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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.