GLOSSARY A ACCESS ­ (v) Read, write, or update information stored on a disk or other medium. (n) The operation of reading, writing, or updating stored information. ACCESS TIME ­ The interval between the time a request is made by the system and the time the data is available from the drive. Includes the seek time, rotational latency, and command processing overhead time. (See also seek, rotational latency, and overhead.) ACTUATOR ­ Also known as the positioner. The internal mechanism that moves the read/write head to the proper track. The Quantum actuator consists of a rotor connected to head mounting arms that position the heads over the desired cylinder. Also known as rotary actuator. AIRLOCK ­ A patented Quantum feature that ensures durable and reliable data storage. Upon removal of power from the drive for any reason, the read/write heads automatically park and lock in a non data area called the landing zone. Airlock allows the drive to withstand high levels of non-operating shock. When power is applied to the drive, airflow created from the spinning disks causes the Airlock arm to swing back and unlock the actuator, allowing the heads to move from the landing zone. Upon power down, the Airlock swings back to the locked position, locking the heads in the landing zone. A park utility is not required to park the heads on drives equipped with Airlock (all Quantum drives). ALLOCATION ­ The process of assigning particular areas of the disk to specific data or instructions. An allocation unit is a group of sectors on the disk reserved for specified information. On hard disks for small computer systems, the allocation unit is usually in the form of a sector, block, or cluster. (See also allocation unit.) ALLOCATION UNIT ­ An allocation unit, also known as a cluster, is a group of sectors on the disk that can be reserved for the use of a particular file. ASIC ­ Acronym for Application Specific Integrated Circuit. AVERAGE SEEK TIME ­ The average time it takes for the read/write head to move to a specific location. Calculated by dividing the time it takes to complete a large number of random seeks by the number of seeks performed. B BACKUP ­ A copy of a file, directory, or volume on a separate storage device from the original, for the purpose of retrieval in case the original is accidentally erased, damaged, or destroyed. BAD BLOCK ­ A block (usually the size of a sector) that cannot reliably hold data due to a physical flaw or damaged format markings. BAD TRACK TABLE ­ A label affixed to the casing of a hard disk drive stating which tracks are flawed and cannot hold data. This list is typed into the low-level formatting program when the drive is installed. Quantum users can ignore bad track tables since Quantum's built-in defectmanagement protections compensate for these flaws automatically. BEZEL ­ A plastic panel that extends the face of a drive so that it covers a computer's drive bay opening. The bezel usually contains a driveactivity LED. Also known as the faceplate. BIT ­ Abbreviation for binary digit. A binary digit may have one of two values--1 or 0. This contrasts with a decimal digit, which may have a value from 0 to 9. A bit is one of the logic 1or logic 0 binary settings that make up a byte of data. (See also byte.) BLOCK ­ In UNIX workstation environments, the smallest contiguous area that can be allocated for the storage of data. UNIX blocks are generally 8 Kbytes (16 sectors) in size. In DOS environments, the block is referred to as a cluster. (Note: This usage of the term block at the operating system level is different from its meaning in relation to the physical configuration of the hard drive. See sector for comparison.) Viking 2.27/4.55S G-1 Thi d t t d ith F Mk 404 Glossary BPI ­ Bits Per Inch. A measure of how densely information is packed on a storage medium. (See also FCI.) BUFFER ­ An area of RAM reserved for temporary storage of data that is waiting to be sent to a device that is not yet ready to receive it. The data is usually on its way to or from the hard disk drive or some other peripheral device. BUS ­ The part of a chip, circuit board, or interface designed to send and receive data. BYTE ­ The basic unit of computer memory, large enough to hold one character of alphanumeric data. Comprised of eight bits. (See also bit.) D DATA SEPARATOR ­ The circuit that extracts data from timing information on drives that store a combined data and clock signal. DEDICATED SERVO ­ A positioning mechanism using a dedicated surface of the disk that contains timing and positioning information only, as compared to surfaces that are also used for data. (See also embedded servo.) DEFECT MANAGEMENT ­ A technique ensuring long-term data integrity. Consists of scanning disk drives both at the factory and during regular use, de-allocating defective sectors before purchase and compensating for new defective sectors afterward. DISK ­ In general, any circular-shaped datastorage medium that stores data on the flat surface of the platter. The most common type of disk is the magnetic disk, which stores data as magnetic patterns in a metal or metal-oxide coating. Magnetic disks come in two forms: floppy and hard. Optical recording is a newer disk technology that gives higher capacity storage but at slower access times. DISK CONTROLLER ­ A plug-in board, or embedded circuitry on the drive, that passes information to and from the disk. The Quantum hard disk drives all have controllers embedded on the drive printed-circuit board. (See also controller.) DMA ­ Direct Memory Access. A process for transferring data directly to and from main memory, without passing through the CPU. DMA improves the speed and efficiency by allowing the system to continue processing even while new data is being retrieved. DOS ­ Disk Operating System. The most common operating system used in IBM PCs. Manages all access to data on the disk. DRIVE ­ Short form of disk drive. C CACHE ­ Specialized High-speed RAM used to optimize data transfers between system elements with different performance characteristics, e.g., disk to main memory or main memory to CPU. CAPACITY ­ The amount of information that can be stored on a hard drive. Also known as storage capacity. (See also formatted capacity.) CLEAN ROOM ­ An environmentally controlled dust-free assembly or repair facility in which hard disk drives are assembled or can be opened for internal servicing. CLUSTER ­ In DOS