER.
MEDIA DEFECT
A media defect can cause a considerable reduction of the read signal
(missing pulse or DROP-OUT), or create an extra pulse (DROP-IN). See
HARD ERROR MAP.
MEGABYTE
One million bytes (usually exactly 1,024,000 bytes). Abbreviation:
MB or Mbyte.
MEMORY
Any device or storage system capable of storing and retrieving
information. See also STORAGE DEFINITIONS.
MICROCOMPUTER
A computer whose central processor unit (CPU) is manufactured as a
chip or a small number of chips. Personal computers are examples of
microcomputers.
MICROINCH
One-millionth of an inch.
MICROSECOND
One-millionth of a second.
MILLISECOND
(Msec) One-thousandth of a second.
MINICOMPUTER
A computer midway in size and processing power between a
MICROCOMPUTER and a MAINFRAME COMPUTER.
MINI-SLIDER HEADS
Manganese/Zinc Ferrite Winchester heads. Smaller, lighter heads with
stiffer load arms
than standard
flying heights, and therefore higher bit and track density, if they are
made with smaller and narrower gaps.
MINI
A
MNEUMONIC
A shortened code for a longer term.
MODIFIED FREQUENCY MODULATION
(MFM). A method of recording digital data, using a particular CODE to
get the flux reversal times from the data pattern. MFM recording is
self-clocking because the CODE guarantees timing information for the
playback process. The controller is thus able to synchronize directly
from the data. This method has a maximum of one bit of data with each
flux reversal. (See NRZ, RLL).
MULTIPROCESSOR
A computer containing two or more processors.
MULTITASKING
The ability of a computer system to execute more than one program or
program task at a time.
MULTIUSER
The ability of a computer system to execute programs for more than
one user at a time.
NOISE
Extraneous electronic signals that interfere with information signals
(similar to radio static or TV interference). Sources of noise in
computers can be power supplies, ground loops, radio interference,
cable routing, etc.
NRZ
NON-RETURN TO ZERO 1) User digital data bits; 2) A method of
magnetic recording of digital data in which a flux reversal denotes
a one bit, and no flux reversal a zero bit, NRZ recording requires an
accompanying synchronization clock to define each cell time unlike
MFM or RLL recording). No Seagate drives use NRZ recording methods.
OFF LINE
Processing or peripheral operations performed while not connected to
the system CPU via the system BUS.
OPEN COLLECTOR
A type of output structure found in certain bipolar logic families.
The device has a transistor that enables it to output to a low
voltage level only. When the device is inactive, an external
resistor holds the device output at a high voltage level.
OPERATING SYSTEM
An operating system is a program which acts as an interface
between the user of a computer and the computer hardware.
The purpose of the operating system is to provide an
environment in which a user may run programs. The goal of
the operating system is to enable the user to conveniently
use the computer's resources such as the CPU, memory, storage
devices and printers.
OUTPUT
Processing data being transferred out of the computer system to
peripherals (i.e. disc, printer, etc.). This includes responses to
user commands or queries.
PARITY
A computer data checking method using an extra bit in which the total
number of binary 1's (or 0's) 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 9th 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.
PARKING
Parking the disc drive heads means the recording heads are moved so
that they are not over the platter's data area. Many drives have an
auto-park feature where the heads are automatically parked when power
to the drive is shut off. Other drives require the user to run some
kind of parking software to park the heads.
PARTITIONING
Method for dividing an area on disc drive for use by more than one disc
operating system or for dividing large disc drives into areas which the
File Allocation Table (FAT) can deal with when in use. The current IBM
DOS maximum partition size is 32 MB for the XT and AT. This limit can
be overridden using partitioning software written expressly for this
purpose.
PATH
The DOS term "path" has three definitions and each definition involves
directories. A PATH may be defined as: 1) the names of the chain of
directories leading to a file; 2) the complete file or directory name;
3) a DOS command.
PERIPHERAL EQUIPMENT
Auxiliary memory, displays, printers, disc drives, and other
equipment usually attached to computer systems' CPU by controllers
and cables (they are often packaged together in a desktop computer).
PLATED THIN FILM DISCS
Magnetic disc memory media having its surface plated with a thin
coating of a metallic alloy instead of being coated with oxide.
PLATTER
The round magnetic disc surfaces used for read/write operations in a
hard disc system.
POLLING
A technique that discerns which of several devices on a connection is
trying to get the processor's attention.
PRECOMPENSATION
Applied to write data by the controller in order to partially
alleviate bit shift which causes adjacent 1's written on magnetic
media physically to move apart. When adjacent 1's are sensed by the
controller, precompensation is used to write them closer together
on the disc, thus fighting the repelling effect caused by the
recording. Precompensation is only required on some oxide media
drives.
