Computer Science and Networking Glossary-H

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H.323
HCI
HCL
HCT
HDSL
hex
HGC
HID
HMA
HMMP
HPFS
HSB
HSSI
HTML
HTTP
Hz

half-duplex mode: Also called half-duplex point-to-point link. This is a type of point-to-point connection in which devices take turns using a single wire. When one device is finished transmitting, it must "turn over" the communication channel so that the other device knows that is is permitted to transmit. When both devices can transmit at the same time, the operation is called full-duplex mode.
half-height drive: A space-saving drive bay that is half the height of the 3" drive bays used in the original IBM PC. Most of today's drives are half-height drives.
hand-held computer: A portable computer that is small enough to be held in one hand.
hand scanner: An optical device used to digitize a relatively small image or artwork.
hard card: A single expansion board that contains a small hard disk and associated controller circuitry. A hard card allows you to add another hard disk, even when all your drive bays are occupied, as long as there is still a single expansion slot available. Hard cards were brought to prominence by Plus Development Corporation.
hard disk: The part of a hard disk drive that stores data, rather than the mechanism for reading and writing to it.
hard disk controller: An expansion board that contains the necessary circuitry to control and coordinate a hard disk drive. Many hard disk controllers are capable of managing more than one hard disk, as well as floppy disks and even tape drives.
hard disk drive: A storage device that uses a set of rotating, magnetically coated disks called platters to store data or programs. In everyday use, the terms "hard disk," "hard disk drive," and "hard drive" are all used interchangeably, because the disk and the drive mechanism are a single unit.
A typical hard disk platter rotates at up to 3600 rpm, and the read/write heads float on a cushion of air from 10 to 25 millionths of an inch thick so that the heads never come into contact with the recording surface. The whole unit is hermetically sealed to prevent airborne contaminants from entering and interferring with these close tolerances.
Hard disks range in capacity from a few tens of megabytes to several gigabytes of storage space; the bigger the disk, the more important a well-thought out backup strategy becomes. Hard disks are very reliable, but they do fail, and usually at the most inconvenient moment.
hard disk interface: A standard way of accessing the data stored on a hard disk. Several different hard-disk interface standards have evolved over time, including the ST-506 interface, enhanced small device interface, integrated drive electronics interface, and small computer-system interface.
hard reset: A system reset made by pressing the computer's reset button, or by turning the power off and then on again. Used only when the system has crashed so badly that a Ctrl-Alt-Del reboot doesn't work.
hardware: All the physical electronic components of a computer system, including peripherals, printed circuit boards, displays and printers.
hardware address: The physical address for the NIC (network interface card). Used by the low-level hardware layers of a network. TCP/IP's ARP protocol translates IP addresses into hardware addresses. The NIC vendor gets the address from the IEEE and assigns it permanently to the NIC. Also called the MAC (Media Access Control) address.
hardware branch: The hardware archive root key in the registry that is a superset of the memory-resident hardware tree. Although the hardware tree contains information only about those devices currently detected and running in the system, the registry contains a complete list of all hardware ever installed on the particular computer. The hardware root key is \\Hkey_Local_Machine\Hardware.
Hardware Compatibility List: A registry of products that have been tested by WHQL and that have passed Microsoft® Windows® compatibility testing.
Hardware Compatibility Tests: A suite of tests from WHQL to verify hardware and device driver operations under a specific operating environment. These tests exercise the combination of a device, a software driver, and an operating system under controlled conditions to verify that all components operate properly.
hardware-dependence: The requirement that a specific hardware component be present for a program to work. Hardware-dependent software is often very difficult to move or port to another computer.
hardware-independence: The ability to produce similar results in a wide variety of environments, without requiring the presence of specific hardware.
The Unix OS and the Post-Script page-description language are both examples fo hardware independence. Unix runs on a wide range of computers, from the PC to a Cray, and PostScript is used by many printer manufacturers.
hardware ID: A vendor-defined string used by the Plug and Play Manager to find an INF-file match for a device.
hardware interrupt: An interrupt or request for service generated by a hardware device such as a keystroke from the keyboard or a tick from the clock. because the processor may receive several such signals simultaneously, hardware interrupts are usually assigned a priority level, and processed according to that priority.
hardware tree: A record in RAM of the current system configuration based on the information for all devices in the hardware branch of the registry. The hardware tree is created each time the system is started or whenever a dynamic change occurs to the system configuration.
hardwired: Describes a system designed in a way that does not allow for flexibility or future expansion. May also refer to a device or computer connected directly to a network.
hashing: A process that help generate aliases for long file names. For instance, Windows NT uses a hashing process to generate characters three through five of the file name, after it encounters four file names having the same first six characters and extension.
