ASME's Mechanical Engine…ing Toolkit 1997 December/ASME's Mechanical Engineering Toolkit 1997 December.iso/edu_comp/glossary.lzh/GLOSSARY.TXT

Text File | 1989-01-09 | 56KB | 2,315 lines

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 Winchester disc drive to make
    copies of files or other data for off line storage, distribution or
    protection against accidental data deletion from the Winchester
    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 LOOP
    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 WINCHESTER DRIVE is direct access.





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., ROM.  It can also refer to devices
    containing such programs.




FIXED DISC
    A disc drive with discs that cannot be removed from the drive by
    the user, e.g., WINCHESTER DISC DRIVE.





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 HEIGHT DRIVE
    Winchester 5-1/4" drive which fits in the same space as full height
    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 HIGH DRIVE
    A Winchester drive which fits in one half of the space of a full height
    mini-floppy drive.





HARD DISC DRIVE
    Commonly called rigid disc drives, or Winchester disc drives.  An
    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 Winchester assembly including discs,
    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 10 to 6th
      Composition  12000       1000          12 X 10 to 6th
      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 Winchester drives, the head is in contact with the platter when the
    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 Winchester
    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 Winchester heads.  They allow smaller
    flying heights, and therefore higher bit and track density, if they are
    made with smaller and narrower gaps.





MINI WINCHESTER
    A Winchester disc drive with 5-1/4 or 3-1/2 inch diameter discs.





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 ROM.





ROTATIONAL SPEED
    The speed at which the media spins.  On a 5-1/4 or 3-1/2" Winchester
    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 MODULE DRIVE
     (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 LOOP.





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.





WINCHESTER DRIVE
    A disc drive with a Winchester head and non-removable (fixed) discs
    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.