PREVENTIVE MAINTENANCE
A method of doing a scheduled routine observation or exchanging a
part, prior to a breakdown of a piece of equipment.
PRINTED CIRCUIT BOARD
(PCB) The circuit board with the chips attached to a drive.
PROCESSING
(DATA PROCESSING) The process of computer handling, manipulating,
and modifying data such as arithmetic calculation, file lookup and
updating, or word processing.
PROGRAM
A sequence of instructions stored in memory and executed by a
processor or microprocessor. See also APPLICATIONS PROGRAMS.
PROTOCOL
A set of conventions governing the format of messages to be exchanged
within a communications system.
RADIAL
A way of connecting multiple drives to one controller. In radial
operation, all output signals are active even if the drive is not
selected. Also see DAISY CHAIN.
RAM DISC
A DOS operation, where part of the computer's random access memory
is used to simulate a disk drive. The RAM disc and its contents
will disappear if power is lost or DOS MAIN MEMORY is restarted.
RAM is far faster (microseconds ACCESS TIME) than discs (milliseconds),
so APPLICATIONS PROGRAMS which access the disk run faster.
RANDOM ACCESS MEMORY
(RAM) Memory where any location can be read from or written to in a
random order. Random access memory usually refers to volatile memory
where the contents are lost when power is removed. The user
addressable memory of a computer is random access memory.
READ
To access a storage location and obtain previously recorded data.
RECALIBRATE
Return to Track Zero. A common disc drive function in which the
heads are returned to track 0 (outermost track).
RECORD
1. Software. A record is a single unit made up of logically related
fields.
REDUCED WRITE CURRENT
A signal input (to some older drives) which decreases the amplitude
of the write current at the actual drive head. Normally this signal
is specified to be used during inner track write operations to
lessen the effect of adjacent bit "crowding." Most drives today
provide this internally and do not require controller intervention.
RESOLUTION
With regards to magnetic recording, the band width (or frequency
response) of the recording heads.
RLL
(RUN LENGTH LIMITED CODE). 1) A method of recording digital data,
whereby the combinations of flux reversals are coded/decoded to allow
greater than one (1) bit of information per flux reversal. This
compaction of information increases data capacity by approximately
50 percent; 2) a scheme of encoding designed to operate with the
ST412 interface at a dial transfer rate of 7.5 megabit/sec. The
technical name of the specific RLL CODE used is "two, seven".
ROM
(READ ONLY MEMORY) A chip that can be programmed once with bits of
information. This chip retains this information even if the power is
turned off. When this information is programmed into the ROM, it is
called burning the
ROTATIONAL SPEED
The speed at which
the media spins. On a 5-1/4 or
3-1/2"
drive it is usually 3600 rpm.
SCSI
Small Computer Systems Interface. The current "high end" CPU-to-drive
interface.
SECTOR
A sector is a section of a track whose size is determined by
formatting. When used as an address component, sector and location
refer to the sequence number of the sector around the track.
Typically, one sector stores one user record of data. Drives typically
are formatted from 17 to 26 sectors per track. Determining how many
sectors per track to use depends on the system type, the controller
capabilities and the drive encoding method and interface.
SECTOR-SLIP
Sector-slip allows any sector with a defect to be mapped and
bypassed. The next contiguous sector is given that sector address.
SEEK
The radial movement of the heads to a specified track address.
SEEK COMPLETE
An ST506 interface signal from drive to controller which indicates that
read/write heads have settled on the desired track and completed the
seek.
SEQUENTIAL ACCESS
Writing or reading data in a sequential order, such as reading data
blocks stored one after the other on magnetic tape (the opposite of
random access).
SERVO TRACK
A prerecorded reference track on the dedicated servo surface of a
closed-loop disc drive. All data track positions are compared to
their corresponding servo track to determine "off-track/on-track"
position.
SILICON
Semiconductor substrate material generally used to manufacture micro-
processors and other integrated circuit chips.
SMD
SURFACE MOUNTED DEVICE. A CHIP in a smaller integrated surface
package, without connection leads.
SOFT ERROR
A bit error during playback which can be corrected by repeated
attempts to read.
SOFTWARE
APPLICATION PROGRAMS, disc operating systems and other programs (as
opposed to HARDWARE). The instructions or programs, usually stored
on floppy or hard discs, which are used to direct the operations
of a computer, or other hardware.
SOFTWARE PATCH
Software modification which allows or adds functions not otherwise
available using the standard software program.
SPINDLE
The rotating hub structure to which the discs are attached.
SPINDLE MOTOR
The spindle motor is the electro-mechanical part of the disc drive
that rotates the platters.
ST-506/ST-412 INTERFACE
One of several industry standard interfaces between a hard disc and
hard disc controller. In the ST-506/ST-412 interface, the
"intellegence" is on the controller rather than the drive.