Hayes-compatible modem: Any modem that recognizes the commands in the industry-standard AT-command set, defined by Hayes Microcomputer Products,Inc.
HDSL: High Bit-rate Digital Subscriber Line (HDSL) is generally used as a substitute for T1/E1. HDSL is becoming popular as a way to provide full-duplex symmetric data communication at rates up to 1.544 Mbps (2.048 Mbps in Europe) over moderate distances via conventional telephone twisted-pair wires. Traditional T1 (E1 in Europe) requires repeaters every 6000 ft. to boost the signal strength. HDSL has a longer range than T1/E1 without the use of repeaters to allow transmission over distances up to 12,000 feet. It uses pulse amplitude modulation (PAM) on a 4-wire loop.
head: The electromagnetic device used to read and write to and from magnetic media such as hard and floppy disks, tape drives, and compact discs. The head converts the information read into electrical pulses sent tot he computer for processing.
head crash: An unexpected collision between a hard disk head and the rapidly rotating magnetic recording surface of the disk resulting in damage to the disk surface,and in some severe cases resulting in damage to the head itself.
a head crash in the file allocation table(FAT) area of a disk can be especially devastating because the FAT contains instructions for the OS on how to find all the other directories and files ont he disk, and if it is damaged, the other files and directories may become completely inaccessible.
: An aluminum radiator that dissipates heat. Fast CPU's like the 586/Pentium have heatsinks, and often also cooling fans.
helical-scan recording: A technique used by the 8mm and 4 mm data backup cartridges to increase data density on tapes. The tape moves at a relatively slow speed pasta drum that contains the recording head and revolves at high speed. The drum is tilted, and the recording head records data in diagonal stripes on the tape.
Helical-scan recording increases data density by revolving the recording head more rapidly. Tape speeds are low and tape stress is minimized. See also serpentine recording.
hertz (Hz): A unit of frequency measurement; 1 hertz equals one cycle per second.
hexidecimal (hex): The base-16 numbering system that uses the digits 0 to 9, followed by the letters A to F (equivalent tot he decimal numbers 10 through 15).
Hex is a very convenient way to represent the binary numbers computers use internally, because it fits neatly into the 8-bit byte. All of the 16 hex digits 0 to f can be represented in four bits, and so two hex digits (one digit for each set of four bits) can be stored in a single byte. This means that one byte can contain any one of 256 different hex numbers, from 0 through FF. Hex numbers are often labled with a lower-case h (for example,1234h) to distinguish them from decimal numbers.
HGC: Hercules Graphics Card. A video adapter for DOS computers, introduced by Hercules Computer Technology. HGC provides monochrome graphics with 720 horizontal pixels and 348 vertical pixels.
high-contrast support: Part of Accessibility options set by users to indicate that they require a high degree of contrast to improve screen legibility.
high definition television: A proposed standard that recommends doubling the current 525 lines per picture to 1050 lines, and increasing the screen aspect ratio (that is, width to height) from the current 12:9 to 16:9, which creates a television screen shaped more like a movie screen.
high-density disk: A floppy disk with more recording density and storage capacity than a double-density disk. In the Macintosh, high-density disks contain 1.44MB. In IBM-compatible computers, high-density 5.25" floppy disks contain 1.2MB, while high-density 3.5" floppy disks contain either 1.44MB or 2.88MB of storage space.
high-level format: The process of preparing a floppy disk or a hard disk partition for use by the operating system. In the case of DOS, a high-level format ceates the boot sector, the file allocation table (FAT), and the root directory.
high memory area (HMA): In an IBM-compatible compter, the first 64K of extended memory above the 1 MB limit of 8086 and 8088 addresses. Programs that conform to the extended memory specification can use this memory as an extension of conventional memory although only one program can use or control HMA at a time; DOS, Microsoft Windows, or an application. If you load DOS into the HMA, you can recover approximately 50K of conventional memory for use by your applications.
high performance file system (HPFS): A file system available in OS/2 and WIN NT that supports long, mixed-case file names up to 256 characters, up to 64K of extended attributes per file, faster disk access with an advanced disk cache for caching files and directory information, highly contiguous file allocation that eliminates file fragmentation, and support for hard disks up to 64GB in size. DOS does not recognize the HPFS file structure. HPFS cannot be used on a floppy disk.
high-persistence phosphor: In a monitor, a phosphor that glows for a relatively long time after being energized by electrons. This can lead to ghost images on the screen.
high resolution: In monitors and printers, a description of high-quality output; resolution refers to the sharpness and detail of the image.