environments, the smallest contiguous area that can be allocated for the storage of data. DOS clusters are usually 2 Kbytes (4 sectors) in size. CONTROLLER ­ The chip or circuit that translates computer data and commands into a form suitable for use by the hard drive. Also known as disk controller. CONTROLLER CARD ­ An adapter containing the control electronics for one or more hard disks. Usually installed in a slot in the computer. CPU ­ Central Processing Unit. The microprocessor chip that performs the bulk of data processing in a computer. CRC ­ Cyclic Redundancy Check. An error detection procedure that identifies incomplete or faulty data in each sector. CYLINDER ­ When disks are placed directly above one another along the shaft, the circular, vertical "slice" consisting of all the tracks located in a particular position. DRIVE GEOMETRY ­ The functional dimensions of a drive, including the number of heads, cylinders, and sectors per track. (See also logical format.) G-2 Viking 2.27/4.55S Glossary E ECC ­ Error Correction Code. The incorporation of extra parity bits in transmitted data in order to detect errors that can be corrected by the controller. EMBEDDED SERVO ­ A timing or location signal placed on tracks that store data. These signals allow the actuator to fine-tune the position of the read/write heads. ENCODING ­ The conversion of data into a pattern of On/Off or 1/0 signals prior to being written on the disk surface. (See also RLL and MFM.) EPROM ­ Erasable Programmable Read-Only Memory. An integrated circuit memory chip that can store programs and data in a non-volatile state. These devices can be erased by ultraviolet light and reprogrammed with new data. EXTERNAL DRIVE ­ A drive mounted in an enclosure separate from the computer system enclosure, with its own power supply and fan, and connected to the system by a cable. FLYING HEIGHT ­ The distance between the read/write head and the disk surface, made up of a cushion of air that keeps the two objects from touching. Smaller flying heights permit denser data storage but require more precise mechanical designs. Also known as fly height. FORMAT ­ To write a magnetic track pattern onto a disk surface, specifying the locations of the tracks and sectors. This information must exist on a disk before it can store data. FORMATTED CAPACITY ­ The amount of room left to store data on a disk after writing sector headers, boundary definitions, and timing information during a format operation. The size of a Quantum drive is always expressed in formatted capacity, accurately reflecting the usable space required. FORM FACTOR ­ The industry standard that defines the physical, external dimensions of a particular device. For example, most Quantum hard disk drives use a 3 1/2-inch form factor. F FCI ­ Flux Changes per Inch. The number of magnetic field patterns that can be stored on a given area of disk surface, used as a measure of data density. (See also BPI.) FILE SERVER ­ A computer that provides network stations with controlled access to shareable resources. The network operating system is loaded on the file server, and most shareable devices (disk subsystems, printers) are attached to it. The file server controls system security and monitors station-to-station communications. A dedicated file server can be used only as a file server while it is on the network. A non dedicated file server can be used simultaneously as a file server and a workstation. FIRMWARE ­ Permanent instructions and data programmed directly into the circuitry of readonly memory for controlling the operation of the computer. Distinct from software, which can be altered by programmers. FLUX DENSITY ­ The number of magnetic field patterns that can be stored in a given length of disk surface. The number is usually stated as flux changes per inch (FCI), with typical values in the thousands. (See also FCI.) G GIGABYTE (GB) ­ One billion bytes (one thousand megabytes). GUIDE RAILS ­ Plastic strips attached to the sides of a hard disk drive in an IBM PC/AT or compatible computer so that the drive easily slides into place. H HALF-HEIGHT ­ Standard drive size equivalent to half the vertical space of a 5 1/4-inch drive. HARD DISK ­ A type of storage medium that retains data as magnetic patterns on a rigid disk, usually made of an iron oxide or alloy over a magnesium or aluminum platter. Because hard disks spin more rapidly than floppy disks, and the head flies closer to the disk, hard disks can transfer data faster and store more in the same volume. HARD ERROR ­ A data error that persists when the disk is re-read, usually caused by defects in the physical surface. HARD-SECTORED ­ The most common method of indicating the start of each sector on a disk, based on information located in the embedded Viking 2.27/4.55S G-3 Glossary servo. This method is more precise than softsectored techniques and results in lower overhead. (See also soft-sectored.) HEAD ­ The tiny electromagnetic coil and metal pole used to create and read back magnetic patterns on the disk. Also known as read/write head. HEAD CRASH ­ Damage to the read/write head, usually caused by sudden contact with the disk surface. Head crash can also be caused by dust and other particles. HIGH-CAPACITY DRIVE ­ By industry conventions typically a drive of 100 megabytes or more. HIGH-LEVEL FORMATTING ­ Formatting performed by the operating system to create the root directory, file allocation tables and other basic configurations. (See also low-level formatting.) HOME ­ Reference track used for recalibration of the actuator. Usually the outermost track (track 0). HOST ADAPTER ­ A plug-in board that acts as the interface between a computer system bus and the disk drive. by, and then the next is read. It would take three revolutions of the disk to access a full track of data. Quantum drives have an interleave of 1:1, so a full track of data can be accessed within one revolution of the disk, thus offering the highest data throughput possible. INTERNAL DRIVE ­ A drive mounted inside one of a computer's drive bays, or a hard disk on a card installed in one of the computer's expansion slots. J JUMPER ­ A tiny box that slips over two pins on a circuit board, connecting the pins electrically. Some board manufacturers use Dual In-Line Package (DIP) switches instead of jumpers. K KILOBYTE (K) ­ A unit of measure consisting of 1,024 (210) bytes. L LANDING ZONE ­ A non-data area on th ...