SEE INTERFACE STANDARD, ESDI AND SCSI.
STEP
An increment or decrement of the head positioning arm to move the
heads in or out, respectively, one track from their current
position. In buffered mode (open loop drives), the head motion
is postponed until the last of a string of step pulses has been
received.
STEPPER MOTOR
The stepper motor is the electro-mechanical part of the disc drive that
positions the heads by step pulse on the tracks of the disc to read and
write data.
STEP PULSE
The pulse sent from the controller to the stepper motor on the step
interface signal line to initiate a step operation.
STEP TIME
The time required by the drive to step the heads from the current
cylinder position to a target cylinder.
STORAGE CAPACITY
Amount of data that can be stored in a memory, usually specified in
kilobytes (KB) for main memory and floppy disc drives and megabytes
(MB) for hard disc and tape drives.
STORAGE DENSITY
Usually refers to recording density (BPI, TPI, or their product,
AREAL DENSITY).
STORAGE LOCATION
A memory location, identified by an ADDRESS, where information is to
be read or written.
STORAGE
(SMD). Storage module drive interface. An interface, used in larger
disc drives, e.g., 14" drives.
SYNCHRONOUS DATA
Data sent, usually in serial mode, with a clock pulse.
TAPE DRIVE
A sequential access memory device whose magnetic media is tape in a
cassette, reel or continuous loop.
THIN FILM HEADS
A read/write head whose read/write element is deposited using
integrated circuit techniques rather than being manually fabricated
by grinding ferrite and hand winding coils.
TPI
Tracks per inch.
TRACK
The radial position of the heads over the disc surface. A track is
the circular ring traced over the disc surface by a head as the
disc rotates under the heads.
TRACK ACCESS TIME
See AVERAGE ACCESS TIME.
TRACK DENSITY
See TPI.
TRACK FOLLOWING SERVO
A closed-loop positioner control system that continuously corrects
the position of the disc drive's heads by utilizing a reference
track and a feedback loop in the head positioning system. See also
CLOSED
TRACK PITCH
Distance from centerline to centerline of adjacent tracks (TPI
divided into 1.0).
TRACKS PER INCH
Track density, number of tracks per inch.
TRACK WIDTH
Width of data track. Also called core width of Read/Write Head.
TRACK ZERO
Track zero is the outermost data track on a disc drive. In the ST
506 INTERFACE, the interface signal denotes that the heads are
positioned at the outermost cylinder.
TRACK ZERO DETECTOR
An obsolete technology that RECALIBRATES by sensing when infrared
beams between a LED and infrared sensitive photo-transistor are
blocked by the track zero interrupter (TZI).
TUNNEL ERASE
An erase scheme where both sides of the recorded data is erased when
writing data to eliminate track to track interference. This is
primarily used on floppy disk drives.
UNFORMATTED
(Capacity) Drive byte capacity before formatting. Maximum capacity of
a disc drive before formatting = (bits per track) x number of heads x #
of cylinders. See MEGABYTE.
UPGRADE PATH
Generally, with disc products, a family having multiple products with
varying capacities such that the system storage capacity can increase
with changing application requirements simply using a different disc
drive within the product family.
VERIFICATION
This feature lets the computer go back and read what it just wrote to
disc to ensure the data was written correctly.
VOICE COIL MOTOR
An electro-magnetic positioning motor in the rigid disk drive similar
to that used in audio speakers. A wire coil is placed in a stationary
magnetic field. When current is passed through the coil, the resultant
flux causes the coil to move. In a disc drive, the CARRIAGE ASSEMBLY
is attached to the voice coil motor. Either a straight line (linear)
or circular (rotary) design may be employed to position the heads on
the disc's surface.
VOLATILE
Memory that will be erased if power is lost. Typically, MAIN MEMORY
is volatile, and AUXILIARY MEMORY is non-volatile and can be used
for permanent (but changeable at will) storage of programs and data.
WAN
Wide Area Network
WEDGE SERVO SYSTEM
A certain part of each CYLINDER contains servo positioning data. Gap
spacing between each sector contains servo data to maintain position
on that cylinder.
A disc drive with
a
sealed in a contaminant-free housing.
WORD
Number of bits processed in parallel (in a single operation) by a
CPU. Standard word lengths are 8, 16, 32, and 64 (1, 2, 4 or 8 bytes).
WRITE
To access a storage location and store data on the magnetic surface.
WRITE CURRENT
The optimum HEAD write current necessary to saturate the magnetic
media in a cell location.
WRITE FAULT
Disc drive interface signal to the controller used to inhibit further
writing when a condition exists in the drive which, if not detected,
would cause improper writing on the disc.
XSMD
Extended storage module drive interface.