HIMEM.SYS: The DOS and Microsoft Windows device that manages the use of extended memory and the high memory area on IBM-compatible computers. HIMEM.SYS not only allows your application programs to access extended memory, it also oversees that area to prevent other programs from trying to use the same space at the same time.
HIMEM.SYS must be loaded by a DEVICE command in your CONFIG.SYS file; do not use DEVICEHIGH.
HKEY_CLASSES_ROOT: One of the five subtrees found in a Windows registry. HKEY_CLASSES_ROOT contains object linking and embedding (OLE) and file-class association data. Information in this subtree is duplicated in HKEY_LOCAL_MACHINE.
HKEY_CURRENT_CONFIG: One of the five subtress found in a Windows registry, HKEY_CURRENT_CONFIG contains current hardware configuration, derived from the configuration used to boot the system. This is the area of the registry that is used and modified during the current session. When the system shuts down, the current configuration is copied back to HKEY_LOCAL_MACHINE under the appropriate area.
HKEY_CURRENT_USER: One of the five subtrees found in a Windows registry, HKEY_CURRENT_USER contains user-profile data for the currently logged-on user.
HKEY_LOCAL_MACHINE: This subtree contains computer hardware information. Part of this subtree is reconstructed each time the computer is started to reflect the current hardware configuration.
HKEY_USERS: This subtree contains all actively loaded user profiles, including the default profile and a duplicate of information in HKEY_CURRENT_USER. Profiles fro remotely logged-on users are stored in the Registries of their local computers
HMMS See CIM.
HMOM See CIMOM.
h-node resolution names: One of the four name resolution modes used on Windows networks. H-node is a hybrid mode wich favors WINS. First, an attempt is made to use p-node to resolve a name via WINS(Windows Internet Naming Service). Because it typically results in the best network utilization, h-node is the default mode of operation for MS TCP/IP clients configured to use WINS for name resolution.
horizontal scanning frequency: In a monitor, the frequency at which the monitor repaints the horizontal lines that make up an image. Horizontal scanning frequency is measured in kHz, and is standardized at 31.5 kHz for a VGA. For SuperVGA, this frequency ranges from 35 to 48 kHz, depending on the refresh rate of the video adapter.
host: The central or controlling computer in a networked or distributed processing environment, providing services that other computers or terminals can access via the network.
Computers connected to the Internet are also described as hosts, and can be accessed using ftp, telnet, Gopher, or a World Wide Web browser.
hosts file: A text file that lists host names and their IP addresses on a network. For small networks, the hosts file is an alternative to DNS. Also called a host table by some TCP/IP vendors.
HSSI: High-Speed Serial Interface (HSSI) is a short distance communications standard for data rates from 2Mbps to 52 Mbps.Common uses include connecting a LAN router to a high-speed line (like T3 or OC-3), host-to-host linking, image transmission, or connecting web servers or e-commerce servers to the Internet.
HTML: HyperText Markup Language. A standardized hypertext language used to create WWW pages and other hypertext documents.
When you access an Internet HTML document using a WWW browser, you will see a mixture of text, graphics, and links to other documents. When you click on a link, the related document will open automatically, no matter where on the Internet that documents is actually located. Normally, you don't see the individual elements that make up HTML when you view a document, although certain browsers have a special mode that displays both the text and the HTML in a document.
HTTP: HyperText Transport Protocol. The protocol used to manage the links between one hypertext document and another.
HTTP is the mechanism that opens the related document when you click on a hypertext link, no matter where on the Internet that related document happens to be.
H.323: An ITU-T(International Telecommunications Union) standard that defines how a flexible, real-time, interactive set of multimedia communications can be exchanged on packet-based networks. For more information www.itca.org
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